Stabilising mine workings with pfa grouts
Stabilising mine workings with pfa grouts
Environmental Code of Practice
Environmental Code of Practice Since the 1970s, hundreds of thousands of cubic metres of disused underground mine workings have been successfully stabilised by filling with grouts based on pulverized-fuel ash (pfa) and Portland cement in order to prevent surface collapse. Similar volumes of disused workings will need to be filled in coming years as pressure grows on land for development. Pfa is particularly favoured for such work as it is cost-effective, technically effective and consumes a nationally available and abundant by-product. However, concerns have been expressed over the potential for contamination of controlled groundwaters by bleed water and leachate released from pfa grout, disruption and dust arising from the work and the effect on land and property prices, and the risk of subsequent classification of the land above the mine workings as being contaminated. This Code of Practice on the use of pfa grouts for filling disused underground mine workings addresses these concerns by providing guidance on the selection of environmentally compatible and cost-effective materials and techniques, with authoritative guidance on good practice. The information and guidance are based on information in the literature, laboratory studies carried out at BRE, data from the use of pfa grouts and expertise from an industry steering group. A detailed report describing an in-depth laboratory study carried out at BRE to assess the leaching characteristics, permeability and physical properties of pfa grouts at different levels and from different sources, is included on the accompanying CD Rom in pdf format. It draws on considerable field experience, and includes a review of groundwater risk assessment models and a specification for mine infilling works.
BRE Press Garston, Watford WD25 9XX
BR 488 ISBN 1 86081 909 5
Stabilising mine workings with pfa grouts Environmental Code of Practice
BRE Construction Division
This Code of Practice is accompanied by a CD Rom containing a detailed Technical Report (BRE Report 220192) and four supplementary documents in pdf format. The Technical Report describes an in-depth laboratory study carried out at BRE to assess the leaching characteristics, permeability and physical properties of pfa grouts at different levels and from different sources. The supplementary documents cover: • A review of groundwater risk assessment models • Leaching tests • Long-term effects of pfa grouts on the environment • A sample specification for mine infilling works.
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Stabilising mine workings with pfa grouts
BRE is committed to providing impartial and authoritative information on all aspects of the built environment for clients, designers, contractors, engineers, manufacturers and owners. We make every effort to ensure the accuracy and quality of information and guidance when it is published. However, we can take no responsibility for the subsequent use of this information, nor for any errors or omissions it may contain. BRE is the UK’s leading centre of expertise on the built environment, construction, sustainability, energy, fire and many associated issues. Contact BRE for information about its services, or for technical advice: BRE, Garston, Watford WD25 9XX Tel: 01923 664000
[email protected] www.bre.co.uk BRE publications are available from www.brepress.com or IHS ATP (BRE Press) Willoughby Road Bracknell RG12 8FB Tel: 01344 328038 Fax: 01344 328005
[email protected] Requests to copy any part of this publication should be made to the publisher: IHS BRE Press Garston, Watford WD25 9XX Tel: 01923 664761
[email protected]
The photos on the front cover show grouting of the limestone caverns at Mons Hill, Dudley in the West Midlands. Over 200,000 tonnes of pfa were used on the contract in a 50 : 1 pfa : PC grout. An environmental impact assessment and monitoring programme was carried out as part of the project. More information is at www.ukqaa.org.uk. Top: The grout production plant. Middle: Drilling a grout injection point. Bottom: Injecting the grout into a cavern. Photos courtesy of UKQAA. The background image is a micrograph of pfa particles, courtesy Dr Gretchen Hoffman, New Mexico Bureau of Geology.
BR 488 © Copyright BRE 2006 First published 2006 ISBN 1 86081 909 5
Stabilising mine workings with pfa grouts
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Contents 1
Introduction ..............................................................................................................................9 1.1 Background...................................................................................................................9 1.2 Pfa production and use.................................................................................................9 1.3 Advantages of using pfa grouts in remediation works ................................................11 1.4 Potential contamination of groundwater .....................................................................11
2
Overall assessment methodology .......................................................................................13
3
Conceptual model for the release of leachates to groundwater.......................................15 3.1 Background.................................................................................................................15 3.2 Source–pathway–receptor concept ............................................................................15 3.3 Water quality limits......................................................................................................16
4
Preliminary risk assessment ................................................................................................17 4.1 Introduction .................................................................................................................17 4.2 Overall preliminary risk assessment ...........................................................................19
5
Simple risk assessment ........................................................................................................21 5.1 General .......................................................................................................................21 5.2 Leachate composition and required dilution factors ...................................................21 5.3 Extent of leachate dilution available from groundwater ..............................................23 5.4 Estimation of dilution factors.......................................................................................23 5.5 Assessment of risk......................................................................................................24
6
Complex risk assessment.....................................................................................................26 6.1 Conceptual model for complex risk assessment ........................................................26 6.2 Groundwater risk assessment models........................................................................28
7
Assessment and characterisation of the site .....................................................................30 7.1 Introduction .................................................................................................................30 7.2 Size of void .................................................................................................................30 7.3 Hydrogeology..............................................................................................................30 7.4 Groundwater ...............................................................................................................31
8
Assessment of material properties......................................................................................32 8.1 Introduction .................................................................................................................32 8.2 Assessment of pfa/cement grout mixes......................................................................32 8.3 Materials variables that can be used to control release from pfa grout......................32
9
Engineered measures for minimising contamination of controlled groundwater..........35 9.1 Control of the groundwater regime .............................................................................35 9.2 Engineered barriers ....................................................................................................35
10 Guidance on site practice .....................................................................................................37 10.1 Safety..........................................................................................................................37 10.2 Nuisance .....................................................................................................................37 10.3 Storage of materials....................................................................................................37 10.4 Grout mix ....................................................................................................................37 10.5 Backfilling boreholes ...................................................................................................38 10.6 Cost of filling operations .............................................................................................38 11 Guidance on environmental monitoring during and after filling ......................................39 11.1 Approaches to monitoring...........................................................................................39 11.2 Monitoring groundwater – quality and flow .................................................................39 11.3 Groundwater monitoring devices ................................................................................39 12 Assessment of other risks associated with the use of pfa grouts ...................................41 12.1 Events potentially leading to environmental risk.........................................................41 12.2 Risk screening ............................................................................................................42 13 References..............................................................................................................................44
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Stabilising mine workings with pfa grouts
CONTENTS OF DOCUMENTS ON ACCOMPANYING CD ROM Laboratory and field data for pfa grouts Technical Report (BRE Report 220192) 1
Introduction
2
Test methods, raw materials and mixes studied 2.1 Test methods 2.2 Materials 2.3 Compositional variables studied
3
Properties of wet grout and compressive strength development 3.1 Flow characteristics 3.2 Compressive strength 3.3 Bleeding tests
4
Permeability, pH and conductivity measurements 4.1 Permeability 4.2 pH measurements 4.3 Conductivity measurements
5
Leaching 5.1 Sample preparation 5.2 Results 5.3 NRA tests
6
Field data
7
Conclusions
8
References
APPENDICES Appendix A. Tables Appendix B. Graphs (strength development, bleed, pH, conductivity and permeability) Appendix C. Graphs of element concentration (mg/l) v No of sample volumes permeated for all elements Appendix D. NRA test results Appendix E. Field data E1 Introduction E2 Use of grouts for mine stabilisation E2.1 Data from Dudley Metropolitan Borough Council E2.2 Cow Pasture Mine E2.3 Castlefields, Metropolitan Borough of Dudley E2.4 Wrens Nest and Castle Hill, Metropolitan Borough of Dudley E2.5 Hurst Hill, Metropolitan Borough of Dudley E2.6 Mons Hill Limestone infilling works, Dudley, West Midlands E2.7 Lilleshall Limestone Mine E2.8 Northwich Salt Mines E2.9 Edinburgh Pentland E2.10 Bream Land Stabilisation Programme E2.11 Ash deposited in old gravel pits E2.12 Pfa lagoon effluent E3 References
Stabilising mine workings with pfa grouts
Supplementary Documents Supplementary Document 1. Groundwater risk assessment models 1 Remedial Targets Methodology (P20) 2 Key features of the groundwater modelling capability 3 ConSim 4 The RBCA tool kit for chemical releases 5 BP RISC 6 AEA Technology approach Supplementary Document 2. Leaching tests 1 Background 2 Maximum leachable tests 3 NRA leaching test 4 NEN 7341 leaching test 5 Water Research Unit (WRU) leaching test 6 CEN procedure 7 DIN 38414 S4 leaching test 8 Limitations of maximum leachability tests 9 Percolation tests 10 Triaxial permeation leaching tests 11 Lysimeters Supplementary Document 3. Long-term effects on the environment 1 Background 2 Impact on ecology 3 Classification of materials and land after filling Supplementary Document 4. Sample specification for mine infilling works 1 General specification 1.1 General description of the works 1.2 Site location and description 1.3 Brief site history 1.4 Geology 1.5 Ground investigations 1.6 Method statement 1.7 General requirements 1.8 Site control and safety 2 Particular specification for mine stabilisation works 2.1 General requirements mine stabilisation works 2.2 Layout of boreholes 2.3 Area of treatment 2.4 Naked lights 2.5 Drilling and pressure grouting procedure 2.6 Exploratory open-hole drilling procedure to locate mine shafts 2.7 Drilling and pressure grouting procedure for abandoned mine shafts and adits 3 Noise control
5
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Stabilising mine workings with pfa grouts
Acknowledgements This work was funded by the Department of Trade and Industry, English Partnerships and the UK Quality Ash Association. Contributions to the work were made by the following BRE staff: Tony Fisher Ian Longworth John Matthews Philip Nixon Keith Quillin Hilary Skinner Paul Tedd Ken Watts
Support and technical assistance was provided by an industry steering group: Brian Bone
Environment Agency
Mark Briggs
Parkman
David Keeton
English Partnerships
Duncan McFadyean
Halcrow
Roger Morgan
Dudley MBC
Alan Robinson
M&J Drilling
Lindon Sear
UK Quality Ash Association
Phil Shelton
Wardell Armstrong
Mike Smith
MA Smith Environmental Consultancy
Steve Somerfield
Forkers Ltd
Peter Watson
Forkers Ltd
Steve Waygood
Innogy
Neil Williams
WS Atkins
Jim Wilson
WS Atkins (DTI representative)
Stabilising mine workings with pfa grouts
7
Executive Summary
Since the 1970s, hundreds of thousands of cubic metres of disused underground mine workings have been successfully stabilised by filling with grouts based on pulverized-fuel ash (pfa) and Portland cement (PC) in order to prevent surface collapse. Similar volumes of disused workings will need to be filled in coming years as pressure grows on land for development. Pfa is particularly favoured for such remediation work as: • It is cost-effective (especially where the volumes to be filled are very large), • It is technically effective (extensive experience exists of producing low permeability materials which can be readily pumped into partially collapsed cavities) • It consumes a nationally available and abundant by-product. However, concerns have been expressed over the potential for contamination of controlled groundwaters by bleed water and leachate released from pfa grout, disruption and dust arising from the work and the effect on land and property prices, and the risk of subsequent classification of the land above the mine workings as being contaminated. This Code of Practice on the use of pfa grouts for filling disused underground mine workings seeks to address these concerns by providing guidance on the selection of environmentally compatible and cost-effective materials and techniques for stabilisation of underground cavities, together with authoritative guidance on good practice. The information and guidance in this Code of Practice is based on: • Information in the literature • Laboratory studies carried out at BRE under a research programme funded by DTI and industry • Data from the use of pfa grouts • Expertise from an industry steering group. Available field data on the use of pfa grouts do not indicate that there are significant effects on the groundwater. However, laboratory studies show that leachates from pfa grouts, without further dilution, exceed water quality standard (WQS) values for certain elements. The risk of contaminating groundwater through the use of pfa grout can be reduced through: • Appropriate materials selection • Appropriate design of the pfa grout. • Measures such as controlling the groundwater regime and using engineered barriers. The appropriate approach will be dependent on the nature and size of the site. Guidance is given in this document to allow the site and the materials being considered to be characterised, and for appropriate measures for safe filling to be taken. In addition to this Code of Practice a detailed Technical Report (BRE Report 2201921) is included in pdf format on a CD Rom included with this publication. It describes an in-depth laboratory study carried out at BRE to assess the leaching characteristics, permeability and physical properties of grouts made using different levels of pfa addition, and the effects of using pfa from different sources.
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Stabilising mine workings with pfa grouts
Also included on the CD Rom are four supplementary documents: • A review of groundwater risk assessment models • Leaching tests • Long-term effects of pfa grouts on the environment • A sample specification for mine infilling works.
Section 1 Introduction
1
Introduction
1.1
Background
9
Mining has taken place at some time in every county in England and Wales over many centuries. However, records are available only from the latter part of the 19th century and, in some cases, surface subsidence provides the first evidence of the presence of such mine workings. Abandoned mine workings are often unstable and liable to collapse without warning. They can also blight land by deterring investment and maintenance. Many occur in built-up areas where they constitute a significant risk to property and public safety. Similar problems can arise from other man-made cavities as well as natural voids. Stabilisation works on disused mine workings may be carried out to remove the threat posed by the mines to the safety and wellbeing of those living, working and travelling in the area, the built environment, including infrastructure, housing and public buildings, and the local economy. Since the 1970s, hundreds of thousands of cubic metres of disused underground mine workings have been successfully stabilised by filling with grouts based on pulverized-fuel ash (pfa) and Portland cement (PC) in order to prevent surface collapse. Similar volumes of disused workings will need to be filled in coming years as pressure grows on land for development.
1.2
Pfa production and use
Pulverised-fuel ash (pfa) is a product of the burning of pulverised bituminous coal at electricity power stations2. About 20–25% of the ash fuses together to form ‘furnace bottom ash’ which has particle sizes in the range 50 mm down to less than 50 µm. The remainder, pfa, (also known as fly ash) is a fine dust which is collected from the combustion gases. This material has particle sizes in the range 150 µm down to 0.5 µm. The composition of the pfa depends on the nature of the coal. The main components are oxides of silicon, aluminium and iron together with a wide range of minor and trace elements including heavy metals. However, potentially toxic trace elements are predominantly contained within the insoluble glassy matrix that constitutes the major part of pfa.
Micrograph of pfa particles.
ASTM Standard C6183 divides ashes into two types: • siliceous or low calcium ashes (designated Class F) produced by the combustion of bituminous coals •
calcareous or high calcium ashes (CaO generally >10%, designated Class C) produced from sub-bituminous coals or lignite.
Only siliceous pfa is produced in the UK. Typical compositions of UK pfa are given in Tables 1 and 2.
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Stabilising mine workings with pfa grouts
The current annual UK production of pfa is about 4.5 million tonnes. Of this about 50% is used in construction (in some parts of Europe it is fully used). There is also a stockpile of some 250 million tonnes, including landfilled material, of which about 55 million tonnes are estimated to be readily available and about 100 million tonnes available ‘with some difficulty’. Table 1. Typical composition of pfa (from Woolley et al4, and Sear and Coombs5) Range of content, % by weight Component
Woolley et al Range
Sear and Coombs Typical
Range
SiO2
45 – 51
48
45 – 52
Al2O3
24 – 32
27
24 – 32
Fe2O3
7 – 11
9
7 – 15
CaO
1.1 – 5.4
3.3
1.8 – 5.3
MgO
1.5 – 4.4
2.0
1.2 – 2.1
Na2O
0.9 – 1.7
1.2
0.8 – 1.8
K2O
2.8 – 4.5
3.8
2.3 – 4.5
TiO2
0.8 – 1.1
0.9
0.9 – 1.1
0.005 – 0.015
0.008
0.01 – 0.2
–
–
< 0.1 – 1.0
0.09 – 0.65
0.2
–
0.3 – 1.3
0.6
0.35 – 1.7
–
–
1.3 – 4.0
Cl Free calcium oxide P2O5 Sulfate as SO3 Water soluble sulfate (g/L as SO4) 2:1 water: solid extract Water soluble alkalis (% as Na2Oeq)
2.0 – 5.5
pH of water extract
9 – 12
Table 2. Trace constituents (after Sear and Coombs5) Trace constituents Arsenic Boron Barium Cadmium Chloride Cobalt Chromium Copper Fluoride
Typical range, mg/kg 40 – 109 5 – 310 0 – 36,000 0–4 0 – 2990 2 – 115 97 – 192 119 – 474 0 – 200
Mercury
0.01 – 0.61
Manganese
103 – 1555
Molybdenum Nickel Phosphorus
3 – 81 108 – 583 372 – 2818
Lead
1 – 976
Antimony
1 – 325
Selenium
4 – 162
Tin
933 – 1847
Vanadium
292 – 1339
Zinc
148 – 918
Section 1 Introduction
11
Pfa is used in a range of engineering applications including engineered fills, landfill reclamation and restoration, as a constituent of cementitious injection grouts, for block and lightweight aggregate manufacture, as a cement replacement and addition to concrete, and in brick manufacture.
1.3
Advantages of using pfa grouts in remediation works
Pfa is particularly favoured for remediation work as: • It is cost-effective (especially where the volumes to be filled are very large), • It is technically effective (extensive experience exists of producing low permeability materials which can be readily pumped into partially collapsed cavities) • It consumes a nationally available and abundant by-product. The technical advantages of using pfa grout for infilling mine workings include: • Its suitability for pumping • Reduced bleeding in comparison with Portland cement • Its reaction with free lime in cement to produce a stable and durable material • Its development of strength over time • Its low permeability • A long-established record of use However, despite industry assurances on safe usage based on many years successful use in practice changing regulations have meant that previously unconsidered issues have become important. Concerns have been expressed over the potential for contamination of groundwater through the release of bleed water (during mixing and placing) and leachates (throughout the life of the grout, but particularly at early stages). Concerns have also been expressed regarding disruption and formation of dust during the work, on the effects on land and property prices, the risk of subsequent classification of the land above the mine workings as being contaminated and the consequent responsibilities for the land owner under the “polluter pays principle”.
1.4
Potential contamination of groundwater
The main risk of adverse effects from the use of pfa in grouts and filling arises from the presence of small amounts of soluble salts. Dissolution of this soluble component leads to an alkaline solution containing mainly calcium and sulfate ions, together with smaller quantities of other metal ions. These soluble components constitute about 2% of pfa and may be mobilised through contact with groundwater. Pollution of controlled waters is only likely if those contaminants are in a mobile form and the receiving water body is sufficiently sensitive. However, work1 has shown that contaminant concentrations in the initial bleed water and contaminated leachate due to longterm rain infiltration and groundwater migration through and around the grout could potentially exceed the Drinking Water and Environmental Quality Standards. Three potential mechanisms for polluting groundwater can be identified during the use of pfa/cement grout and need to be addressed in any risk assessment: 1. Release of bleed water upon emplacement 2. Initial mixing of the grout with mine water when pumped into water-filled mines 3. Leaching of contaminants from the set grout monolith, either from its surface or by permeating through its bulk. Mechanisms 1 and 2 will arise during the grouting operation itself and may consequently only have a short-term effect in the near field, particularly if measures are taken to control its flow. Leaching of any contaminants from the set grout either from surface contact or from groundwater permeating through the bulk material is a longer-term phenomenon. The severity and extent of any pollution during the grouting process and in the longer term will depend to a great extent on the hydrogeology of the site.
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Stabilising mine workings with pfa grouts
Bleed water can arise from segregation of the grout due to the inability of the solid constituents to hold all of the mixing water as they settle downwards, water having the lowest specific gravity of all the constituents. The volume of bleed water can be up to 15% of the total volume of the grout. Such high bleed volumes result from the requirement for a high water-cement ratio in order to achieve the flow characteristics that are required to enable the grout to be pumped over long distances and to flow through fissures within the mine. The bleed water contaminant concentration is unlikely to reflect the long-term composition of the leachate. Bleed water will contain elevated concentrations of certain species as a result of leaching soluble salts from the pfa and the early stages of the cement hydration process. In some circumstances it may be possible to remove bleed water by pumping after emplacement of the grout in order to mitigate its effect on the groundwater. Mixing of pumped grout with mine water in water-filled mines is largely a function of the pumping process. Ideally, the grout will be pumped to the lowest part of the cavity such that the mine water is displaced upwards with the minimum of turbulence. It is standard practice to withdraw mine water for use in the grout mixing plant in order to minimise disposal issues. However, flushing of grout pipes with water into the cavity at the end of pumping is common, but poor, site practice likely to exacerbate contamination issues. Leaching of contaminants from the pfa/PC grout can arise as a result of rainfall or groundwater infiltrating the site. The direction of flow of the leachate will depend on the hydrogeology and it could move down into an underlying important aquifer. The concentration of contaminants in the groundwater depends on a number of factors including: • Groundwater flows and hydraulic gradient across the site • Rain infiltration • Grout permeability • The rate at which soluble species are released from the grout Total contaminant concentrations in the grout may indicate elevated levels, although pollution of controlled waters is only likely if those contaminants are in a mobile (i.e. dissolved or suspended) form.
Section 2 Overall assessment methodology
2
13
Overall assessment methodology
This Code of Practice presents a systematic approach to assessing and minimising environmental risks associated with the use of pfa. Its main focus is the potential for pollution of groundwater due to leaching of contaminants from a grout monolith. However, there are other events with potential environmental consequences and these are also briefly discussed. The level of risk assessment required will depend on a number of factors including the size of filling operation, proximity to controlled groundwaters, the properties of the materials selected and the nature of the specific site being considereda. A tiered approach is therefore used; the level of detail required increases at each tier as the assessment focuses on those risks identified as having the highest priority at the previous stage. This approach is broadly consistent with those used in a number of Environment Agency documentsb and allows the level of detail required in conducting the assessment to be proportionate to the nature and complexity of the risk being addressed. The approach used in this Code of Practice can effectively be broken into the following stages: (a) Risk of release of leachates to groundwater I. Conceptual model II. Preliminary risk assessment III. Simple risk assessment IV. Complex risk assessment (b) Consideration of appropriate additional protective measures (c) Carry out site work in accordance with best practice (d) Carry out site monitoring during and after filling (e) Assessment of other risks associated with the use of pfa grouts It is important to bear in mind that the assessment process is iterative. The level and detail of information available and the understanding of the site and the properties of the materials to be used will develop throughout the risk assessment process. Some of this information may need to be fed back into the conceptual model developed at the start of the process (see Section 4), requiring a new iteration of the risk process. The approach adopted is outlined in a simplified way in Figure 1 and discussed in more detail in subsequent sections. a
b
The Environment Agency should be involved in the process throughout.
For example Environment Agency R&D Publication 206 uses the following tiers: I. Contaminant concentrations in the pore water are compared with target concentrations to establish whether they are sufficient to impact on the receptor, ignoring dilution, dispersion and attenuation along the pathway. II. Dilution by the receiving groundwater upstream of the compliance point is considered. The assessment determines whether there is sufficient dilution to reduce contaminant concentrations to an acceptable level. III. (and Tier IV) Attenuation along the pathway is also considered in addition to dilution. Tiers III and IV are distinguished by the level of sophistication in the modelling and prediction processes used.
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Stabilising mine workings with pfa grouts
Figure 1. Overview of methodology for assessing risk to groundwater
Section 3 Conceptual model for the release of leachates to groundwater
3
Conceptual model for the release of leachates to groundwater
3.1
Background
15
Although other issues are addressed, this code of practice is primarily concerned with the possible effects on controlled waters arising from the release of leachates from a grout monolith. Potentially harmful substances may be leached from a fill or grout and can move into the groundwater. Groundwater may be abstracted for drinking water and river regulation purposes and also naturally feeds surface waters through springs and seepages to rivers. Harmful leachates from materials used for infilling disused mine workings could therefore affect water quality and the attainment of water quality standards. Contaminated groundwater can be defined as that which has been affected by human activities to the extent that it has higher concentrations of dissolved or suspended constituents than the maximum admissible concentrations formulated by national or international standards for drinking, industrial or agricultural purposes7 (Hiscock (1995)). It is an offence to pollute surface and groundwaters (controlled waters) under the Water Resources Act 19918. The European Community Directive on Groundwater9 (80/68/EEC) requires that specific measures be taken to prevent pollution by chemicals in two categories: those that should be prevented from entering groundwaters (List I), and those that “should be minimised” and could have a harmful effect (List II). Further details are given in Supplementary Document 3 ‘Long-term effects on the environment’, which is included on the accompanying CD Rom. The conceptual model should be developed to the standards recommended in the good practice guide10 (McMahon et al. 2001). For many situations the development of a conceptual model and modelling approach will be iterative (it is discussed further in Section 7).
3.2
Source–pathway–receptor concept
The overall approach used in this code of practice involves the determination of a target concentration at the compliance point or receptor and whether leaching from the grout monolith will lead to the target concentration being exceeded. The target concentration will depend on the contaminant in question, the background water quality, and relevant water quality standards for the present or intended use of water from the receptor. A conceptual model for the release of leachates from a grout monolith can be developed using the source–pathway–receptor concept as follows: 3.2.1
Source
The source is the grout monolith. Definition of the source term in a quantitative risk assessment requires the following to be established: • Source contaminants to be expected from the grout used. This will need to consider available materials sources, compositions, physical properties (including durability), • Source geometry – volume and extent may determine total potential • Composition and concentration of bleed water and leachate • Rate of release of leachate/bleed water and change in concentration over time. • The availability of alternative materials should also be considered.
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Stabilising mine workings with pfa grouts
The source and contaminants are reasonably well defined for pfa grout applications compared with many contaminated land and pollution incidents. The contaminants are largely in the dissolved phase either from the bleed water or from groundwater leaching the contaminants from the pfa. Some knowledge of the extent of the pfa grouting will be known depending on the nature of the ground being grouted. Where large open workings such as limestone and chalk mines, or only partially collapsed workings, are grouted the volume and extent should be reasonably well defined. For collapsed workings where relatively small volumes of grout are being injected and are widely dispersed, it may be very difficult to identify the extent. 3.2.2
Pathway
The investigation of possible pathways to the groundwater receptor will require an understanding of the hydrological and geological regimes at the site, such that the groundwater flow and its alteration following grouting can be assessed. Dilution processes must be taken into account in the transport of contaminants from the grout monolith. Contaminant pathways need to be identified in terms of6: • Distance to the receptor or compliance point (e.g. the site boundary or a point 50–100 m downstream) • Time to reach the receptor • Character of the hydrogeological pathway • Processes affecting contaminant concentrations along the pathway, including: o Diffusion/dispersion o Dilution o Attenuation (including sorption and degradation) • Chemical environment • Groundwater/surface water interaction • Microbiological environment • Potential transfer between environmental compartments (e.g. aqueous to sediment phases) • Background water quality • Physical and chemical properties • Possible changes to pathways over time (seasonal abstractions, rates of infiltration etc). The level of information required depends on the level of risk assessment being carried out. Information will need to be collected in increasing detail on progressing from risk screening to simple and complex risk assessments as outlined in Sections 4–6. 3.2.3
Receptor
A receptor, in this context, is any protected water or location down gradient from the grouted site which may be at risk from contamination. Concentration limits for different elements or species are defined by Water Quality Standards. These may be Drinking Water Standards, or Environmental Quality Standards (see below).
3.3
Water quality limits
Two standards are relevant to the potential pollution of groundwater; the Drinking Water Standard (DWS) and the Environmental Quality Standards (EQS). These set specific targets for water quality. For some substances, the EQS may be more stringent (because fish are particularly sensitive to a particular contaminant) while for other substances, where the human is more sensitive, the DWS are more stringent. In such a case the more stringent of the limits should be used.
Section 4 Preliminary risk assessment
4
Preliminary risk assessment
4.1
Introduction
17
A preliminary assessment of the risk of unacceptable pollution of controlled groundwaters can be carried out using an empirical approach based around an indicative risk factor, R, which is a product of factors related to the source (size of site and materials properties), pathway (site hydrology) and the consequences of leachate from a grout monolith entering groundwater: R = Fsource × Fpath × Freceptor where: Fsource is a factor relating to the size of the site to be filled and the materials to be used. It can take values of between 0 and 10. Fpath is a factor relating to the hydrology of the site. It can take values of 1 or 2 for preliminary risk assessment. Freceptor is a factor relating to the consequences arising from contamination of groundwater (receptor) and Source Protection Zone (SPZ) level (where applicable)c. It can take values of between 2 and 10. The indicative risk factor can therefore vary between 0 and 200, depending on the values selected for these factors (see below). The actual value produced should be treated as a guide only; the overall approach and data used may need to be agreed with regulatory bodies. This approach to risk assessment also helps to guide the user towards areas where more information is required. The factors identified can also be refined as the process moves from preliminary risk assessment to simple and complex risk assessments. 4.1.1
Source
The source factor, Fsource, is related to two main characteristics; the size of the site and the nature of the materials used. These are summarised in Table 3 and an approach for combining these is given in Table 4. This code of practice assumes that the risk of an unacceptable effect on groundwater from filling workings with pfa grout on minor sites (less than 250 m3) is, in most cases, likely to be low. This is, after all, equivalent to a void space of just 6.3 m x 6.3 m x 6.3 m. However, a risk assessment should still be carried out in accordance with the process given below although, in general, it is likely that no further precautions will be necessary beyond those related to good site practice (see Section 6) where materials of a known composition are used. The materials classification is based on the level of data available for the material, both generically and for the specific materials and mixes proposed for the site in question. Where the materials properties can be demonstrated to be such that leachate compositions are within appropriate limits (WQS values or other agreed limits) then the risk of groundwater contamination will be acceptably low, irrespective of the nature of the site, provided appropriate best practice is followed during the filling operations.
c
The Environment Agency identifies Source Protection Zones to protect groundwater (especially public water supply) from developments that may damage its quality.
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Stabilising mine workings with pfa grouts
Table 3. Classification of source Issue
Classification
Comments
Factor
Size of the void/grout monolith
Minor
Less than approx. 250 m3 of grout needed.
1
Major
3 More than approx. 250 m of grout needed
2
Materials properties
V
No materials data available
5
IV
Materials from tried/tested source. No List 1 substances in leachate. No data on List 2
III
No list 1 substances. Peak leachate concentration in column tests < 20 times WQS (or other target concentration)
1
II
No list 1 substances. Peak leachate concentration in column tests < 10 times WQS (or other target concentration). Data providing correlation of scale-up available
0.5
I
No list 1 substances. Peak leachate concentrations in full suite of leaching tests < WQS (or other target concentration)
0
2.5
Note: The information required for classifying the materials may not be available at this stage. If this is the case, the highest risk factor given in Table 3 should be taken.
Table 4. Combined source term factor Materials classification Size
V
IV
III
II
I
< 250m3
5
2.5
1
0.5
0
> 250m3
10
5
2
1
0
4.1.2
Pathway
For the purposes of this assessment the factor Fpath can be assessed in terms of the classification of the site as a major, minor or non-aquifer (see Table 5). The depth of the unsaturated zone may also be a factor in determining the amount of attenuation occurring before the leachate reaches the water table.
Section 4 Preliminary risk assessment
19
Table 5. Pathway terms Issue
Classification
Comments
Factor
Non-aquifers
These are formations with negligible permeability that are generally regarded as not containing groundwater in exploitable quantities. However, groundwater flow through such rocks, although imperceptible, does take place, and needs to be considered when assessing the risk associated with very slowly degrading pollutants.
1
These can be fractured or potentially fractured rocks which do not have a high primary permeability, or other formations of variable permeability. Although these aquifers will seldom produce large quantities of water for abstractions, they are important both for local supplies and in supplying base flow for rivers. In certain local circumstances minor aquifers can be highly vulnerable to pollution.
1–2
Hydrogeology Minor aquifers of the site
Major aquifers
These are highly permeable formations usually with a known or probable presence of significant fracturing. They may be highly 2 productive and able to support large abstractions for public supply and other purposes. 2
No data Depth of unsaturated zone
4.1.3
This factor can be used to modify the overall risk factor to take account of attenuation of leachates arising from indirect discharge into groundwater.
0.5 – 1
Receptor
For the purposes of this assessment the term Freceptor is defined in terms of the suitability of the groundwater for human consumption and the site’s position with respect to Source Protection Zones (see Table 6). Table 6. Receptor terms Issue
Classification
Comments
Groundwater Poor quality Permanently unsuitable for human consumption quality Medium quality Suitable for human consumption after extensive treatment High quality
4.2
Factor 2 4
Suitable for human consumption after minimal treatment (not within SPZ)
6
SPZ 3
6
SPZ 2
8
SPZ 1
10
No data
10
Overall preliminary risk assessment
The proposed preliminary risk assessment framework provided here allows risk to be presented empirically, as an indicative risk factor, on a scale of 0 to 100 (200 where no information on the materials used is available). The relative factors are: • Source 0 – 5 (minor void) or 0 – 10 (major void) • Pathway 1 – 2 • Receptor 2 – 10
20
Stabilising mine workings with pfa grouts
Consequently it is assumed that, at the preliminary risk assessment stage, the main emphasis will be on the source and receptor. The effects of the pathway can be addressed more specifically at the simple and complex risk assessment stages. If the material is assumed to be inert (supported by adequate leaching data – see Section 8) then the indicative risk factor can be 0 in all cases. In this case no further assessments will be required. Where no data are available the highest value within any category should be assumed. Where the indicative risk factor is less than or equal to 20 the preliminary risk assessment process may be sufficient to provide confidence that the risks are acceptable. This can only be achieved where at least one of the following can be demonstrated: • Leachate concentrations arising from the source are low • The site is small • The groundwater is already of poor quality Where the indicative risk factor exceeds 20 it is recommended that a simple risk assessment is carried out. Under these circumstances the preliminary risk assessment process provides a steer towards priority factors in reducing the level of risk. The simple risk assessment may provide better quality data, which, when used in the preliminary risk assessment procedure, indicates a reduced level of risk.
Section 5 Simple risk assessment
5
5.1
21
Simple risk assessment
General
Where the preliminary risk assessment is insufficient to enable a decision to be made concerning the suitability of using pfa grout it will be necessary to proceed to the next tier – Simple risk assessment. The objective is to determine a target concentration at the compliance point or receptor and to determine whether leaching from the grout monolith will lead to the target concentration being exceeded. The target concentration will depend on the contaminant in question, the background water quality and relevant water quality standards for the present or intended use of water from the receptor. For groundwater drinking water standards may be most appropriate, although others, such as environmental quality standards, may need to be considered in certain circumstances (see R&D 20 for further details6). In general the target concentration should be set to provide the greatest level of protection to the receptor. Where a quality standard is used in deriving the target concentration it is possible that, even though the standard is met, a significant deterioration in groundwater quality will result. R&D 20 recommends that the acceptability of this needs to be assessed in relation to the extent to which more onerous standards could be met, the sensitivity of the receptor and conservativeness of the approach used in the assessment. The simple risk assessment process will involve a more detailed understanding of the source and pathways. It may involve characterisation of the actual materials proposed for use in the filling operation, including an assessment of the elements that are likely to leach from them in use. It may also be necessary to collect more information on the site itself. Where the conservative assumptions used in the simple risk assessment lead to the prediction of unacceptable impacts it may be possible to complete a further iteration of the assessment using less conservative inputs, provided these can be justified. High priority risks may require more detailed investigation in a Complex risk assessment. The preliminary risk assessment approach outlined above is likely to indicate that most sites greater than 250 m3 will require at least a simple risk assessment, unless there is already a substantial body of data available on the properties of the grout material and the geology of the site. This process will therefore require the collection of data not available at the preliminary assessment stage. More information will be required concerning the materials used and mix compositions as well as the hydrogeology of the site. The criteria used need to be conservative. Where this is the case the simple risk assessment may provide sufficient confidence in the use of the material without the need for a complex risk assessment. There are a number of modelling packages available that can be used for carrying out the calculations that form the basis of the simple risk assessment although these are not essential at this stage (see section on complex risk assessments).
5.2
Leachate composition and required dilution factors
The composition of the leachate from a grout will depend on the composition of its basic ingredients and the mix design. This issue is discussed in more detail in Section 8. There are two ways in which data on leachate compositions could be determined:
22
Stabilising mine workings with pfa grouts
1. Carry out laboratory tests on the actual materials and mixes to be used in the filling operation. 2. Use existing data • Materials from a known and quality assured source • Worst case laboratory test data, with appropriate safety margins 5.2.1
Use of existing data
Some leaching data for pfa grouts is available (see BRE Report 2201921) but it is, in many cases, unlikely that it will provide sufficient evidence in making a case to the Environment Agency given the inherent compositional variability of pfa (and other materials). Whilst, in the longer term, a more substantial database may be developed it is likely that laboratory testing will be required in most cases at present. However, it may be sufficient, in certain circumstances to use existing worst case data with an acceptable and agreed safety factor to reflect the fact that the limited number of data available may not cover the worst case for any particular element. Table 7 summarises available BRE data and shows that this approach would require a dilution factor of 300 to provide sufficient dilution for the peak aluminium concentration to meet the water quality standard for that element. Table 7. Peak ion concentrations in leachate (see BRE Report 2201921 for further details) Element/ species
WQS (mg/l)
Highest experimental leachate concentration (µg/l)
Required dilution
Al
200
58,600
293
As
10
299
30
B
1000
900
–
Ba
1000
33,100
33
Ca
250,000
778,000
3
Cd
5
40
8
Cr
50
549
11
Fe
200
120
–
Hg
1
0.5
–
K
12,000
2640,000
220
Li
25,000
27,600
–
Mg
50,000
250
–
Mn
50
15
–
Mo
70
7000
100
Na
200,000
857,000
4.3
Ni
20
101
5
P
2200
800
–
Pb
25
2070
83
Sb
5
11
2
Se
10
265
26
Sn
25
7
–
Sr
–
74,100
V
20
1030
50
Zn
5000
1830
–
Cl-
250,000
3,450,000
13
SO42-
250,000
213,000
–
50,000
10,300
–
NO3 pH
-
10
–
Section 5 Simple risk assessment
5.2.2
23
Carry out tests on the material to be used.
Evidence from BRE laboratory work indicates that there is significant variability in leachate concentrations between pfa sources. This is shown for grouts made using pfa from six sources (A to F) in Table 8. Where the actual materials to be used are studied no safety factor should be required. Table 8. Effect of pfa source on peakd leachate concentration (see BRE 2201921 for further details) Ion concentration (mg/l) Source
A
B
C
D
E
F
K
1450
1330
2640
917
1090
1000
Al
55.3
7.6
19.4
3.85
2.89
10
Mo
5.8
0.1
0.31
0.12
0.05
0.31
Peak concentrations also depend on the mix design used; both the pfa : Portland cement ratio and the water: solid ratio can affect leachate concentrations and the permeability of the material (see Section 9). Details of leaching tests are included in BRE Report 2201921.
5.3
Extent of leachate dilution available from groundwater
For the purposes of this Code of Practice an approach similar to Tier 2 used in R&D Publication 206 is proposed. This approach considers the potential dilution of contaminants by groundwater flowing beneath (or around) the monolith. Other forms of attenuation are not considered at this stage. A dilution factor appropriate for the specific site can be estimated. This can be used, together with water quality standards (or some other measure of acceptable maximum concentrations for species in the groundwater) to produce a target concentration for species in the leachate from the monolith itself.
5.4
Estimation of dilution factors
There are a number of equations available for estimating dilution factors for different scenarios. The appropriate equation can be selected on a site-specific basis. 5.4.1
Grout monolith above the water table
R&D Publication 206 (page 38) provides the following equation for the dilution factor, DF, for groundwater flow beneath a contaminated site: DF =
(k.i.Mz + L.Inf)Cc ki .Mz.Cu + L.Inf .Cc
or
( k .i .Mz + L.Inf )CT − k .i .Mz.Cu L.Inf .Cu
where: Mz = Mixing zone thickness (m) = (0.0112L2)0.5 + da(1 – exp(–L.Inf / kIda)) (note: where da < 20 m then Mz = da). L
= length of contaminant source (grout monolith) in the direction of groundwater flow (m)
da
= aquifer thickness
Inf = infiltration to the aquifer from the contaminated site (m/day) k d
= hydraulic conductivity (m/day)
The data in Table 6 derive from samples using similar mix designs. They differ from peak values in Table 5 which represent a wider range of samples
24
I
Stabilising mine workings with pfa grouts
= hydraulic gradient
Cc = concentration of contaminant in contaminated discharge (mg/l) CT = target concentration (mg/l) Cu = Background concentration of contaminant in receiving water (mg/l) Where the background concentration Cu is assumed to be zero the following equation can be derived: DF =
k .i .Mz +1 L.Inf
The values used in this equation can be obtained from laboratory measurement (see Section 5.4.3) and site measurement6. 5.4.2
Grout monolith below the water table
In this situation the critical short-term issues relate to site practice during the injection phase (see Section 1.4). Dilution of any leachate from the hardened monolith is likely to be significantly greater and will be related to water flow, which is dependent on the hydrogeology of the site in question. 5.4.3
Permeability and rate of flow of leachate
The rate of infiltration from the grout monolith to the groundwater (used in estimating the dilution factor) will depend on the permeability of the grout. Grout permeability can be estimated using laboratory techniques that measure the volume of fluid passing through a sample of known dimensions under a known pressure and the time required to collect that volume (see BRE Report 2201921). The rate of flow of leachate, Q, through a grout sample or monolith is given by: Q = A.K.i Where K is the permeability (hydraulic conductivity) in m/s, A is the cross-sectional area and i is the hydraulic gradient (the ratio between the hydraulic head and the distance over which the flow occurs). If it is assumed that the monolith is not below the water table the value of Q can be divided by the cross-sectional area, A, to produce a value for the infiltration rate (inf) for use in the equations in Section 5.4.1. This simple relationship given above also shows the effect of sample geometry on the rate of release of leachate (although the effects of water flowing around the monolith will also be a factor). Assuming that the hydraulic head and the volume of the monolith remain constant the rate of flow of leachate will be greater for a monolith with a smaller dimension in the direction of flow. It is important, therefore, that the nature and shape of the void being filled is known as well as the hydrology of the site. The equation also shows the potential benefits of varying the material composition in order to reduce the permeability and consequently the rate of leachate release from a grout. Reducing the rate of release of leachate will serve to reduce the concentration of leachate at extraction points from the groundwater.
5.5
Assessment of risk
The simple risk assessment process will essentially involve the comparison of the required dilution needed to reduce the peak leachate concentration to the target concentration (assumed here to be the water quality standard) as described in Section 5.1 with the dilution available in the groundwater.
Section 5 Simple risk assessment
25
Where the simple risk assessment indicates that dilution alone will not be sufficient to reduce the concentrations in the groundwater to below the target concentrations then there are two options: • Carry out a complex risk assessment (see Section 5). • Modify the materials properties (leachate composition and permeability) using the factors summarised in Section 9 and in BRE Report 2201921 and retest. • Apply engineered measures as outlined in Section 10. The revised data can also be used in the preliminary risk assessment framework to identify areas where further information is needed.
26
6
Stabilising mine workings with pfa grouts
Complex risk assessment
Complex risk assessments need to focus on source–pathway–receptor linkages where risks have not been screened out at the preliminary and simple risk assessment stages. The level of complexity may therefore depend on the nature of the risk. Complex risk assessments are likely to require more detailed information on the site (such as local hydrogeology). The conceptual understanding of the flow and transport processes operating around the site is developed from that required at the simple risk assessment stage.
6.1
Conceptual model for complex risk assessment
The source–pathway–receptor approach to developing a conceptual model, discussed in Section 3, may need to be developed further in carrying out a complex risk assessment. At this stage additional consideration is given to any attenuation that may arise as the contaminant moves through unsaturated and saturated zones to the receptor. The compliance point is taken as a point down gradient of the site being assessed, e.g. a bore hole or aquifer. Currently, available data from a standard site investigation are rarely sufficient for contaminant transport modelling purposes and additional or more detailed data are likely to be required to construct and test a mathematical model. 6.1.1
Translation of the source term to the mathematical model
In translating the source term to a mathematical model it will be necessarily to define: • Dimensions of the contaminant source (length, width and depth). Many analytical models require the assumption of a simple source geometry, a linear source term • Initial contaminant concentrations at the source • Change in contaminant concentration with time. For pfa grout this should be represented by a declining source represented by a first order declining decay term (see BRE Report 2201921). 6.1.2
Defining the pathway
The conceptual model should aim to define the pathways or routes by which contaminants can migrate through the sub-surface from the source to a receptor. Conceptual models produced by the Environment Agency as part of the regional flow models or Source Protection Zone delineation may be available. They often include Agency held data, a description of the important flow mechanism operating at the site and some water balances. Geology It is essential to determine the lithology in the area of study, the geometry of the different lithologies and the stratigraphy. The lithology determines the flow and transport properties of the strata. It may vary laterally and vertically over a short distance and within a single stratigraphic unit. Water table The water table in relation to the grouted area needs to be established. Historical fluctuations and the existence of perched water tables need to be determined. The effect of grouting on the water table and regional flows needs to be considered in the model. Rising groundwater table is common in many urban areas and this also needs to be considered in the long-term evaluation of the model.
Section 6 Complex risk assessment
27
Unsaturated zone flow and transport Movement of water through ground above the water table, the unsaturated zone, is complex and a function of infiltration, degree of saturation, hydraulic conductivity, soil suction and the properties of the polluted fluid. There are many situations where grouting does take place above the water table. Mathematical expressions have been developed to describe water movement through unsaturated ground but these are complicated and still require simplifications about the system behaviour. Flow of water within the unsaturated zone is characterised by unsaturated permeability and may be very much slower than in a saturated zone. Where a ‘wetting front’ moves through an unsaturated zone the permeation may vary widely depending on the degree of saturation. Groundwater flow in saturated ground The conceptual model should seek to describe the groundwater flow system in terms of: • Horizontal and vertical hydraulic gradient • Seasonal variations in groundwater and flow directions • Possible heterogeneity of the flow system, multiple porosity and fractures • Inflows, recharge, river leakage and outflow (abstraction, spring discharge) This information provides the basis for understanding how contaminants dissolved in the groundwater are likely to move through the groundwater system. Fate and transport processes The conceptual model should describe the processes that control the movement of contaminants in the soil, unsaturated and saturated zones. These processes include: • Advection • Dilution • Dispersion • Diffusion • Sorption • Degradation (biotic and abiotic). Each of these transport processes can be described by different driving equations. 6.1.3
Defining the receptor
A receptor, in this context, is any protected water or location down groundwater gradient from the grouted site which may be at risk from contamination. The potential receptors that must be considered in the conceptual model include: • Groundwater including aquifers below the site and any deeper aquifers • Groundwater abstraction boreholes • Natural groundwater discharge e.g. springs, wetlands surface watercourses • Compliance point defined by the Environment Agency, e.g. a compliance borehole located down hydraulic gradient of the site Identification of the groundwater receptor should take account of the location and designation of Major, Minor and Non-aquifers and Source Protection Zone I, II and III under the Policy and practice for the protection of groundwater11 (Environment Agency, 1998). Where groundwater is classed as permanently unsuitable for use then provisions for its protection are not required. Information will need to be gathered concerning: • The importance of the groundwater as a resource (major, minor, non-aquifer) • Aquifer vulnerability • Proximity of source to (or continuity with) surface water • Proximity to groundwater abstractions (including whether the site falls within a SPZ) • Actual use of groundwater resource • Historical, current and planned land use, ownership etc. • Site services.
28
6.2
Stabilising mine workings with pfa grouts
Groundwater risk assessment models
Increasingly, computer models are being used to aid the assessment of risks to groundwater sources from contaminated soils. Limits and assumptions made in applying the model also need to be taken into account in the risk assessment process. It is suggested that these models could be used to undertake a complex risk assessment if required. Consultation with the Environment Agency (or the Scottish Environmental Protection Agency) should be carried out. It is essential that any modelling study has well defined objectives and that the model itself will meet the objectives and provide the necessary results to enable decisions to be made. A modelling approach is only appropriate if a robust conceptual model has been developed that can adequately be described by a mathematical relationship and in which sensible values of input parameters can be used. The following groundwater models are briefly described and their value in evaluating groundwater pollution arising from pfa-based grouts is assessed. The first two have been published by the Environment Agency. They have been developed to be applicable to the legislative and policy framework that applies in the UK and are understood to represent the Agency’s recommended approach to assessing risks to water resources from land contamination. •
Remedial Targets Worksheet v1.1. The remedial targets worksheet and manual may be downloaded from the Environment Agency website. A password is obtained following purchase of R&D Publication 206.
•
ConSim. A demonstration copy of ConSim is available from the Agency website and a full copy can be obtained from Golder Associates, email:
[email protected].
•
The RBCA Tool Kit for Chemical releases. This is obtained from the GSI website, www.gsi-net.com.
•
BP RISC v3.09. Further information is given on www.bprisc.com and the software may be obtained from www.groundwatersoftware.com.
The various methodologies for the assessment of risks and derivation of remedial targets for the protection of groundwater make use of the various models available. These models are, of necessity, a simplified mathematical description of the governing and chemical and physical processes. Significant simplifications in modelling the contaminant source and transport through pathways are made that are likely to lead to conservative results. In relation to source modelling, it is important to analyse a range of scenarios that represent reductions in source contaminant concentration and availability. Each of the software tools contains its own tiers or levels. Only the Remedial Targets Worksheets is designed to be compatible with the Remedial Target Methodology. ConSim deals only with contamination sources in the unsaturated zone and is not suitable for many applications of water-filled mines. These software tools have only limited or no capabilities for representation of fissured flow in the ground. ConSim includes a dual porosity model for the unsaturated zone, which is designed for modelling chalk aquifer conditions. It is not intended for general use with all types of fissured aquifer, since flow in the fissures is assumed to be laminar. Turbulent flow cannot therefore be modelled. A benchmarking exercise undertaken by Whittaker et al12 showed that the four software tools considered take similar approaches to modelling groundwater flow and contaminant transport. Their conceptual models are based on comparable assumptions and geometries. Of the four
Section 6 Complex risk assessment
tools considered, only ConSim allows a probabilistic approach to groundwater risk assessments. The ASTM RBCA protocol and therefore the GSI RBCA Tool Kit were primarily developed to assess the chronic risks to human health and do not consider a water resource to be a receptor in its own right. Nevertheless The RBCA Tool Kit and the BP RISC may be used to consider impacts on water quality but it needs care of the default set-up of those models. These models are summarised in Supplementary Document 1 ‘Groundwater risk assessment models’, which is included on the accompanying CD Rom.
29
30
7
7.1
Stabilising mine workings with pfa grouts
Assessment and characterisation of the site
Introduction
Effective characterisation of the site is a necessary prerequisite for assessing the risk of pollution. It will be an essential component in the development of a conceptual model and will provide a basis for the risk assessment process (although the amount of information required will depend on the level of risk assessment; less information will be required for risk screening than at the complex risk assessment level). Characterisation of the site will establish three important points: 1. The size and shape of the void to be filled and hence of the grout monolith that will be created. 2. The hydrogeology of the site and the likely amounts of rainfall which will influence the pathway and pollutant linkage to the groundwater receptor. 3. The nature of the groundwater. The investigation suggested here gives an indication of how the information required for the classification can be obtained. If the site is likely to be classified as a 'major' site consultation of an expert is strongly recommended. An awareness of the current state and any historical incidents of contamination is also recommended. A simple characterisation of the hydrogeological regime is unlikely to prove easy for most sites requiring infilling. Further useful information on the investigation of groundwater can be found in CIRIA C515 Groundwater control – design and practice13.
7.2
Size of void
The demarcation at 250 m3 used in this Code of Practice is made on the basis that sites of this size or smaller should include all the single plot workings where the risk of an unacceptable effect on groundwater is minimal. Information on the local geology can be obtained from solid and drift geology maps and geological features can be inferred from aerial photographs. These sources, together with mining and other historical records may give an indication of the potential void size. Drilling can also help locate voids and areas of solid ground although access constraints can make the physical identification of void sizes very difficult.
7.3
Hydrogeology
The direction of flow of the leachate will depend on the hydrogeology which will govern the likelihood of it reaching an underlying aquifer. After the leachate has left the grout it could move through unsaturated rock towards an aquifer from where the groundwater provides a medium of transport to a location down-gradient of the site where water may be extracted. Whether the bleed water or leachate from the set grout reaches the controlled groundwater depends upon whether there is a hydrological pathway and on the nature of that pathway. A pathway may not exist. If the pathway is long and tortuous, then natural attenuation and dilution by unpolluted infiltration and groundwater could reduce the contaminant concentration to below the required standards. Dilution of the leachate from the grout monolith will also depend on the level of rainfall.
Section 7 Assessment and characterisation of the site
31
An understanding of the local geology is an essential prerequisite. Likely water-bearing strata, their geometry and structure (composition, faults and fissures), and therefore potential for ground water flow can be identified from geological maps and records and may be confirmed by drilling and sampling. Details of abstraction and recharges to the groundwater can be found in abstraction licences (including historical), discharge consents and well records. Evidence of more surficial flows and likely infiltration can be found in the topography and meteorological records. Local hydrogeological maps may also be available.
7.4
Groundwater
Identification of the groundwater receptor should take account of the location and designation of Major, Minor and Non-aquifers and Source Protection Zone I, II and III under the Policy and practice for the protection of groundwater11 (Environment Agency, 1998). Where groundwater is classed as permanently unsuitable for use then provisions for its protection are not required. Groundwater quality can be classified using: • Environment Agency source protection zones and vulnerability maps, which define areas surrounding drinking water abstraction points and dangers from pollution. Sites within a source protection zone will be of greatest concern and groundwater should generally be of ‘high quality’. • Sites outwith an SPZ, and which do not show significant linkage with other water resources may be classified as ‘medium quality’. Detailed information on groundwater can be obtained from the ground investigation. Groundwater levels should be obtained via standpipes at suitable intervals across the site – a greater number of measuring points are required to characterise a potentially complex site. Hydrostatic water pressures can be found from piezometers at different depths. An estimate of the local permeability can be found by in situ permeability tests in the boreholes. On sites with simple geology, flow may be inferred from measurements of water pressure across the site, coupled with knowledge of the intervening geology. Where the situation is more complex, groundwater flows can be measured directly in a number of boreholes.
32
8
8.1
Stabilising mine workings with pfa grouts
Assessment of material properties
Introduction
In carrying out a risk assessment it is likely that the properties of the materials to be used will need to be characterised to allow the likely concentrations and volumes of leachate to be assessed. This could be achieved through: • Using approved materials with known properties (if available) • Characterising the proposed materials through leaching and permeability tests This section reviews these techniques and summarises the materials-related variables that can be controlled in order to minimise leaching. The information presented is derived from an experimental programme aimed at assessing the properties of pfa grouts and assisting in the design of mixes which limit the leaching of soluble species from the grout (summarised in BRE Report 2201921), together with other information available in the literature and from practical use of pfa grouts has been used here. Leaching tests are summarised in Supplementary Document 2, which is included on the accompanying CD Rom.
8.2
Assessment of pfa/cement grout mixes
The best way of assessing the leaching properties of a pfa/cement grouts is by a laboratory leaching test. Leaching tests have been developed to identify the rate and extent to which the component species can be removed from a material in contact with a liquid. Key factors include: • The nature of the solid o chemical properties (such as speciation, the manner in which ions/species are bound to the solid, whether they are at the surface, homogeneity, solubility, lability, changes over time etc) o physical properties (permeability, porosity, cracking, particle size etc) o water content o durability • The nature of the liquid (composition, pH etc) • The surface area exposed to the liquid • The solid : liquid volume ratio • The flow rate • The effectiveness of the contact between the solid and liquid (e.g. does the liquid percolate through the whole of the solid or concentrate in channels?, does pore clogging occur?) • Temperature • Time – degree of curing before leaching
8.3
Materials variables that can be used to control release from pfa grout
Experimental work has shown that contaminant concentrations in the bleed water and leachate could exceed the Drinking Water and Environmental Quality Standards. However, there are a number of factors that can be used to control the rate of leaching and the concentration of species. These include: • Composition and source of pfa • Pfa : cement ratio • Water : solids ratio • Engineering methods
Section 8 Assessment of material properties
33
These are described below and summarised in Table 9. Further details are given in BRE Report 2201921. Table 9. Summary of mitigation of leaching from pfa grouts Factor
Permeability
Leachate Comments concentration
Increase Decrease cement content
Composition changed
10 : 1 pfa : PC mixes are more than an order of magnitude less permeable than 30 : 1 mixes
Ash source
Little effect
Significant influence
The ash source has a significant influence on the composition of the leachate and peak values observed in laboratory tests
Lagooned ash
No effect
Reduction
Removal of soluble species through the washing resulting from the lagooning process reduces peak concentrations
Reduce water : solids ratio
Reduction
No effect
Order of magnitude change in permeability between mixes with water : solids ratio of 0.45 and 0.55.
Leaching
Significant falls
Leachate concentrations generally fall significantly after about one sample volume has been leached. Some species solubility increases through common ion effects
Sample age Reduction prior to leaching with age
Variable
Available data show that concentrations of different species depend on the age of the pfa
Bentonite
Composition changed
Laboratory data show no reduction in permeability from the addition of bentonite to pfa grout
8.3.1
No effect
Source and composition of pfa
The composition of the ash can have a major influence on the concentrations of specific ions in the leachate from a pfa grout. In general, ashes from lagoons or from other sources where some initial leaching has already taken place, will be more suitable than fresh ashes for use in stabilisation work. Apart from this preference for lagoon ash, the compositions of different pfa sources are so variable that no general advice can be given and it will be necessary to assess the individual source intended to be used. 8.3.2
Pfa/cement ratios
The pfa/cement ratio will influence the leachate composition. In cement-rich mixes the peak levels of elements associated with cement such as lithium, sodium, potassium, calcium, strontium, and barium will be highest, whereas in pfa-rich mixes, elements such as chromium, aluminium, boron, selenium, vanadium, molybdenum, antimony and arsenic plus the ions sulphate and nitrate are highest. 8.3.3
Water/solids ratio
Variations in the water/solids ratio seem to have little consistent effect on the leachate composition but will influence the propensity to develop bleed water. 8.3.4
Permeability
Permeability generally increases with increasing water/solids ratio and with increasing pfa/cement ratio. Ash sources have little effect. Reducing the permeability of the grout will reduce the rate of leaching and consequently increase the degree of dilution. 8.3.5
Bleed water
Bleed water can arise from segregation of the grout due to the inability of the solid constituents to hold all of the mixing water as they settle downwards, water having the lowest specific gravity of all the constituents. The volume of bleed water can be up to 15% of the total volume of the grout. Such high bleed volumes result from the requirement for a high water-
34
Stabilising mine workings with pfa grouts
cement ratio in order to achieve the flow characteristics that are required to be able to pump the grout over long distances and to flow through fissures within the mine. The bleed water contaminant concentration is unlikely to reflect the long-term composition of the leachate. Bleed water will contain elevated concentrations of certain species as a result of leaching soluble salts from the pfa and the early stages of the cement hydration process. The concentrations of these species in the leachate will fall rapidly as the small amounts of soluble salts are removed. The pozzolanic reaction between the cement and the pfa will also reduce the concentrations of certain species. Control of the bleed water in terms of volume produced or managing its migration from mine workings could be important in controlling any potential contamination arising from the use of a pfa/PC grout. Bleed water volume can be reduced by changing the grout mix components and proportions. Migration of the bleed water could also potentially be regulated by hydraulically containing the site, and pumping out and discharging excess groundwater. The volume of bleed water liberated from a pfa grout is dependent not only on the water-solids ratio but also on the nature of the pfa. 8.3.6
The use of additions such as bentonite
In laboratory experiments the use of bentonite has not led to a reduction in the permeability of the grout. Some benefits may arise from the use of bentonite via chemical stabilisation. In BRE experiments the addition of bentonite reduced peak leachate concentrations for calcium, lead, molybdenum, selenium, chromium, barium, chromium and potassium. However, arsenic, vanadium, boron and sodium ion concentrations in leachates from bentonite-containing grouts were significantly higher than those from bentonite-free grouts.
Section 9 Engineered measures for minimising contamination of controlled groundwater
9
35
Engineered measures for minimising contamination of controlled groundwater
Contact between the groundwater and leachate can be minimised by means of engineered barriers, control of the local groundwater regime, or both. Barrier systems are often associated with permanent measures, whereas shorter term solutions are more likely to be effected through pumping. In contrast to other kinds of potential releases of contaminants, the leachate from a pfa monolith arises from a source with relatively low permeability that may be similar to the barrier itself. A benefit of an engineered barrier solution is that it will be easier to control the overall permeability of the barrier when compared with the grout monolith.
9.1
Control of the groundwater regime
As it is groundwater flow, and in particular the flow around and through the grout monolith, that gives rise to concerns over contamination, direct control of groundwater flows would seem attractive and can provide opportunities for regular sampling of groundwater in the vicinity of the monolith. However, control of the groundwater is not generally a simple operation and requires an understanding of the sequence and characteristics of each stratum. Groundwater control exercised over what may be very large areas in some cases can impact significant volumes of ground. Local soil behaviour and hence the performance of local buildings and other structures can be sensitive to changes in the groundwater regime and this should be considered in the design of such measures. As construction techniques and analytical methods have evolved to control groundwater during other forms of construction, so control can be exercised through the use of barriers (see below) and pumped wells in the same way for grouting operations. Groundwater could be lowered beneath the grouting operation level, or the site hydraulic gradients could be managed to ensure that no adverse flows occur. Guidance can be found in CIRIA C515 Groundwater control – design and practice13 and in SP124 Barriers, liners and cover systems for containment and control of land contamination14. Consent of either the Environment Agency or the local water company will be required for any recharges into controlled waters or sewer systems.
9.2
Engineered barriers
Vertical barriers can be used to contain contaminants and redirect groundwater flows. In the case of leachate from a pfa grout monolith, the barrier can impede the flow of groundwater in and around the pfa in order to minimise leachate production. A vertical barrier in general consists of a low permeability barrier installed to sufficient depth or keyed into an impermeable stratum. The barrier may be constructed of many materials but it is likely that cement bentonite or cement-based grouted walls would most readily provide permeabilities of lower than 10–7m/s. It will be important that the barrier materials do not give rise to similar concerns over leachate contamination as the grout monolith itself.
36
Stabilising mine workings with pfa grouts
In plan, barriers could: • Encircle the grout monolith. The entire grouted mass is surrounded by a barrier wall that prevents ingress or egress of groundwater. This has the advantage that uncertain groundwater flows are catered for, but may require a very long wall. Extraction wells can be positioned within the barrier in order to lower the groundwater table and force the preferential flow direction towards rather than away from the grout monolith. • Be positioned up-gradient. The barrier is positioned such that the groundwater flow is intercepted before entering and flows preferentially around the grout monolith. Extraction wells positioned down-gradient of the barrier can be used to intercept the groundwater flow that may still arise. • Be positioned down-gradient. A barrier is positioned to intercept leachate flows from the grout monolith. Extraction wells positioned down-gradient of the monolith but inside the barrier can be used to intercept the groundwater flow. For further information see: • CIRIA, Groundwater control – design and practice, C51513 • CIRIA, Barriers, liners and cover systems for containment and control of land contamination, SP12414 • Institution of Civil Engineers, Specification for the construction of slurry trench cut-off walls, Thomas Telford15
Section 10 Guidance on site practice
10
37
Guidance on site practice
Grouting operations should be carried out so as to minimise the risks posed to site personnel, visitors, members of the public and the environment.
10.1
Safety
It should be assumed that all materials encountered in boreholes and trial excavations at the site are potentially contaminated, hence workers should operate with care at all times. All site personnel and visitors should be made aware of hazards and health and safety procedures involved to ensure safe operation. Monitoring of gases may be required in boreholes or excavations. When odours are perceived or identified all drilling or excavation should cease in the locality until adequate testing has been carried out and it is confirmed by the testing that it is safe to continue.
10.2
Nuisance
All stored materials, such as pulverised-fuel ash, should be sheeted or treated so that no nuisance is caused by wind borne dust as a result of this operation. It may be necessary to minimise the production of dust by the wetting of access roads and excavated materials. During operations, noise and vibration should be reduced to a level compatible with the particular environment. Recommendations are made in BS 5228, Noise and vibration control on construction and open sites16.
10.3
Storage of materials
Cement should be stored on a raised platform and should be protected by a waterproof covering; the sequence of deliveries should be recorded so that the cement can be used in rotation. Pfa and other grouting materials should be kept free from contact with water or other materials. Any surface runoff should be controlled.
10.4
Grout mix
The composition of the grout mix should be maintained within the specification (materials, consistency and slump), using only the minimum quantity of water to render the grout workable, to minimise bleed water. There is a need to control waters and slurries produced and used on site while conducting drilling and grouting operations and this should include safe and controlled discharge for all waters and slurries produced on site including surface runoff.
Grout production plant at Mons Hill, Dudley, West Midlands
38
10.5
Stabilising mine workings with pfa grouts
Backfilling boreholes
Upon completion of the grouting procedure, and following the subsequent withdrawal of the borehole casing, boreholes should be backfilled to the surface with grout.
10.6
Cost of filling operations
The cost of the filling operations is dependent on the materials used, especially where the volumes to be filled are very large, and include the cost of: • Basic materials • Materials storage and processing • Any materials-specific quality and safety requirements • Any additional measures required in utilising the material during the infilling operation. The requirement for investigation, risk assessment and consultation with the Environment Agency will add to the cost and duration of the planning stages of the operation. Cost effectiveness and value for money are key requirements for obtaining funding for land stabilisation from English Partnerships. It will also be a key concern for local authorities, landowners or developers responsible for ensuring the stability of the site, and for contractors responsible for carrying out the stabilisation work. Further details on good site practice are included in CIRIA C514, Grouting for ground engineering17. A sample specification for mine infilling works is included in Supplementary Document 4, which is included on the accompanying CD Rom.
Section 11 Guidance on environmental monitoring
11
Guidance on environmental monitoring during and after filling
11.1
Approaches to monitoring
39
Two levels of field monitoring can be used to assess the potential impact on the groundwater of species leached from a grout or fill. 1. Large scale tests using lysimeters in which leaching from pfa or grout is undertaken under controlled conditions. 2. Groundwater monitoring of both elevation and chemical concentration via wells and piezometers around an area that has been grouted. Monitoring of groundwater levels is most commonly carried out by: Standpipe: An open pipe with a perforated length installed in a borehole, standing water levels are read using a simple dip meter. These standpipes are cheap, but have a relatively slow response and cannot indicate where in the ground profile the water is from. Standpipe piezometer: As for the simple standpipe, but the filter element is isolated by grout above and below a sand pocket. Pneumatic piezometer: A porous stone is installed at the measuring level; the pressure required to open a valve against the water pressure at the measurement depth can be read using a standard readout unit. This type of installation has a relatively rapid response, depending on the porous stone and installation conditions. Geophysical techniques can be used to detect leachate plumes.
11.2
Monitoring groundwater – quality and flow
Long-term monitoring of the quality of groundwater before and after grouting remediation provides the most valuable data with regard to any pollution migration from the grout. It needs to be carried out with a comprehensive knowledge of the hydrogeology and the objectives need to be clearly identified. Wells or piezometers provide the simplest method of obtaining samples of groundwater for subsequent analysis. The various methods for groundwater monitoring are briefly described below.
11.3
Groundwater monitoring devices
Standpipe piezometers or wells are most commonly used to gain access to groundwater. They are generally installed in boreholes driven vertically from the surface. Simple standpipe piezometers with a relatively short sandcell can be used to: • Measure piezometric pressure • Measure in-situ permeability at a particular location • Obtain groundwater samples for analysis. All of these parameters are required to build up a model of groundwater flow. Only a limited number of piezometers can be used in any one borehole. Piezometers are not suitable for monitoring flow through fissures in the rock unless the piezometers can be confidently placed within a fissure.
40
Stabilising mine workings with pfa grouts
Screened wells backfilled with gravel over much of their lengths are appropriate to many groundwater situations although a considerable volume of water is required to recharge the well and it is not possible to differentiate at which horizon polluted water is entering. More complex multi-level systems are available (such as the Waterloo multilevel system) but their use for groundwater monitoring in pfa is not known. Geophysical techniques can also be used to detect leachate plumes. 11.3.1
Pre-construction groundwater monitoring
Pre-construction groundwater monitoring can be important where ground parameters, water flows and any seasonal changes affect the infilling operations. Testing of pre-construction samples can define a benchmark level of water quality. 11.3.2
Monitoring during the construction phase
During construction measurements can be used to check on changes to groundwater pressures. Checks on quality can also be made by testing for chemicals of concern. Selection of a suitable interval between readings will depend on the flow rate of water and grout, the soil properties and the level of and locality of concern. The rate of readings should be adequate to identify changes in the local groundwater regime that could be rectified in time to prevent an incident by: • Stopping or changing the grouting regime • Installing and/or enacting groundwater control Leachate testing should be carried out on samples of incoming materials to be used for grouting as discussed in Section 8. 11.3.3
Post-construction monitoring
Post-construction monitoring should be sufficient to ensure that bleed water and, if required, leachate, does not impact the local regime. The rate of monitoring and the length of time post construction should be determined with the same technical priorities in mind as that carried out during construction, in conjunction with the Regulatory Authorities.
Section 12 Assessment of other risks associated with the use of pfa grouts
12
Assessment of other risks associated with the use of pfa grouts
12.1
Events potentially leading to environmental risk
41
The main focus of this Code of Practice is the risk of groundwater pollution arising from the use of pfa grouts in filling disused mine workings. However, there are a number of other scenarios that should be considered and which could lead to environmental harm. These are, in general, not specifically related to the use of pfa grout and would need to be considered for any filling operation. These may not be deemed significant but do need to be addressed in a risk assessment. These are discussed below and summarised in Table 10 in Section 12.3. 12.1.1
Consequences of taking no action
The potential consequences of leaving mine workings in their current state is likely to be the main driver for any stabilisation work. However, there may be instances where the risk of contamination of groundwater is substantial and needs to be weighed against the risk of collapse. Other more extreme forms of remediation (such as digging out the remnant voids) may need to be considered. In some cases the mine workings to be stabilised are themselves a source of contamination. In these cases the stabilisation works should be considered as part of a remedial strategy for the site contamination. 12.1.2
Environmental consequences arising from filling operations
The risk assessment and conceptual model need to address all the risks associated with the use of pfa during the filling operations, for example dust, noise and site access problems. This of course is not unique to pfa and in this Code of Practice it is assumed that such risks can be appropriately managed through the application of normal site practice measures (see Section 10). 12.1.3
Release of leachates to groundwater
The volume of water stored in underground strata (aquifers) greatly exceeds that of fresh surface water. Groundwater provides 35% of current public water supply; in some areas it is the only available future resource. It also feeds surface waters through springs and by base flows to rivers. The quality of groundwater can be seriously affected by pollution sources. The effects of such pollution can last for many years and can be virtually impossible to clean up, even when the source of the problem has been removed. It is therefore important that contamination is minimised or prevented. 12.1.4
Consequences of stability and durability failings
Materials used for stabilisation must have physical properties appropriate to the requirements for stabilisation. Such properties include flow characteristics, compressive strength and dimensional stability. They must also be physically durable both in relation to reactions between the various components making up the grout and between the grout and groundwater. One of the drivers for the use of pfa grout is the suitability of its physical properties, supported by a long track record of use. The durability of pfa grouts is supported by work done over many years on the use of pfa in concrete and the chemistry of the system is well understood. However, in general it is important to consider both the effects of leaching and of reaction between materials used for filling and the groundwater at a specific site.
42
Stabilising mine workings with pfa grouts
12.2
Risk screening
This section provides a simple approach for the assessment of risks associated with the use of pfa grouts in filling disused mineworkings. Risk screening is the first tier of the risk assessment process and involves an initial consideration of all the potential risks associated with the grouting operation. Common criteria used in risk screening include the following: •
Identification and magnitude of consequences: this should be based on an initial evaluation of the likely pathways between the source (i.e. the grout monolith and its constituent materials prior to, and during filling operations) and any potential receptors
•
Probability of consequences: this should be a rough estimate at this stage. It is important that the long-term, as well as short-term consequences are considered.
•
Significance of the consequences: this is a measure of the harm arising if exposure to the hazard actually occurs.
This initial risk screening should allow risks to be prioritised and insignificant risks to be screened out. It should also provide an initial assessment of the possible impacts at the receptor. This approach allows later stages of the risk assessment process, if required, to be focused on priority riskse. It is anticipated that many of the risks identified in Table 10 can be addressed generically or through the application of existing best practice guidance. However, others will be site-specific and can be evaluated using the matrix given in Figure 2. A full risk assessment may be required where specific risks are categorized as high (value 6 or 9 in Figure 2) and additional measures may need to be taken. However, this is outside the scope of the current document which primarily addresses potential contamination of groundwater.
e
It may be appropriate to consult the Environment Agency on the risk screening assessment.
Section 12 Assessment of other risks associated with the use of pfa grouts
43
Table 10. Risks and potential consequences of the use of pfa grout in stabilising disused mine workings Event
Receptor
Collapse due to Site above taking no action filling
Consequence Probability Risk
Comments
Site specific (SS)
SS
The need for the filling operation is site specific. Justification will presumably exist prior to the risk screening process. Pfa grout has excellent physical properties for filling mine workings. There is also a lot of experience. Any risks can be managed using best practice guidance.
SS
Short-term stability failure
Site above filling
High
Low
Low
Long-term stability failure
Site above filling
High
Low
Available evidence suggests that Moderate/ pfa grouts should perform as well low as other cement-based materials.
Changes to environment
SS
SS
SS
SS
Possible changes to groundwater levels in future may occur and need to be considered in a risk assessment
Environmental Environment effects during and individuals Moderate filling operations in vicinity of site
Low
Low
Controlled by application of existing best practice
Leaching and contamination of groundwater
Groundwater
SS
SS
Addressed in detail in this Code of Practice
Future human incursion
Individuals coming into contact with grout
Low
Pfa is currently used in many applications. It is unlikely to have harmful effects based on limited exposure as a grout.
SS
Low
SS
Consequences Probability
Low
Medium
High
Low
1
2
3
Medium
2
4
6
High
3
6
9
Risk
Figure 2. Simple matrix for use in risk screening for risks included in Table 10.
44
13
Stabilising mine workings with pfa grouts
References
1
Matthews J D, Quillin K C, Watts K S and Tedd P, Laboratory and field data for pfa grouts, BRE Report 220192, BRE Press, 2006. [In pdf format on the accompanying CD Rom]
2
Sear L K A, The properties and use of coal fly ash, Thomas Telford, London, 2001.
3
ASTM C618 Standard specification for coal fly ash and raw or calcined natural pozzolan for use as a mineral admixture in concrete, ASTM C618, ASTM International, 2005.
4
Woolley G R, Simpson D T, Quick W and Graham J. Ashes to assets. PowerGen UK plc, 2000.
5
Sear L K A and Coombs R, The use of pfa as a fill material and the environment, Proc. 3rd BGA Geoenvironmental Engineering Conference, Edinburgh, 2001.
6
Environment Agency. Methodology for the derivation of remedial targets for soil and groundwater to protect water resources, Environment Agency, Bristol, R&D Publication 20, 1999. Hiscock K, Groundwater pollution and protection, in Environmental science for environmental management, Ed T O’Riordan, Longman, 1995.
7 8
Water Resources Act 1991 (c. 57), ISBN 0105457914, TSO, 1991.
9
The European Community Directive on Groundwater (80/68/EEC).
10 McMahon A, Heathcote J, Carey M and Erskine A, Guide to good practice for the development of conceptual models and the selection and application of mathematical models of contaminant transport processes in the subsurface. National Groundwater & Contaminated Land Centre report NC/99/38/2, Solihull, 2001. 11 Environment Agency, Policy and practice for the protection of groundwater, 1998. 12 Whittaker J J, Buss S R, Herbert A W and Fermor M. Benchmarking and guidance on the comparison of selected groundwater risk-assessment models. National Groundwater and Contaminated Land Centre report NC/00/14, 2000. 13 CIRIA Groundwater control – design and practice. C515. 2000. 14 CIRIA. Barriers, liners and cover systems for containment and control of land contamination. SP124. 1996. 15 Institution of Civil Engineers. Specification for the construction of slurry trench cut-off walls. Thomas Telford, London, 1999. 80pp. 16 BS 5228: 1997, Noise and vibration control on construction and open sites. (5 parts) 17 CIRIA Grouting for civil engineering. C514. 2000
Sources of further information •
The Environment Agency, www.Environment-Agency.gov.uk
•
The Scottish Environmental Protection Agency, www.SEPA.org.UK
•
The UK Quality Ash Association, www.UKQAA.org.uk
Stabilising mine workings with pfa grouts: Supplementary Documents
1
Stabilising mine workings with pfa grouts Supplementary Documents Supplementary Document 1. Groundwater risk assessment models Supplementary Document 2. Leaching tests Supplementary Document 3. Long-term effects on the environment Supplementary Document 4. Sample specification for mine infilling works
BRE is committed to providing impartial and authoritative information on all aspects of the built environment for clients, designers, contractors, engineers, manufacturers and owners. We make every effort to ensure the accuracy and quality of information and guidance when it is published. However, we can take no responsibility for the subsequent use of this information, nor for any errors or omissions it may contain. BRE is the UK’s leading centre of expertise on the built environment, construction, sustainability, energy, fire and many associated issues. Contact BRE for information about its services, or for technical advice: BRE, Garston, Watford WD25 9XX. Tel: 01923 664000.
[email protected]. www.bre.co.uk BRE publications are available from www.brepress.com or IHS ATP (BRE Press), Willoughby Road, Bracknell RG12 8FB. Tel: 01344 328038 Fax: 01344 328005.
[email protected] Requests to copy any part of this publication should be made to the publisher: IHS BRE Press, Garston, Watford WD25 9XX Tel: 01923 664761
[email protected]
© Copyright BRE 2006 First published 2006
2
Stabilising mine workings with pfa grouts: Supplementary Documents
CONTENTS Supplementary Document 1
Groundwater risk assessment models ......................................... 5 1 2 3 4 5 6
Remedial Targets Methodology (P20) ................................................................................ 5 Key features of the groundwater modelling capability ........................................................ 6 ConSim ..................................................................................................................... 7 The RBCA tool kit for chemical releases ............................................................................ 8 4.1 Key features of the groundwater modelling capability 9 BP RISC ..................................................................................................................... 9 5.1 Key features of the ground water modelling capability 10 AEA Technology approach ............................................................................................... 10
Supplementary Document 2
Leaching tests............................................................................... 11 1 2 3 4 5 6 7 8 9 10 11
Background ................................................................................................................... 11 Maximum leachable tests.................................................................................................. 11 NRA leaching test ............................................................................................................. 11 NEN 7341 leaching test .................................................................................................... 12 Water Research Unit (WRU) leaching test ....................................................................... 12 CEN procedure ................................................................................................................. 12 DIN 38414 S4 leaching test .............................................................................................. 13 Limitations of maximum leachability tests......................................................................... 13 Percolation tests................................................................................................................ 13 Triaxial permeation leaching tests .................................................................................... 14 Lysimeters ................................................................................................................... 14
Supplementary Document 3
Long-term effects on the environment ....................................... 15 1 2
3
Background ................................................................................................................... 15 Impact on ecology ............................................................................................................. 15 2.1 Basis for determining whether pfa/cement grout can cause pollution of controlled waters 16 2.2 Groundwater regulations 1998 18 Classification of materials and land after filling ................................................................. 19
Supplementary Document 4
Sample specification for mine infilling works............................ 20 1. 1.1 1.2 1.3 1.4 1.5 1.6 1.7
General specification...................................................................................................... 21 General description of the works ...................................................................................... 21 Site location and description ............................................................................................. 21 Brief site history................................................................................................................. 21 Geology ................................................................................................................... 21 Ground investigations ....................................................................................................... 21 Method statement ............................................................................................................. 21 General requirements ....................................................................................................... 21 1.7.1 1.7.2 1.7.3 1.7.4
British standard specifications Definitions Progress report Responsibility for the execution and performance
Stabilising mine workings with pfa grouts: Supplementary Documents
1.7.5 1.7.6 1.7.7 1.7.8 1.7.9 1.7.10 1.7.11 1.7.12 1.7.13 1.7.14 1.7.15 1.7.16 1.7.17 1.7.18 1.7.19 1.7.20 1.7.21 1.7.22
1.8
Description Method statement
Layout of boreholes........................................................................................................... 27 Area of treatment .............................................................................................................. 27 Naked lights ................................................................................................................... 27 Drilling and pressure grouting procedure.......................................................................... 27 2.5.1 2.5.2 2.5.3 2.5.4 2.5.5 2.5.6 2.5.7 2.5.8 2.5.9 2.5.10 2.5.11 2.5.12 2.5.13 2.5.14 2.5.15 2.5.16
2.6
COSHH and transfer to waste General Safety policy Monitoring of gases General site protocol Drilling operations and shaft search operations Drilling in the vicinity of buried cables, gas mains, pipes and sewers etc. Visitors
Particular specification for mine stabilisation works.................................................. 27 General requirements mine stabilisation works ................................................................ 27 2.1.1 2.1.2
2.2 2.3 2.4 2.5
Personnel and relevant experience Materials Suitability of equipment Setting out Tolerances Programme Services Site facilities Site cleanliness and making good working areas Site electricity supply Site water supply Control of nuisance Highways to be kept clean Lighting Site security Water spray Water and slurries Site clearance
Site control and safety....................................................................................................... 25 1.8.1 1.8.2 1.8.3 1.8.4 1.8.5 1.8.6 1.8.7 1.8.8
2 2.1
3
Method of drilling Diameter of borehole Casing to rockhead Depth of boreholes Plumbing of boreholes Redrilling of boreholes Grouting tubes Grout mix Grout materials Storage of materials Grouting equipment and pressure Grouting procedure Backfilling boreholes Abandoned casing Drilling and grouting sequence Daily record
Exploratory open-hole drilling procedure to locate mine shafts ........................................ 31 2.6.1 2.6.2 2.6.3 2.6.4 2.6.5 2.6.6 2.6.7 2.6.8 2.6.9 2.6.10
Orientation of boreholes Drilling flush and borehole diameter Loss of flush Depth of boreholes Borehole grid Sealing of boreholes Sampling of superficial deposits Sample storage and ownership Drilling records Standing time, claims etc
4
2.7
Stabilising mine workings with pfa grouts: Supplementary Documents
Drilling and pressure grouting procedure for abandoned mine shafts and adits .............. 33 2.7.1 2.7.2 2.7.3 2.7.4 2.7.5 2.7.6 2.7.7
3
Shaft boreholes Grouting tubes Grouting procedure Grout mix Recovery of materials Drilling platform Group caps
Noise control ..................................................................................................... 35
Supplementary Document 1: Groundwater risk assessment models
5
Supplementary Document 1
Groundwater risk assessment models
1
Remedial Targets Methodology (P20)
The Remedial Targets Worksheet is a Microsoft Excel based spreadsheet model which calculates solutions to the equations presented in the R&D Publication 20. The methodology was developed specifically to take account of the risks to groundwater resources from soil and groundwater contaminant sources. The procedure for determining the site-specific remedial targets of the source contamination is summarised below: 1) Determine a target concentration at the receptor or compliance point in relation to its use. 2) Undertake the tier assessment to determine whether the contaminant source would result in the target concentration being exceeded at the compliance point. At each tier, a remedial target is determined. 3) If the contaminant concentrations on-site exceed the remedial target, then the decision whether it is appropriate to upgrade the tier analysis is based on: •
What additional information is required and can be obtained
•
Cost-benefit analysis
A four tiered approach is used to determine risk-based remedial targets for soil and groundwater contamination. At each tier, remedial targets are derived which become less onerous as additional processes such as attenuation and dilution of the contamination are taken into account. A summary of the four assessment tiers for soil and groundwater contamination are summarised in Table 1. The pfa grout could be considered to be the contaminated soil from which soluble contaminants are leached and bleed water is the contaminated groundwater. It is possible to calculate remedial targets for one contaminant per spreadsheet. Two sources may be modelled, a soil source and a groundwater source. The source may be in the unsaturated zone. Then dilution may be considered for the three types of receptor: the groundwater, an abstraction borehole or a receiving stream. The groundwater source is defined by a plume of specified concentration. The receptor is a point in the aquifer at some location downstream of the site. The primary outputs at each Tier are remedial target concentrations. Forward calculation of the concentration at the receptor is also possible for groundwater sources (Tier 3) and soil sources. At Tier 3 some manual calculation is necessary.
6
Supplementary Document 1: Groundwater risk assessment models
Table 1. Summary of assessment tiers Soil (pfa grout) For each tier, the pore water concentration determined for the soil zone is compared to the remedial target to determine the need for remedial action. Tier 1 determines whether the concentrations of contaminants in the pore water of the contaminated ground (the pfa grout) are larger and therefore likely to impact the groundwater ignoring the effect of the dilution, dispersion and attenuation along the pathway. Target concentrations at the compliance point (usually the water quality standard or the background groundwater quality) are compared with pore water contaminant concentrations. For pfa, this involves comparing the concentration of contaminants from leaching tests with the Target concentration. Table 3 shows that concentrations from leaching tests exceed WQS values for some elements. Tier 2 considers dilution by the receiving groundwater of surface water. The remedial target concentration is defined by the target concentration multiplied by the dilution factor. The dilution factor is typically calculated as the ratio between the groundwater flow below the site and the infiltration through the contaminated soil. This can only be calculated if the permeability of the contaminated soil is known. Tiers 3 and 4 consider whether attenuation of the contaminant as it moves through the unsaturated and saturated zone to the receptor is sufficient to reduce the concentration to an acceptable level. The remedial target is defined by a dilution factor and an attenuation factor. In Tier 3, simple analytical models are used to calculate the significance of attenuation whereas in Tier 4 more sophisticated numerical models are used. The compliance point is taken as a point down gradient of the site. Groundwater (pfa bleed water) The assessment of contaminated groundwater commences at Tier 2, so that the only processes of significance are degradation, retardation, dispersion and dilution in the saturated zone as it moves towards the receptor. For each tier, the observed contaminant concentration is compared to the remedial target to determine the need for remediation. Tier 2 The observed groundwater concentration below the site is compared directly with the target concentration. The compliance point is taken as groundwater below the site. Tiers 3 and 4 The observed groundwater concentration below the site is compared directly to the target concentration multiplied by an attenuation factor. As with the soil tiered assessments, Tiers 3 and 4 are distinguished by the sophistication of the modelling and prediction processes.
2
Key features of the groundwater modelling capability
Software usability Installation of the Remedial Targets Worksheet is straightforward since it exists as a spreadsheet. The Excel ‘Analysis ToolPak’ add-in must be loaded in order for the error function used by the model to operate. The worksheet is structured so that a risk assessment may be conducted by progression through the tiers. All of the calculations that the spreadsheet performs are available for view by the user. The cells are protected so that no changes can be made to the formulae. Data input sheets are easily printed out in order to check input parameters. The user manual starts with an overview of the product and is followed by a sheet by sheet description of the worksheets. The overview provides a description of the conceptual model and references to design and justify the conceptual model. There are no case studies in the manual, but those in R&D Publication 20 may be used for training purposes.
Supplementary Document 1: Groundwater risk assessment models
7
Contamination source characterisation The Remedial Targets Worksheets allows consideration of sources in the unsaturated zone (soil source) and the saturated zone (groundwater source). Calculations for the soil source derive remedial target concentrations in terms of pore water concentrations (mg/l; Tiers 2 and 3) and soil contaminant concentrations (mg/kg; Tiers 1–3). For the groundwater source remedial targets are specified in terms of aqueous concentrations (mg/l; Tier 3). For both types of source, the release of contaminant is assumed to be at a continuous constant concentration, which is equivalent to an infinite mass of contaminant and is therefore a very conservative assumption. Solubility of the contaminants is not taken into account and therefore pore water concentrations are overestimated, leading to overcautious remedial targets. Only one contaminant may be entered into a single worksheet. Because the worksheet calculates the remedial target concentrations, it is unnecessary to input contaminant concentrations, but it is possible in Tiers 2 and 3 (soil source) and Tier 3 (groundwater source) analyses to enter aqueous source concentrations and derive concentrations at the receptor. Pathway characterisation The conceptual model of aquifer flow is that of intergranular flow. Fissure flow characteristics are not modelled and are very difficult to model realistically. Much of the ground being treated by pfa grouting is in fissured ground and therefore this approach may be unsuitable for those circumstances. Receptor characterisation The receptor type is defined in the Tier 2 soil analysis from a choice of three available: the groundwater below the site, a groundwater abstraction and a receiving point. If the receptor is intended to be the water table, then a Tier 1 analysis is appropriate. All Tier 2 receptors involve dilution within the receptor. Software limitations The Remedial Targets Worksheets has no probabilistic capability to allow consideration of the effects of parameter uncertainty.
3
ConSim
ConSim (Contamination Impact on Groundwater: Simulation by Monte Carlo Method) was commissioned by the Environment Agency and released at the same time as R&D Publication 20. The software is a dedicated program that simulates the transport of contaminants from a source in an unsaturated zone to a receptor within the groundwater. It provides a tool for assessing the risks that are posed to groundwater quality by leaching contaminants from the source. Issues that may be addressed using ConSim include the following: •
To help assess the risk of pollution of controlled waters by leaching of dissolved contaminants from land contamination;
•
To assess whether or not the collection of additional site investigation data is required to quantify the risk to groundwater posed by land contamination;
•
To determine the extent of remediation that is required to reduce the risk of contamination of controlled waters;
•
To compare the viability of various remedial techniques to successfully reduce the risks of pollution to controlled waters.
ConSim is designed primarily for risk assessments using a probabilistic approach, but there is an option to obtain deterministic results. Although the modelling approach is similar to the Remedial Targets Methodology the tiered system is defined differently. ConSim’s three tier approach refers to points along the transport pathway progressively further away from the source. There is no restriction on the processes that may be considered at any level. Therefore, in Level 2
8
Supplementary Document 1: Groundwater risk assessment models
it is possible to model attenuation processes in the unsaturated zone at the same time as including dilution in the underlying aquifer. A summary of the three ConSim levels is as follows: Level 1: Comparison of contaminant source concentration with target concentration. Level 2: Assessment of concentrations at the water table. Consideration of Level 1 processes together with advection and attenuation (dispersion, retardation degradation) in the unsaturated zone. Assessment of concentration at the point of maximum dilution in the aquifer. Dilution may consider dilution due to vertical spreading down gradient of the site as well as mixing under the site. Level 3: Assessment of concentrations at a receptor at some distance from the site. Consideration of the Level 1 and 2 processes, together with advection and attenuation processes (dispersion, retardation, degradation) in the saturated zone. The software models contaminant mobilisation and transport, and is intended to use commonly available site investigation data. The probabilistic methodology allows full incorporation of data uncertainty such that assessment may be rational and consistent. ConSim requires knowledge of hydrogeology and contaminant chemistry, and is likely to be used by consultants, regulators and stakeholders experienced in these disciplines. It should be used in conjunction with Environment Agency guidance on risk assessments for land contamination where there is a threat to groundwater. Contamination source characterisation The contaminants species may be selected from a list in a contaminant inventory. New species may be added if required. The software is designed for sources in the unsaturated zone only and therefore it is not appropriate for use in backfilling of mines beneath the water table. The conceptual model for the source is a rectangular volume of contaminated soil and groundwater in the unsaturated zone. The contaminant concentration is assumed to be constant for the duration of the simulation, therefore conservatively representing a source of infinite mass. The software was not designed for groundwater pollution sources although some users try to simulate this using fictitious soil sources. Pathway characterisation The unsaturated flow zone is treated either as a porous medium or as a dual porosity medium, combining matrix and fissure flow. The conceptual model for flow in the unsaturated zone is simply plug flow, based on a constant value of moisture content and vertical conductivity with depth. The calculation of the concentration at the base of the unsaturated zone includes longitudinal dispersion, decay and retardation. Receptor characterisation One receptor may be specified. No dilution or attenuation processes due to a receptor may be considered, therefore the receptor is always equivalent to a point in the saturated aquifer. The only parameter specific to the receptor is its position with respect to the site, defined by distance in the direction of the flow and direction perpendicular to the flow.
4
The RBCA tool kit for chemical releases
The RBCA tool kit for chemical releases, developed by Groundwater Services Inc., is a spreadsheet-based application that runs using Microsoft Excel. It is designed to complete all calculations required by for Tiers 1 and 2 of the RBCA process as defined in ASTM (1995 & 1998). Exposure-based generic or site specific clean-up levels are deterministically calculated for air, soil and groundwater. The two tiers determine the availability of analytical methods and the data requirements. Tier 1 determines target concentrations and/or baseline risks using generic exposure and a number of conservative assumptions about the transport process. The latter assumptions include the application of the maximum contaminant concentration over the whole site, and that the receptor is on the same site as the source. Tier 2 introduces the use of
Supplementary Document 1: Groundwater risk assessment models
9
statistically more relevant concentrations and the receptor may be distant from the site, necessitating the application of transport models in the risk assessment. 4.1
Key features of the groundwater modelling capability
Contamination source characterisation Within the groundwater context, there are two available source types: contaminated ground water and contaminated soils leaching to groundwater. A model may contain a source of each type. The fate and transport calculations are independent, but the final assessments can be based on the greater risk or the cumulative risk. Contaminants of concern are added by selection from a list or custom data can be used. GSI’s RBCA Tool Kit only uses a constant source term with infinite capacity, so there is no declining source option available which makes the results of modelling very conservative. There is no option for a solubility-limited source term. Soil source geometry specification includes a number of options, such as area of contamination, depth to base and top of contamination, capillary zone thickness, soil thickness and length parallel to the assumed groundwater flow direction. Addition of a Soil Attenuation Model option allows for the inclusion of an attenuating zone of clean soil between the source and the groundwater. Pathway characterisation Both saturated and unsaturated intergranular flow may be modelled, although equations for the former are more sophisticated. Fissure flow is not modelled explicitly. An infiltration rate, the porosity and degree of saturation determine the advective flow rate. Dilution of the leachate beneath the site is calculated by determining the thickness of a mixing zone, and reducing leachate concentration according to the ratio of the groundwater flux through the mixing zone and the flux of the leachate. The total thickness of the aquifer, length of the source zone and the infiltration rate are used to determine the thickness of this mixing zone. The mixing zone thickness may also be specified for an unsaturated aquifer by using the annual range.
5
BP RISC
RISC (Risk-Integrated Software for Cleanups) is a dedicated program that models the fate of contaminants transported on a combination of different pathways. The software has been designed to fit within the ASTM RBCA framework and therefore calculation of risk to humans forms a significant component of the software. The transport of contaminants is modelled deterministically in BP RISC, but risks to humans may be analysed using probabilistic techniques. Features include: •
A customizable chemical database with 82 chemicals.
•
An Excel spreadsheet based on the RBCA algorithms that can be used to replicate the tiered RBCA process.
•
A detailed user's manual with three in-depth example problems.
•
The ability to determine risk-based TPH (total petroleum hydrocarbon) targets using the TPH fractions proposed by the U.S. Air Force-led TPH Working Group.
•
Calculates additive risk due to multiple pathways, compounds and receptors (such as a resident exposed as both a child and an adult). Monte Carlo capabilities for probabilistic risk evaluation.
•
Fate and transport models distinguish between presence or absence of phase-separated product (NAPL) in the source zone.
The main BP RISC package is a software tool for carrying out ASTM RBCA Tier 2 modelling for those contaminants identified as potentially posing a risk. BP RISC allows for consideration of attenuation processes in the unsaturated and saturated zones and dilution at a receptor. It also includes calculation of clean-up levels, but these are derived on the basis of human risk rather the groundwater target concentrations used in the Remedial Targets Methodology.
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Supplementary Document 1: Groundwater risk assessment models
The fate and transport models relevant to groundwater risk assessments are: •
Dissolved concentrations only (no soils data): groundwater source specified as porewater concentration, transport in the saturated zone.
•
Vadose zone source to groundwater model: source in the soil zone, specified as soil contaminant concentration, transport in the saturated and unsaturated zones.
•
Saturated soil source to groundwater model: source situated in the region of seasonal fluctuation of the water table. Two water levels are modelled annually, the duration of which is specified by the user. The source is specified as soil contaminant concentration. The region of the source below the water table enters the groundwater directly according to partitioning properties. The region above the water table may be transported to the groundwater by infiltration.
5.1
Key features of the ground water modelling capability
Contaminant source contamination Twenty chemical of concern may be selected for a modelling scenario by selection from the chemicals database. For all three models the source concentration is a single value or concentration sample database. For each of the models the contaminant source is represented as a rectangular zone. For dissolved concentrations the source concentration is defined by a pore water concentration which is constant for a pulse length duration of up to the simulation time. For the other two models the source is assumed to decline. Pathway characterisation Each of the three groundwater models simulates intergranular flow by numerical evaluation of analytical contaminant transport equations. The aquifer is assumed to be unbounded having infinite length, width and depth. Receptor characterisation Each of the three groundwater models gives concentrations at the well receptor. The concentration is calculated as an average of the concentration at a number of points. The receptor concentration may be very dependent on the number of average points and the length of the screen. If the screen is short in the near the centreline of the plume, it will represent a maximum point concentration. If it is long and extends beyond the plume additional dilution by groundwater flow will be incorporated by the user. This is not necessarily the same amount of dilution as might be achieved by abstraction at the well. Background concentrations are not taken into account.
6
AEA Technology approach
AEA Technology (1994) carried out an assessment of possible leaching scenarios from pfa grout monoliths (such as infilled mine workings) specific to conditions found at the Combe Down stone mines near Bath and based on laboratory leaching and permeability data summarised above. The dilution factors that needed to be applied to the leachate from the monolith for EQS/WQS standards to be met depended on the level of rainfall. The AEA Technology report considered a number of rainfall scenarios. It concluded that, if a pfa grout were used, the water quality would meet EQS/WQS standards in periods of medium and high rainfall. However, the analysis suggested that concentrations of certain elements in the leachate could exceed the EQS/WQS levels where the amount of rainfall, and the subsequent dilution of the leachate, was low. They also showed that PC/sand grout would produce acceptable water quality in all rainfall scenarios. Of the species present in leachates from PC/pfa grout in the AEA Technology work, phenol required the greatest degree of dilution to meet the WQS values. A dilution factor of 180 was considered to be necessary to meet the WQS values for phenol. However, phenol is believed to have been present only as a contaminant. If so the highest dilution factor required would be 80 (for aluminium).
Supplementary Document 2: Leaching tests
11
Supplementary Document 2
Leaching tests
1
Background
Leaching tests provide information on: •
The maximum concentrations in primary leachate
•
The pattern of leaching behaviour over time
•
Factors affecting the concentrations
•
The total amounts of species released over time.
Leaching tests allow an assessment of the soluble fraction of the material. Only the leachable contaminants are likely to pollute the groundwater. Various severities of laboratory leaching tests have been devised to assess the levels of contamination and rates of leaching. These can be divided into three broad types, which are discussed in more detail below: 1. Maximum availability leaching test 2. Permeation leaching test – more representative of field environment 3. Surface leaching test – again more representative of field environment
2
Maximum leachable tests
Maximum availability leaching tests are designed to determine the potential amounts of components that may be leached from the material under the worst case environmental conditions. These techniques employ leachants that are, for example, acid or alkaline in nature. Similar techniques are also applied to assess leachability under milder conditions. These procedures provide an estimate of the amounts of each species that may be removed by leaching over the long term and include: •
Extraction with hot HCl followed by hot aqua regia
•
Extraction with 5% HCl
•
Extraction with 1N NaOH
•
Extraction with Analar water
•
Extraction with aqueous phase representative of the local groundwater
•
Extraction with organic solvents
The amount extracted will depend on the solubility of individual species in the leachant, any chemical reaction between the leachant and the solid and the kinetics of the processes involved. The methods are described below and summarised in Table 1.
3
NRA leaching test
To obtain the available leachable fraction of soil contaminants, the Environment Agency recommends the NRA Leaching test (NRA, 1994). This test involves mixing 1000 g of material ground to pass a 2 mm sieve with one litre of water and shaking for 24 hours, filtering the liquid prior to analysis.
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Supplementary Document 2: Leaching tests
Table 1. Comparison of main features of leaching tests Test type
Sample Ground Leachant volume or Leachant type size (g) size liquid to solid ratio
Time shaken
NRA/EA
1000
Water
24 hours
Water
Repetitive
pH = 7 pH = 3
3 hours 3 hours 24
2 mm
1litre
WRU NEN
1
CEN (prEN12457) DIN 38414
100
TCLP
200
4
< 0.125
50
< 4 mm
2 or 10
10 mm
1litre
Demineralised water
2litre
Distilled water with 18 11.4 g glacial acetic acid
Repetitive
NEN 7341 leaching test
In the leaching test produced by the Nederlands Normalisatie-instituut NEN 7341 the material is fineground to a grain size of < 125 µm. The sample is extracted twice in succession with leachant using a liquid/solid ratio of 50. In the first stage leaching is continued for 3 hours at a constant pH = 7. The second stage comprises leaching for 3 hours at a constant pH = 4, or a lower pH if the material itself sets a lower pH. The eluates are then filtered, mixed and analysed. The results of this test give the maximum quantity of each component that can leach. In addition the test data can be used to calculate the acid-neutralising capacity of the material.
5
Water Research Unit (WRU) leaching test
The Water Research Unit (WRU) leaching test is dynamic in that the leachant is intermittently replaced to provide fresh opportunity for leaching (Young and Wilson, 1987). It has been used in studying leaching from industrial by-products (including pfa) that may be used in construction and stabilisation applications. It is a repetitive batchwise shaking test in which a sample of the solid (of known mass) is shaken with a ‘bed volume’ – i.e. the amount of liquid required to just cover and render mobile the solid. Small samples are taken after known time intervals (and replaced by fresh fluid) to establish the time required to attain equilibrium. A new sample of waste is then extracted five times using fresh fluid. A portion of the remaining waste is then depleted using ten bed volumes to give an average of the leaching expected between six and fifteen bed volumes. For each bed volume the leachate is analysed for a range of species using appropriate techniques1. Other similar methods include the TCLP (Toxicity Characteristic Leaching Procedure) in which a known amount of solid (200 g) is mixed with a known volume of distilled water (2 litres) together with 11.4 g glacial acetic acid and shaken continuously for 18 hours at 22°C prior to filtering and analysis.
6
CEN procedure
The CEN procedure (prEN12457 Parts 1–4, Characterisation of waste – Leaching - Compliance test for leaching of granular waste materials and sludges, CEN 1999) offers the choice between four procedures depending on the properties of the waste and the purpose of the test. Demineralised water is used as the leachant. For the first three procedures, the material is reduced in size to a grain size < 4 mm. For the one stage-test, leaching is conducted for 24 hours using a liquid: solid ratio of 2 (prEN12457-1) or 10 1
Note: The WRU leaching test allows equilibrium to be reached for a period of up to about 80 hours only. This may be insufficient to provide a realistic assessment of the effects of the interaction between the pfa and the PC components of the grout. BRE work has shown that, even under accelerated conditions of high water:cement ratios and constant agitation, the reactions are not complete over this timescale. The interaction between the two components of the grout is likely to affect the concentrations of species in solution.
Supplementary Document 2: Leaching tests
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(prEN12457-2). In the two-stage test, leaching is conducted first for six hours using a liquid: solid ratio of 2, then for 18 hours using a liquid : solid ratio of 8 (prEN12457-3). The results are added in order to determine details of leaching using a liquid : solid ratio of 10. Information concerning the leaching mechanism can be derived from the results of the two-stage test. The fourth procedure (prEN12457-4) is a one-stage test using a liquid : solid ratio of 10, but with a grain size < 10 mm.
7
DIN 38414 S4 leaching test
In the German standard method for determining leaching with water (DIN 38414 S4) 100 g of sample are mixed in 1 litre of demineralised water (L/S = 10) and agitated for 24 hours. The sample is then filtered or centrifuged and the eluate analysed. The sample is analysed in the form in which it is presented for disposal or reuse. Reduction in size is only carried out if required for sample taking, or for conducting the test. In general the sample is only reduced in size if the particle size exceeds 10 mm. The sample is not pulverised.
8
Limitations of maximum leachability tests
Maximum leachability tests only provide information on the maximum amount of each species that can be removed from a solid. Although this information is useful it should be treated with caution in supporting a potential pollution scenario. It is particularly important to bear in mind that these tests provide no information of the rate of release of these species and consequently the likely concentrations in the leachate either from pfa fill or grout.
9
Percolation tests
Leaching by sequential flushing of a column of material allows the rate of leaching of various contaminants to be assessed as a function of time or of the solid : liquid ratio. For example the column leach test NEN 7343 can be used to determine the leaching characteristics of contaminated soil, industrial waste, fly ash and other related materials. However, this test is only suitable for high permeability materials and is not appropriate to leaching of pfa/cement grout. Deionised water amended with nitric acid is transported through the sample using a laboratory pump with different water : liquid ratios being employed. This acidification is carried out to simulate percolation of the material by acid rain. The test is carried out in a column filled with sample up to the bed height of at least four times the bed diameter. The maximum percolation rate of the liquid is 2 cm per hour. The water flows through the column from bottom to top in order to avoid channelling. The total liquid/solid substance ratio (L/S) is 10. Different fractions are collected in order to be able to follow the leaching behaviour as a function of the time. A considerable amount of work in preparing test methods for characterising wastes has been carried out within CEN Technical Committee TC292, Characterisation of Wastes, with some 38 European standards either in preparation or published. The basic criterion of TC292 standards seems to be to analyse the ‘waste’ for all possible elements irrespective of the potential for leachates to escape and then test the potential for leaching from the material in various size fractions. These tests are based on using de-ionised water as the extracting medium, excepting where acid extraction and/or microwave digestion is employed. For pfa, an accelerated percolation method is felt to be the most appropriate leaching test, as this reflects the situation when pfa is used as a fill material. However, the CEN TC292 ‘Upflow Percolation’ test was extensively tested and found not to be suitable for such materials. Compacted pfa naturally has a low permeability, typically ~1 × 10–7 m/s, and very little leachant will percolate through the material. Applying pressure tends to lead to the water escaping around the pfa sample rather than through it. Such problems have been found in a number of studies, for example it was reported by Baldwin et al (1997) that compacted pfa would not saturate with water within the permeameter that was used and consequently no leachate could be extracted. Indeed, where gravity is used to extract leachates, such as in permeameters, no leachate may be found for periods of up to 3 years.
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Supplementary Document 2: Leaching tests
It was for this reason that Dawson of the University of Nottingham developed the so-called DoRLaP (Double Ring Leachate Permeameter) test. This allows leachant to be forced through the material at high pressure without encountering problems of by-passing the sample. For pfa grouts, the permeability would be expected to be significantly lower than in compacted fill. Consequently, extraction of leachants from the bulk, solid material could prove very difficult using percolation methods. The repeatability and reproducibility of many of the CEN TC 292 test methods has not yet been established. TC292 was unable to harmonise many of the testing standards because data did not exist on the repeatability or reproducibility of the methods. A validation exercise on these methods indicated some highly variable results. Values for the precision of leachates from EN12457 Part 1 to 4 for repeatability ranging from 26 to 50% and reproducibility from 82 to 126% have been reported2. Clearly, this could have a significant effect on the classification of materials should finite limits be set. While CEN TC292 has developed a great number of test methods, various sectors of the materials industry have not accepted that tests for categorising wastes are appropriate for their products. The main problems are not those associated with the validity or applicability of the method, but relate to the test methods being entitled ‘Characterisation of Waste’. Material producers and suppliers do not wish to have their products associated with the word ‘waste’. It is for this reason that other CEN Technical Committees have developed their own test methods.
10
Triaxial permeation leaching tests
This type of test involves permeating water or other leachant through a pfa/cement grout sample and measuring the permeability of the material and the concentration of contaminants in the leachate. Such tests provide information on the rate at which contaminants can be leached by groundwater permeating the grout mass and the permeability of the sample. This type of test has been used extensively to measure the long term permeability and durability of the set slurry used for slurry trench cut-off walls (Tedd et al, 1997). Some tests have lasted for up to three years. Samples of fluid grout are cast into nominally 100 mm diameter by 0.45 m long plastic tubes. Care is required when pouring grout into the sample tubes to avoid air bubbles being trapped in the samples. When set a 100 mm or 200 mm length sample of the grout is extruded and placed in a triaxial cell. The test is carried out in accordance with BS 1377: Part 6 (BSI, 1990). With an hydraulic gradient of 20, and for a test sample with a permeability of 1 x 10–9 m/s, the flow rate is only 0.6 ml per hour such that it takes a week to obtain a sufficient quantity (100 ml) of leachate for chemical analysis3. The triaxial equipment used should not include alkali sensitive parts as the leachate may be very alkaline. Interchange units are used on the inlet and outlet lines such that the type of leachant can be controlled (deionised or deaired water) and the leachate can be collected.
11
Lysimeters
There are two types of lysimeter; •
Collection lysimeter
•
Suction lysimeter.
Lysimeters have been used to monitor the behaviour of leachate from landfilled wastes and unsaturated contaminated ground. However, although they may be suitable for monitoring pore water in a pfa fill, they may not be suitable for use in pfa/grout due to the low permeability of the material. 2
Validation of CEN/TC292 leaching tests and eluate analysis methods EN12457 parts 1-4, EN13370 and EN12506 in co-operation with CEN/TC308, Part 7, Performance characteristics of EN12457, 1-4, September 2001 draft. 3 During the permeation stage, the slurry wall specification recommends a minimum mean effective stress of 100 kPa and a hydraulic gradient of 10 to 20.
Supplementary Document 3: Long-term effects on the environment
15
Supplementary Document 3
Long-term effects on the environment
1
Background
It is an offence to pollute groundwaters under the Water Resources Act 1991. The European Community Directive on Groundwater (80/68/EEC) requires that specific measures be taken to prevent pollution by chemicals in two categories: those that should be prevented from entering groundwaters (List I), and those that “should be minimised” and could have a harmful effect (List II). In England and Wales the Environment Agency is responsible for preserving the quality of groundwater through pollution control powers including the regulation of discharge consents (Sear 2002). In Scotland the responsibility lies with the Scottish Environment Protection Agency. Contaminated groundwater can be defined as that which has been affected by human activities to the extent that it has higher concentrations of dissolved or suspended constituents than the maximum admissible concentrations formulated by national or international standards for drinking, industrial or agricultural purposes (Hiscock, 1995). Groundwater makes up 35% of the total freshwater supply in England and Wales (Hiscock 1995). The Drinking Water Inspectorate (DWI) is responsible for regulating public water supplies in England and Wales including the assessment of the quality of drinking water. The DWI can take enforcement action if standards are not being met, and appropriate action when water is unfit for human consumption. Drinking water quality is also a concern for local water companies and residents/local businesses. Local residents’ concerns over the possible effects arising from the use of pfa for filling disused mine workings (specifically on drinking water quality and heavy metals ‘migrating to surface’) are mentioned in Environmental Statement on the Combe Down stonemines stabilisation works (Volume 3, Annexes, p 18 Table B4, 1996). An additional concern is the possibility of landowners being held responsible for remedial measures where pollution of controlled waters occurs. The Environment Agency have defined Source Protection Zones (SPZ) for nearly 2000 groundwater sources used for public drinking water supplies4. The SPZ provide an indication of the potential risk of pollution. Generally the closer a polluting activity or release is to a groundwater source the greater the risk. Three zones (an inner, outer and total catchment) are usually defined although a fourth zone (zone of special interest) is possible. Where the mines lie within an SPZ (such as those at Combe Down) it is essential that the materials used in filling the mines do not compromise the quality of drinking water.
2
Impact on ecology
Groundwater is often important in supporting wetlands and their ecosystems and, consequently, any pollution of groundwater can have adverse effects in such areas. Adverse impacts on ecology could also arise during the mine filling operations themselves. In the case of the proposed filling of the mines at Combe Down such issues have included the effects on bat colonies that currently inhabit the mines and the loss of limestone grasslands to allow access to the mines. English Nature would be responsible for any ecological mitigation. 4
Guide to Groundwater protection zones in England and Wales (NRA 1995).
16
Supplementary Document 3: Long-term effects on the environment
2.1 Basis for determining whether pfa/cement grout can cause pollution of controlled waters The principal concern with the use of pfa/cement grouts to stabilise disused mine workings is that in certain potentially environmentally sensitive situations pollution of controlled waters could be caused and that the site owners could be required to carry out appropriate remedial measures under the “polluter pays principle”. The Control of Pollution Act 1974 is the principal legislation controlling discharges of poisonous, noxious and polluting matter to controlled waters. Section 30 of the Act provides the definition of the controlled waters which includes groundwaters contained in underground strata, including water in wells, boreholes and excavations into underground strata. In addition, Part IIA of the Environmental Protection Act 1990 introduced by section 57 of the Environment Act 1995 – provides a framework for the identification and remediation of historically contaminated land. Contaminated land is identified on the basis of risk assessment. In accordance with the provisions of Part IIA, land is only contaminated where: “... it appears to the local authority in whose area it is situated to be in such condition, by reason of substances in, on or under the land, that: •
significant harm is being caused or there is a significant possibility of such harm being caused, or
•
pollution of controlled water is being, or is likely to be, caused.”
Guidance on what constitutes significant harm and significant possibility of harm are provided in the Statutory Guidance. Pollution of Controlled Waters is defined under Part IIA as “the entry into controlled waters of any poisonous, noxious or polluting matter”. Part IIA uses the “polluter pays principle” and requires remediation of the land and controlled water to a standard suitable for current use. Under Part IIA, Local Authorities are the principal regulators for the identification, determination and remediation of contaminated land. The Environment Agency has a role to provide technical guidance, particularly where pollution of controlled waters is an issue, and in relation to special sites. They have developed a standardised, practical and reasonable approach for the protection of water resources that can be applied on a site-by-site basis. The approach is described in the Environment Agency Report “Methodology for the derivation of remedial targets for soil and groundwater resources” (Environment Agency, 1999). The methodology is based on a risk assessment approach incorporating a source-pathway-receptor analysis. In December 2001, the Environment Agency produced a document “Environment Agency advice to third parties on pollution of controlled waters for Part IIA, EPA 1990” (Environment Agency 2001). Although the document has been produced by the Environment Agency for those parties, principally Local Authorities, who are involved with inspecting, investigating and determining contaminated land under Part IIA, it seems likely that such guidance will be used by Local Authorities to determine whether the use of pfa/cement grout poses a significant risk to controlled groundwater. Under Part IIA, land (and therefore substances injected into the ground) may only be determined as contaminated where a significant pollutant leakage can be identified between the source of pollution (e.g. bleed water or leachate from the grout) and the receptor (the controlled water). Table 1 summarises a range of potential pollutant linkages that might be identified in the conceptual model for a site.
Supplementary Document 3: Long-term effects on the environment
17
Table 1. Potential pollutant linkages that could give rise to determination of pfa grout as contaminated land on the basis of pollution of controlled waters Sources
Pathways
Receptors
Bleed water Surface leaching Grout body leaching
Existing joint system Through ground mass
Groundwater in an aquifer Surface water body
To determine whether pollution of controlled waters is likely to occur, the first step is to identify the species and concentration of contaminants likely to come from the pollution source and compare them against the relevant water standards as shown in Table H2. The judgement should be made against the most stringent of the relevant standards. For example, if a contaminated site is located above a potable aquifer and close to a watercourse it would be appropriate to consider both the Drinking Water Standard (DWS) and Environmental Quality Standards (EQS) in deciding on the significance of the pollution. For some substances, the EQS may be more stringent (because fish are particularly sensitive to a contaminant), while for other substances the human may be the most sensitive receptor and the drinking water standard is more stringent. In this situation, the more stringent of the EQS and the DWS for each substance should be used for comparative purposes. This may mean that a DWS is used to assess the significance of one substance, while the EQS is used for another substance on the same site. In assessing the risks to controlled waters, the key issue is the mobility or leachability of the contaminant, rather than the total contaminant concentration in the soil. The risk is dependent on the mobility of the polluting substances either as dissolved contaminants, vapours, free-phase liquids or solids in suspension. Table 2 Water quality standards for assessing the presence of pollution in controlled waters Contaminant Issues to consider property
Relevant environmental standards
Poisonous
Potential to cause harm to living organisms (including humans) by exposure to toxins. Implies requirement to consider relevant dose
EU Drinking Water Standards 1998; Water Supply (Water Quality) Regs 1989; Private Water Supplies Regs 1991; World Health Organisation (Health) Guideline Values; Environmental Quality Standards.
Noxious
Potential to cause nuisance or harm, e.g. by World Health Organisation (ATO) Guideline taste, odour or discolouration Values; Taste and odour threshold data.
Polluting
Potential to cause environmental detriment (to water quality and ecosystems)
Solid waste matter
Amenity and aesthetic detriment, or harm to Suspended solid threshold limits. aquatic organisms (e.g. by attrition of fish gills or reduced light to aquatic flora)
Environmental Quality Standards (Freshwater and Saltwater); Presence of solid waste.
Note: The Drinking Water Inspectorate (DWI) is the competent authority for the purpose of enforcing the drinking water standards at the point of supply.
The Environment Agency has also developed risk assessment tools and guidance, including tools for assessing risks to controlled waters. ConSim (Environment Agency, 1999) is an Environment Agency land contamination risk assessment model for controlled waters, which has been developed for use within the framework set out in the Methodology for the Derivation of Remedial Targets for Soil and Groundwater to Protect Water Resources (Environment Agency, 1999). The Agency expects and recommends that risk assessments for pollution of controlled waters from existing contamination should be undertaken in accordance with this framework and recommends the use of ConSim where it adequately simulates conditions and geometries on the site. If other methods or approaches are
18
Supplementary Document 3: Long-term effects on the environment
adopted, the basis and assumptions within those models must be fully documented to the regulatory authorities. The use of risk assessment tools will be addressed at a later stage in the project. Further details are given in Appendix E of BRE Report 220192.
2.2
Groundwater Regulations 1998
The Environment Agency could also resort to the use of the Groundwater Regulations 1998 to assess the use of pfa/cement grout for remedial works to disused mine workings. The Groundwater Directive 80/68/EEC is the main European legislation. This is expected to be replaced over coming years by a new Groundwater Directive to be made pursuant to Article 17 of the Water Framework Directive 2000/60/EC. In the UK the main legislation implementing the Directive is the Water Resources Act 1991 and the Groundwater Regulations 1998. The regulations implement the EC Directive 80/68/EEC and supplement Regulation 15 of the Water Management Licensing Regulations 1994 and existing water pollution legislation. The regulations mainly relate to controlling the disposal, or tipping for disposal, of List I or II substances where there might be an indirect discharge of those substances to groundwater, and to controlling other activities which might lead to an indirect discharge of List I substances to groundwater and to pollution of groundwater by List II substances. Draft guidance on the implementation of the regulations is given on the DEFRA website. Extracts from this guidance are given below where they appear to be relevant to the use of pfa/grout in disused mines. The regulations came into force on 1 April 2000. They effectively extend existing controls, contained in the Water Resources Act 1991, over the discharge of polluting matter to controlled waters (including groundwater). They require that disposal, or tipping for the purposes of disposal, to land of certain listed substances may be carried out only if prior authorisation has been given by the Environment Agency. The regulations place a duty on the Environment Agency to protect groundwater, in effect by prohibiting discharges of List I substances to groundwater, and preventing pollution of groundwater by List II substances. Under the Regulations the Environment Agency also has powers to issue notices to control activities other than disposal, where these are likely to result in an indirect discharge of a listed substance to groundwater. The regulations empower the Agency to issue notices which may prohibit, or impose conditions on activities other than disposals which could result in an indirect discharge of listed substances to groundwater. Regulation 2 – Exclusions from the Groundwater Regulations 1998 Regulation 2 provides that the Groundwater Regulations do not apply in certain circumstances. One exclusion that may be relevant is the following: “Discharges containing List I and II substances in quantities and concentration so small as to pose no present or future threat to groundwater quality” The regulations do not apply to any discharge containing a quantity and concentration of listed substances found by the Environment Agency to be “so small as to obviate any present or future danger of deterioration in the quality of the receiving water”. This is not a numeric standard, and the European Court has ruled that “if the quantity in List I or II substances contained in discharges of other substances is such that risk of pollution cannot automatically be excluded, the Directive is applicable”. The Environment Agency’s consideration of this issue will depend on the nature of the discharge, but any quantities or concentrations which may be appropriate under regulation 2(1) (c) are likely to be very small indeed, and likely to be no less stringent than drinking water standards. In practice this is likely to mean quantities so small that it is obvious that they will not cause a rise in concentrations of that substance in the underlying groundwater body.
Supplementary Document 3: Long-term effects on the environment
19
It would seem that where concentrations of substances in List II from either bleed water or leachate resulting from pfa/cement grout are above drinking water standards dilution effects will be critical in approval being given for the use of pfa grout. Regulation 4 – Prohibition of List I substances reaching groundwater Regulation 4 sets out rules for authorisations which are designed to prevent direct or indirect discharges of List I substances to groundwater. Reference is made to “prior investigation” in which the hydrogeological conditions in the area, the possible purifying powers of the soil or sub-soil and the risk of the discharges altering the quality of the groundwater are established. The groups and families of List I and II substances to which the Regulations apply are set out in the Schedule to the Regulations which is reproduced in Appendix A. This is taken from the Schedule to the Groundwater Directive and is similar in its overall coverage to the relevant part of the Annex to the Dangerous Substances Directive (76/464/EEC). Circumstances in which discharges to groundwater may be authorised Regulation 4(5) provides that in exceptional circumstances the Agency may authorise a discharge of a List I substance into the groundwater where the prior investigation shows that: • the groundwater is permanently unsuitable for other uses, especially domestic or agricultural uses and • the presence of the substances does not impede the use of the ground resources • where an authorisation is granted, conditions are imposed requiring that all technical precautions are observed to prevent substances reaching other aquatic systems or harming other ecosystems. Regulation 5 – Limiting List II discharges to prevent pollution of groundwater The procedures for assessment of discharges containing List II substances are similar to those for List I. Reference is made to Regulation 19 where the Environment Agency considers that an activity, in or on the land, may result in indirect discharge and may prohibit the activity or allow it to continue subject to conditions. Regulation 19 – Notices It would appear that this regulation could be used by the Environment Agency to prohibit the use of pfa/cement grout where it is decided that either the bleed water or leachable substances posed a threat to the groundwater. Laboratory work (see BRE Client Report No. 206470, 2002) has shown that List II Substances are present both in the bleed water and in the leachate from pfa/cement grouts.
3
Classification of materials and land after filling
Part IIA of the Environment Protection Act (1990) identifies contaminated land as “any land which appears to the local authority in whose area it is situated to be in such a condition, by reason of substances in, on or under the land that: •
significant harm is being caused or there is a significant possibility of such harm being caused; or
•
pollution of controlled waters is being, or is likely to be caused.”
Harm is defined as “harm to the health of living organisms or other interference with the ecological systems of which they form a part, and in the case of man includes harm to his property”. Substance is defined as “any natural or artificial substance whether in solid or liquid form or in the form of gas or vapour”.
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Supplementary Document 3: Long-term effects on the environment
The standard definition of land applies, which includes land under water. The term pollution of controlled waters is as defined in section 78A (9) of the Act as “the entry into controlled waters of any poisonous, noxious or polluting matter or any solid waste matter”. Controlled waters have the same meaning as in section 30A of the Control of Pollution Act 1974 (COPA) and include: •
relevant territorial waters, extending seaward for three miles from the baseline from which the breadth of the territorial sea adjacent to Scotland is measured
•
coastal waters, extending from the baselines above as far as the limit of the highest tide or as far as the fresh-water limit of the river or watercourse which adjoins waters within that area
•
inland waters, including the waters of any relevant loch or pond and rivers and other watercourses above the fresh-water limit
•
groundwaters contained in underground strata, wells, boreholes, excavation into underground strata or similar.
By virtue of the definition, there will be chemically contaminated land which does not meet the statutory definition. The definition relies heavily on the concept of a pollutant linkage i.e. the presence of a source of contamination which has the potential to impact on a receptor by means of a pathway. Contaminated land is of concern if it presents a threat to the environment or if it poses risks to users of the land. Such land is seen to have potential environmental liabilities, which are also of concern to land owners due to their financial and legal implications. Financial liabilities include reduced land values or the requirement to fund remediation. Under Part IIA of the Environmental Protection Act 1990, Local Authorities are the principal regulators for the identification, determination and remediation of contaminated land. The Environment Agency has a role to provide technical guidance, particularly where pollution of controlled waters is an issue, and in relation to special sites. They have developed a standardised, practical and reasonable approach for the protection of water resources that can be applied on a site by site basis. The methodology is based on a risk assessment approach incorporating a source-pathway-receptor analysis. In December 2001, the Environment Agency produced a document “Environment Agency advice to third parties on pollution of controlled waters for Part IIA, EPA 1990” (Environment Agency 2001). This document has been produced by the Environment Agency for those parties, principally Local Authorities, who are involved with inspecting, investigating and determining contaminated land under Part IIA. However, it seems likely that such guidance will be used by Local Authorities to determine whether the use of pfa/cement grout poses a significant risk to controlled groundwater.
Supplementary Document 4: Sample specification for mine infilling works
Supplementary Document 4
Sample specification for mine infilling works
1. 1.1 1.2 1.3 1.4 1.5 1.6 1.7
1.8
GENERAL SPECIFICATION General description of the works Site location and description Brief site history Geology Ground investigations Method statement General requirements 1.7.1 British standard specifications 1.7.2 Definitions 1.7.3 Progress report 1.7.4 Responsibility for the execution and performance 1.7.5 Personnel and relevant experience 1.7.6 Materials 1.7.7 Suitability of equipment 1.7.8 Setting out 1.7.9 Tolerances 1.7.10 Programme 1.7.11 Services 1.7.12 Site facilities 1.7.13 Site cleanliness and making good working areas 1.7.14 Site electricity supply 1.7.15 Site water supply 1.7.16 Control of nuisance 1.7.17 Highways to be kept clean 1.7.18 Lighting 1.7.19 Site security 1.7.20 Water spray 1.7.21 Water and slurries 1.7.22 Site clearance Site control and safety 1.8.1 COSHH and transfer to waste 1.8.2 General 1.8.3 Safety policy 1.8.4 Monitoring of gases 1.8.5 General site protocol 1.8.6 Drilling operations and shaft search operations 1.8.7 Drilling in the vicinity of buried cables, gas mains, pipes and sewers etc. 1.8.8 Visitors
2 2.1
2.2 2.3 2.4 2.5
2.6
2.7
3
PARTICULAR SPECIFICATION FOR MINE STABILISATION WORKS General requirements mine stabilisation works 2.1.1 Description 2.1.2 Method statement Layout of boreholes Area of treatment Naked lights Drilling and pressure grouting procedure 2.5.1 Method of drilling 2.5.2 Diameter of borehole 2.5.3 Casing to rockhead 2.5.4 Depth of boreholes 2.5.5 Plumbing of boreholes 2.5.6 Redrilling of boreholes 2.5.7 Grouting tubes 2.5.8 Grout mix 2.5.9 Grout materials 2.5.10 Storage of materials 2.5.11 Grouting equipment and pressure 2.5.12 Grouting procedure 2.5.13 Backfilling boreholes 2.5.14 Abandoned casing 2.5.15 Drilling and grouting sequence 2.5.16 Daily record Exploratory open-hole drilling procedure to locate mine shafts 2.6.1 Orientation of boreholes 2.6.2 Drilling flush and borehole diameter 2.6.3 Loss of flush 2.6.4 Depth of boreholes 2.6.5 Borehole grid 2.6.6 Sealing of boreholes 2.6.7 Sampling of superficial deposits 2.6.8 Sample storage and ownership 2.6.9 Drilling records 2.6.10 Standing time, claims etc Drilling and pressure grouting procedure for abandoned mine shafts and adits 2.7.1 Shaft boreholes 2.7.2 Grouting tubes 2.7.3 Grouting procedure 2.7.4 Grout mix 2.7.5 Recovery of materials 2.7.6 Drilling platform 2.7.7 Group caps NOISE CONTROL
21
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Supplementary Document 4: Sample specification for mine infilling works
1.
GENERAL SPECIFICATION
1.1
General description of the works
All Permanent and Temporary Works in connection with the stabilisation of underground mine workings at the site. 1.2
Site location and description
The site is located approximately ……….at National Grid Reference XXXXXXX (Refer Figure 1). 1.3
Brief site history
1.4
Geology
The site is immediately underlain by Productive Coal Measures strata comprising interbedded sandstones and shales with workable coal seams. In addition there are varying thicknesses of Made Ground across the site. 1.5
Ground investigations
The results of various ground investigations that have been undertaken on the site in the past have been summarised in the report ……….. That report includes a description of the ground conditions on site and shall be read in conjunction with the present specification. 1.6
Method statement
At the commencement of the Contract, the Contractor shall provide a detailed method statement. This shall include a programme giving full details of both type and quantity of all the plant he proposes to use, the order of carrying out the work, the detailed and dimensioned layout of the works and any further information required in the various subscriptions below. 1.7
General requirements
1.7.1
British standard specifications
All materials and workmanship shall be in general accordance with the appropriate British Standards current at the time of tender, including those listed in this Specification, except that where the requirements of British Standards are in conflict with this Specification, the latter shall take precedence. 1.7.2
Definitions
In this Specification the terms ‘approved’, ‘approval’ and ‘required’ mean ‘approved by the Engineer’, ‘approval of the Engineer’ and ‘required by the Engineer’ respectively. 1.7.3
Progress report
The Contractor shall submit to the Engineer on the first day of each week, or at such longer periods as the Engineer may from time to time direct, a progress report showing the current rate of progress and progress during the previous period on all important items of each section of the works. 1.7.4
Responsibility for the execution and performance
The design, execution and performance of the ground treatment shall be the responsibility of the Contractor who shall, nonetheless, satisfy the Engineer that all treated ground has attained the required degree of improvement.
Supplementary Document 4: Sample specification for mine infilling works
1.7.5
23
Personnel and Relevant experience
Prior to commencement of the Works, the Contractor shall provide a list of key personnel he proposes to employ for each phase of the works, together with a resume of their experience and qualification. The list of key personnel shall include the name of the full-time Contractors Agent. 1.7.6
Materials
Sources of supply The sources of supply of materials shall be approved by the Engineer and shall not be changed without prior approval of the Engineer in writing. Goods and materials used in the execution of this Contract shall, where possible, been produced in the United Kingdom, the British Commonwealth or the European Economic Community. Where a specification issued by the British Standards Institute is current at the date of the tender and is appropriate, goods and materials used in the execution of this Contract shall be in accordance with the specification. Rejected materials Rejected Material shall be removed promptly from the Site. 1.7.7
Suitability of equipment
The Contractor shall satisfy the Engineer regarding the suitability, efficiency and adequacy of the equipment to be employed. The Contractor shall state the type and number of rigs he intends to use. 1.7.8
Setting out
The Contractor shall be responsible for all setting out on the site, including all borehole positions. The Contractor shall be responsible for providing adequate protection of any survey points. The Contractor shall provide and maintain bench marks throughout the duration of the Works. 1.7.9
Tolerances
All treatment points shall be located to the tolerances given in the Specification. 1.7.10 Programme The Contractor shall inform the Engineer at regular mutually agreed intervals of the forward programme of ground works. 1.7.11 Services The Contractor shall be responsible for ensuring that no damage is caused to service installations. If during the execution of the work damage is, or is likely to be, caused to mains, services or adjacent structures, the Contractor shall inform he Engineer of his proposals for avoidance or repair of such damage. The Contractor shall arrange the sequence and timing of the Works to ensure that damage does not occur to treated ground by any subsequent work. 1.7.12 Site facilities Proper sanitary accommodation and washing facilities with an efficient hot water supply shall be provided and the Contractor shall make all necessary arrangements, for the use of the land required. 1.7.13 Site cleanliness and making good working areas The Contractor shall confine his operations to the minimum area of ground required for proper execution of the works and access thereto.
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Supplementary Document 4: Sample specification for mine infilling works
1.7.14 Site electricity supply The Contractor shall provide a suitable electricity supply to suit his requirements. 1.7.15 Site water supply The Contractor shall be responsible for locating a suitable water supply and for providing and paying for all temporary plumbing and connection of the water supply to site. Where a suitable water supply is not available in the locality, the Contractor shall make arrangements for carrying and storing water in quantities as necessary for the Works. All costs incurred in this respect shall be borne by the Contractor. The Contractor is responsible for its sensible use and for the care and the maintenance of his pipework from the supply point. 1.7.16 Control of nuisance General The Contractor shall during the course of the Contract take every precaution to prevent nuisance in the form of noise, dust and water from occurring. In organising the operations to be carried out within the site, the Contractor shall take into consideration the nuisance effect of his proposals and employ means to reduce such effect as necessary. Dust and water In order to abate nuisance the Contractor shall take all necessary precautions to prevent nuisance from dust and water. All stored materials, such as pulverised fuel ash, etc., shall be sheeted or treated such that no nuisance is caused by wind borne dust as a result of this operation. In this regard it may be necessary to erect dust tents for the storage of materials. Noise control The Contractor shall employ the best means to minimise noise and vibration produced by his operations and shall have regard to the recommendations in BS 5228, Noise and vibration control on construction and open sites. The Contractor shall comply in particular with the following requirements: i.
All vehicles and mechanical plant used for the purpose of the Works shall be fitted with effective exhaust silencers and shall be maintained in good efficient working order.
ii.
All compressors shall be ‘sound reduced’ models fitted with properly lined sealed acoustic covers which shall be kept closed whenever the machines are in use, and all ancillary pneumatic percussive tools shall be fitted with mufflers or silencers of the type recommended by the manufacturers.
iii. Machines in intermittent use shall be shut down in the intervening periods between work or throttled down to a minimum. iv. All items of stationary plant, e.g. generators, pumps, etc., shall be housed in acoustic enclosures when in operation. The Contractor shall ensure that all steps are taken to limit noise to a level compatible with the particular environment. 1.7.17 Highways to be kept clean No debris or material resulting from the works shall be allowed to fall onto any public highway.
Supplementary Document 4: Sample specification for mine infilling works
25
1.7.18 Lighting Where works are permitted in hours of darkness all adequate flood lighting at working locations shall be provided for the execution of the works. 1.7.19 Site security The Contractor shall provide adequate site security to protect plant, equipment and offices during the hours of non-working. 1.7.20 Water spray The Contractor shall take all precautions and measure by way of screening and/or suppression to minimise the amount of water spray in all areas on the site. In particular this shall be applicable to drilling works adjacent to any work being carried out by other Contractors and in areas adjacent to or within road or railway networks. 1.7.21 Water and slurries The Contractor shall be responsible for all waters and slurries produced and used on site while conducting his drilling and grouting operations. The Contractor will also be responsible for the safe and controlled discharge for all waters and slurries produced on site. It is however unlikely that permission to discharge into existing sewer systems will be given and the Contractor shall find alternative methods of disposal. 1.7.22 Site clearance On completion of the treatment to the satisfaction of the Engineer, the Contractor shall remove from the site all plant and unwanted material. 1.8
Site control and safety
1.8.1
COSHH and transfer to waste
The Contractor shall carry out comprehensive environmental controls during the works. The Contractor shall submit to the Engineer, such environmental information pertaining to the methods of construction. The Contractor is reminded that he is required to provide his own COSHH assessment tailored to his purpose and method of working. 1.8.2
General
It shall be assumed that all materials encountered in boreholes and trial excavations at the site are potentially contaminated, hence workers shall operate with care at all times. Workers may be affected by contaminants via skin contact, ingestion of contaminated material and the inhalation of gasses such as hydrogen sulphite and hydrogen cyanide. All site personnel and visitors shall be made aware of the contamination hazards and health and safety procedures involved to ensure safe operation. 1.8.3
Safety policy
The Contractor shall provide written details of his safety policy and procedures to be adopted on site prior to the commencement of site works. The procedures shall comply with all prevailing legislation and in any case must include the following as a minimum: (a)
Suitable protective clothing shall be worn at all times. As a minimum this shall include overalls, boots, gloves and safety helmets. Dust masks and suitable eye protection shall be available on site at all times for use as required.
(b)
The Contractor shall provide and maintain on site for the duration of the works, a satisfactory first aid kit to include an eye bath and a supply of clean water.
26
Supplementary Document 4: Sample specification for mine infilling works
(c)
The Contractor shall provide and maintain adequate hot water washing and toilet facilities to the satisfaction of the Engineer.
(d)
The Contractor shall ensure that all activities involving hand/mouth contact such as eating and smoking are restricted to designated areas where hygiene facilities are provided.
(e)
If conditions dictate, the Contractor shall provide equipment to minimise the production of dust by the wetting of access roads and excavated materials.
1.8.4
Monitoring of gases
The Contractor shall arrange for the monitoring of Methane and Carbon Dioxide that may be present in boreholes or excavations. Until such time that borehole can be deemed to be safe from such gas naked lights shall not be permitted within 10m of any boreholes. All gas testing and subsequent standing time associated with this shall be deemed to be included within the drilling rates submitted. When odours are perceived or identified all drilling or excavation shall cease in the locality until adequate testing has been carried out and it is confirmed by the testing that it is safe to continue. 1.8.5
General site protocol
i)
A documented procedure shall exist to ensure that the Contractor’s agent and the Engineer are informed of any unknown materials which are encountered during the works.
ii)
The site shall be secured to restrict access, as far as it is reasonably practicable at the end of each working day.
iii)
No eating, drinking or smoking shall be allowed anywhere on site apart from in the designated areas.
1.8.6
Drilling operations and shaft search operations
Drilling rig stability and operative safety shall have specific consideration. The Contractor shall satisfy himself that the arrangement for general drilling, and probe drilling will be effective and offer a high degree of safety to the personnel. 1.8.7
Drilling in the vicinity of buried cables, gas mains, pipes and sewers etc.
No drilling shall be undertaken at any position until: a)
The appropriate statutory bodies have delineated the position of such buried cables, gas mains etc. and the Contractor has satisfied himself of their position.
b)
Where there is no accurate information available with regard to such utilities the best possible means shall be adopted to locate them. In such circumstances no borehole shall be situated closer than 5 m to the conjectured position of the cable.
c)
Any means of delineation shall be such that mischievous or wilful alteration can either not be made to such alteration will be obvious to the Contractor.
d)
Where delineation is dependent upon surveyors or similar markers, a competent surveyor shall confirm any delineation made.
1.8.8
Visitors
All visitors shall report to a reception at the site offices and be provided with appropriate site protection and site supervision during any visit to the work place.
Supplementary Document 4: Sample specification for mine infilling works
2
PARTICULAR SPECIFICATION FOR MINE STABILISATION WORKS
2.1
General requirements mine stabilisation works
2.1.1
Description
27
The works include the design and construction of measures to stabilise abandoned coal mine workings beneath the site. The mine stabilisation works shall be carried out by means of grout injection through boreholes drilled into the abandoned mine. If specific shafts are identified during the course of the works, these shall be treated according to the procedures outlined in this specification. 2.1.2
Method statement
At the commencement of the Contract, the Contractor shall provide a detailed method statement. This shall include full details of his proposed plant and equipment utilisation along with his general methodology with regard to the following specific operations and restrictions: 1)
Drilling methodology including methods to maintain an open borehole and overcoming problems that may be encountered by interconnection of boreholes and broken ground associated with void migration.
2)
Number, phasing and type of drill rigs.
3)
Drilling flush.
4)
Grouting methodology including techniques to be adopted to maintain open boreholes, insertion of grout tremie pipes and methods of overcoming underground water pressures.
5)
Avoidance and protection of services.
6)
Avoidance and protection of public.
7)
Site security.
8)
Group plant requirements and set up areas.
9)
Methodology for control and storage of materials on site and the capacity of grout mixing and injection plant.
2.2
Layout of boreholes
Centre holes to be drilled and grouted if any primary boreholes accept in excess of 5 tonnes of grout. 2.3
Area of treatment
The grout stabilisation is required below all structures and shall extend to at least 3 m beyond the plan footprint of all structures. Drawing XXXXX shows the proposed site layout. Drawing YYYYY shows the preliminary layout of primary grout boreholes. 2.4
Naked lights
The following safety precautions shall apply when drilling. (i)
Naked lights or smoking shall not be permitted within 15 m of any borehole unless it is full to the top with mud, water or grout.
(ii)
If the hole is to be cased, screw jointed pipes shall be used.
(iii)
The Contractor shall maintain on site apparatus for checking the presence of methane in the atmosphere and shall record any emissions of gas from boreholes.
2.5
Drilling and pressure grouting procedure
2.5.1
Method of drilling
Boreholes drilled for the purpose of pressure grouting shall be drilled by rotary or rotary percussive techniques employing water flush only. All boreholes shall be drilled to a line within 5 degrees of the
28
Supplementary Document 4: Sample specification for mine infilling works
design line and the Contractor shall provide means of demonstrating that the required borehole line has been achieved. 2.5.2
Diameter of borehole
The minimum diameter of borings shall be 70 mm for grout infill and 90 mm for curtain wall boreholes. Where an exploratory hole is to be impossible, or where obstructions are expected, the Contractor shall begin boring at such greater diameter as he may choose. 2.5.3
Casing to rockhead
When each borehole has been drilled to rockhead, drilling shall not continue further until a steel casing of appropriate diameter has been securely placed into the rockhead to prevent the loss of grout into the materials above rockhead during subsequent grouting operations and to allow for the grout to be pressurised. Drilling shall subsequently proceed to the final depth of the borehole. 2.5.4
Depth of boreholes
All holes shall be drilled to not less than 1.0 m below the pavement of the lowest seam to be treated at that location, or though the seat earth. 2.5.5
Plumbing of boreholes
Immediately prior to grouting any borehole the Contractor shall check by plumbing or by other approved techniques that no collapse has occurred within the borehole after the drill rods have been withdrawn and that the hole is open to the design depth. 2.5.6
Redrilling of boreholes
Grouting shall commerce via flexible tubing, or a series of metal rods having a minimum internal diameter of 40 mm placed to the base of the borehole or at such other depth required by the design. Where it is realised that the tubing or rods cannot be placed to the specified depth, the tubing or rods shall be withdrawn and the obstruction removed by re-drilling to the specified depth with casing, if necessary. It should be anticipated that significant mine workings and void migration may be encountered on this side. 2.5.7
Grouting tubes
When the tubing or rods have been placed to the depth specified, and checked for blockages, a Tconnection pipe of approved design shall be attached to the top of the tubing or rods. The connection pipe shall have a free outlet available in order that grout flow may be checked and a pressure meter calibrated in units not greater than 50 kN/m2 (7 psi) up to at least 1050 kN/m2 (150 psi) but not exceeding 2100 kN/m2 (300 psi) must be permanently attached at the inlet during the grouting operations. 2.5.8
Grout mix
(1)
The grout mix to be injected into each borehole normally shall have an initial composition by weight of not less than 1 part sulphate resisting cement to 10 parts pulverised fuel ash (pfa). If the acceptance of such grout exceeds 10 tonnes the Contractor may continue injection with a revised mix which may include sand.
(2)
The minimum amount of water shall be added to the dry grout mix to render it of a workable consistency. High bleed grouts shall be avoided and the mixes shall produce cubes with crushing strengths of not less than 0.7 MN/m2.
2.5.9
Grout materials
The cement shall consist of sulphate resisting Portland Cement complying with BS 12/BS 4027 or Portland blast-furnace cement complying with BS 146.
Supplementary Document 4: Sample specification for mine infilling works
29
(1)
The sand shall comply with BS 882. It shall be of a size and grading, suitable for use with the Contractors plant.
(2)
The pulverised fuel ash shall be conditioned hopper ash or dry powder ash to BS3892: Part 2.
(3)
The water to be used for grouting shall be clear and free from organic or other deleterious substances. If possible, it shall be supplied by the local Water Authority. Where other source is to be used, the Contractor shall arrange for tests of the water to be carried out in accordance with BS 3148.
2.5.10 Storage of materials (1)
Cement shall be stored on a raised platform and shall be protected by a waterproof covering.
(2)
The sequence of deliveries shall be recorded so that the cement can be used in rotation. A complete record of the removals and deliveries of cement shall be maintained.
(3)
Pulverised Fuel Ash and sand shall be kept free from contact with deleterious matter. All materials shall be stored in stock piles adequately separated from each other. The Contractor shall in his method statement state his anticipated source of materials and likely requirements for daily stockpiling of materials.
(4)
The Contractor shall ensure that an adequate supply of tested and approved materials are ready for use at all times during grouting operations.
2.5.11 Grouting equipment and pressure (1)
The arrangements for preparing the grout shall be such that an uninterrupted supply is available at the point of injection.
(2)
The grout shall be mixed in a high speed shearing action grout mixer. From the mixing tank it shall pass to a holding tank where it shall be kept in a continuously agitated state until delivered.
(3)
The grout pump shall be a variable pressure ram type pump or other approved pump, capable of delivering a continuous supply of grout of varying consistency at pressures of up to 1.0 bar at the point of injection. The operating pressure shall be maintained generally at 0.1 bar (10 kN/m2) per one metre depth of temporary casting installed within the borehole.
(4)
All valves in the grout ranges shall be of the ball type for ease of cleaning.
2.5.12 Grouting procedure (1)
The specified grout mix shall be injected under the aforementioned conditions until either grout overflow occurs at the surface or the specified pressure is recorded on the pressure metre attached to the T-connection pipe. When this pressure is reached the outlet tap shall be temporarily opened to ensure that pressure has not been incurred by line blockage.
(2)
If during the operation described above grout overflows at the surface (via the temporary casing), at a similar rate to that being injected, the flexible tube or metal rods shall be withdrawn completely and the borehole pressurised as described below.
(3)
If alternatively, the stipulated pressure is reached on the pressure gauge attached to the Tconnection pipe, and this is not caused by line blockage and no further materials can be injected at this pressure, the injection pipe/tube shall be partially withdrawn until the pressure falls and grout injection shall be recommenced. If the stipulated pressure is again realised this procedure of gradual withdrawal of the grout pipe/tube is to be repeated (hereafter referred to as stage grouting) until grout returns to the surface via the casing. Only when grout appears at the surface in similar quantities to that being injected shall the grout pipe/tube be completely withdrawn from the borehole and the pressurisation process specified below be carried out.
(4)
If it becomes apparent that any grout injection pipe/tube has become blocked (indicated by a constant back pressure on the pressure gauge attached to the T-connection pipe, and
30
Supplementary Document 4: Sample specification for mine infilling works
zero flow of grout) the grout injection pipe/tube shall be removed and only replaced when the blockage has been cleared. (5)
Immediately the grout pipes/tubes are withdrawn the T-connection shall be attached to the top of the metal casing originally inserted into the borehole to rockhead and further grout shall be injected until the specified pressure is recorded on the pressure meter attached to the T-connection pipe/tubes. When this second pressure is reached the outlet pipe shall again be temporarily opened to ensure line blockage has not occurred. When the specified pressure has been maintained such that no further grout can be injected at that pressure, the T-connection pipe shall be removed and the metal casing immediately withdrawn. If on disconnecting the T-connection pipe, an appreciable back flow of grout occurs, a cap shall be screwed onto the casing until sufficient setting time has elapsed for the grout flow to cease and yet allow the casing to be withdrawn.
2.5.13 Backfilling boreholes Upon completion of the pressure grouting procedure, and following the subsequent withdrawal of the borehole casing, the Contractor shall ensure that each borehole is backfilled to the surface with grout. In the event that settlement of each backfill grout subsequently occurs, the Contractor shall return to these boreholes locations to top up the grout level. Only in exceptional circumstances – for example where unacceptably high quantities of grout might flow into loosely filled ground – shall this requirement be relaxed in instances of high grout acceptance the Contractor shall bulk backfill with pea gravel, followed by topping up with grout. 2.5.14 Abandoned casing The Contractor shall make every attempt to recover all casing used in the drilling operations. However, if this becomes impractical than any abandoned casings must be filled with pfa/cement grout and cut at least 2 m below ground level. The positions of such abandoned casings shall be recorded. 2.5.15 Drilling and grouting sequence (1)
The boreholes shall be drilled in a sequence and to depths required by the design. Grout shall be injected into the boreholes in an order and with grout mixes as required by the design. The boreholes shall be drilled and grouted in a systematic manner. Interconnection of boreholes may be anticipated and in this regard it may be necessary to restrict grouting in the proximity of drilling operations to avoid difficulties in the grouting operations.
(2)
The grouting shall where possible commence at the lowest point of the seam to be treated and progress up dip.
(3)
The Contractor shall take all reasonable steps to ensure that any uplift consequent upon the grouting shall be completely obviated and that no disturbance to adjacent structures occurs.
(4)
The Contractor shall attempt to inject grout into each borehole irrespective of whether or not a cavity or old mineworking is suspected. Care shall be taken to ensure that grout is not injected into Made Ground, superficial deposits or pipes, drains, cable ducts etc.
(5)
Where large voids are encountered in the ‘downdip’ zone of the proposed treatment area it may be necessary to drill and form a grout curtain prior to the main infill drilling. Where large voids are located the injection of pea gravel may be required. In these instances to avoid excess pressure causing the grout/pea gravel cone to slump, the injection shall be stopped as soon as practicable after the cone has reached the roof of the void and has formed good interlock. When the perimeter hole grout has set sufficiently hard, the hole shall be filled to the surface with grout and injected.
2.5.16 Daily record (1)
The Contractor shall keep both a Daily Drilling Record and a Daily Grouting Record for each borehole, while work proceeds at that borehole.
Supplementary Document 4: Sample specification for mine infilling works
(2)
(3)
31
The Daily Drilling Record shall record the following information: (a)
Job name and location
(b)
Borehole reference number
(c)
Date
(d)
Contractor’s name
(e)
Plant in use, crew members, and hours worked
(f)
Method of drilling and flushing medium
(g)
Type of bit used
(h)
Diameter and depths of all casing used
(i)
Depth and diameter of borehole at beginning and end of each working day or shift
(j)
Depth to each change of stratum
(k)
Description of each stratum
(l)
Depths at which groundwater is encountered.
(m)
The depths at which samples were taken
(n)
Details of any loose ground or voids penetrated
(o)
Details of any loss of flushing medium
(p)
Details of time spent in overcoming obstructions
(q)
Details of services or drains located
(r)
Details of any emission of gas, water, four air, etc
(s)
Details of any settlement or ground heave adjacent to boreholes
(t)
Daily and cumulative quantities of drilling.
The Daily Grouting Record shall record the following information. (a)
Job name and location
(b)
Borehole reference number
(c)
Date
(d)
Contractors name
(e)
Plant in use, crew members and hours worked
(f)
Details of plumbing of the depth of borehole
(g)
Details of quantity of grout constituents injected into each borehole
(h)
Total daily and cumulative quantities of each constituent of grout mixes
(i)
Details of the grouting pressures recorded, and corresponding depths.
(j)
Details of casing abandoned in boreholes with appropriate reference numbers.
(4)
Particular attention shall be paid to recording the depths of cavities or suspected old mine workings and to recording the loss of water during drilling.
(5)
The Daily Records of Materials and Plant Received shall show in particular each day’s quantities by weight of each type of material and cumulative quantities. The Contractor shall submit to the Engineer with the Daily Records copies of receipts or invoices for all materials delivered and shall keep them on site until the work is complete.
2.6
Exploratory open-hole drilling procedure to locate mine shafts
2.6.1
Orientation of boreholes
The boreholes shall be drilled vertically and shall be maintained within 5 degrees of the vertical throughout their entire length.
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2.6.2
Supplementary Document 4: Sample specification for mine infilling works
Drilling flush and borehole diameter
The drilling fluid shall be clean water. The borehole diameter shall not be less than 70 mm and the drill bit shall be of a design that produces the required open-hole samples. 2.6.3
Loss of flush
If at any time, whilst drilling is in progress, the flushing medium fails to return to the surface, drilling shall only proceed where it is possible to identify the level of rockhead by the determination of ‘hard’ rock drilling from ‘soft’ superficial deposit drilling. If this is not possible and material cuttings borne by the flushing medium are required for the identification of rockhead level then casing must be inserted into the borehole to restore the flush circulation. If at any stage during the drilling the insertion of casing fails to re-instate the flow of the flush to the surface, the Contractor shall stop drilling and contact the Engineer for further instruction. 2.6.4
Depth of boreholes
All probe boreholes will be drilled to a depth of 1.5 m below rockhead. Where a borehole does not encounter rockhead at an expected level, this borehole shall continue to prove ‘soft’ drilling or otherwise to a depth of 5.0 m below previously proved rockhead level and the Engineer informed immediately. No shaft drilling is to take place until a shaft platform can be established. 2.6.5
Borehole grid
The boreholes shall be drilled on a grid to be established and numbered by the Engineer. This grid will be centred on the ‘best plot’ location of the shaft presentation. The Contractor shall commence probe drilling at the shaft location and then drill boreholes at increasing distances from the shaft position until the shaft is located or further instructions are received from the Engineer. In the event that the entire grid is drilled without encountering the shaft, a secondary ‘centre borehole’ grid may be drilled to an appropriate distance around the best plot position of the shaft. 2.6.6
Sealing of boreholes
On completion, to the approval of the Engineer, each borehole shall be carefully sealed with cement slurry and drill cuttings or sand from the bottom upwards so as to prevent any collapse of the borehole and/or surface disturbance at or in the vicinity of the borehole. 2.6.7
Sampling of superficial deposits
Where directed by the Engineer, the Contractor, when drilling through superficial deposits shall provide disturbed samples at two metre intervals and at each change in strata, or as otherwise directed by the Engineer. 2.6.8
Sample storage and ownership
Disturbed samples from individual boreholes shall be presented and stored collectively on site and made available for inspection by the Engineer. All samples shall become the property of the Employer and shall not be removed from site by the Contractor other than at the Engineers instruction. It may be required for samples to be kept for three months after completion of site works and they shall be discarded only on the instructions of the Engineer. 2.6.9
Drilling records
(1)
The Contractor shall keep a written record, in a form approved by the Engineer, while work proceeds at a borehole.
(2)
Drilling records shall record the following information. (a)
Job name and location
(b)
Borehole reference number
Supplementary Document 4: Sample specification for mine infilling works
(c)
Date
(d)
Contractor’s name
(e)
Plant in use, crew members and hours worked
(f)
Method of drilling and flushing medium
(g)
Type of bit used
(h)
Diameter and depths of all casing used
(i)
Depth and diameter of borehole at beginning and end of each working day or shift
(j)
Depth to each change of stratum
(k)
Description of each stratum
(l)
Depths at which groundwater is encountered
(m)
The depths at which samples were taken
(n)
Details of any loose ground or voids penetrated
(o)
Details of any loss of flushing medium
(p)
Details of time spent in overcoming obstructions
(q)
Details of services or drains located
(r)
Details of any emission of gas, water, foul air, etc
(s)
Details of backfilling of holes.
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(3)
The details of the strata shall be based on the driller’s interpretation of the drilling returns and shall note any major colour or stratum changes and the hardness of the strata (as estimated from the rate of penetration). Generally the strata description shall be confined to simple terminology such as made ground, sand, gravel, mudstones, sandstones, shale, coal etc with their appropriate colour and degree of hardness.
(4)
The Contractor shall give written instructions to his site staff defining the nature of the log required for each exploratory hole. Copies of all such instructions shall be forwarded to the Engineer for his information and comment.
(5)
Particular attention shall be paid to recording the depths of cavities or suspected old mine workings and to recording the loss of flushing medium during drilling.
2.6.10 Standing time, claims etc All additional work, standing time etc shall be recorded on the Daily Record Sheets. Any claim for such shall be notified in writing to the Engineer within 72 hours of the same occurring. Entries on Daily Record Sheets alone, will not be accepted as being notification of an intent to claim under this clause. 2.7
Drilling and pressure grouting procedure for abandoned mine shafts and adits
2.7.1
Shaft boreholes
The boreholes shall be drilled by rotary methods employing air flush techniques unless otherwise agreed to by the Engineer. Initially, a vertical borehole of minimum 90 mm diameter will be drilled commencing at the centre of the shaft. Drilling will then continue until solid strata are reached at the base of the shaft. It is proposed that the subsequent grouting of the shaft is to be carried out via the drill string. The minimum internal diameter of the drill string shall be not less than 50 mm and external casing will not be necessary unless it is required to insert the pea gravel. If a cavity is located within the shaft the Engineer may require it to be infilled with pea gravel in which case the Contractor will be required to insert casing of minimum 100 mm diameter to the upper limit of the cavity to facilitate the placing of the pea gravel into the cavity. The casing must then remain in place until the base of the shaft is proved by the drilling. During the drilling the Contractor shall carry out regular checks and any necessary adjustment to the drilling rig to ensure that the drill is vertical.
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Supplementary Document 4: Sample specification for mine infilling works
When the drilling encounters solid strata at the presumed base of the shaft, boring will continue into rockhead for a distance of at least 5 m or as specified by the Engineer. 2.7.2
Grouting tubes
Grouting can then proceed via the drill string. Any casing may at this stage be withdrawn but the drill string must remain at the borehole base. A triple outlet, 1-connection pipe will be attached to the top of the drill string, via a coupling of approved design. The connection pipe will be of such design that a free outlet is available to check the flow of grout and that a pressure meter, calibrated up to at last 1,725 kN/m2 (250 psi) is permanently attached at the inlet location. It is anticipated that pressure in the region of 690 kN/m2 (100 psi) to 1,035 kN/m2 (150 psi) will be required. 2.7.3
Grouting procedure
Grouting will continue via the drill string, previously placed to the base of the borehole until a pressure to be stipulated by the Engineer has been recorded and maintained at the pressure meter, attached to the I-connection pipe, or until grout overflow at the surface occurs. The drill rig will be of such design that up to 3 m (10 feet) lengths can be withdrawn, disconnected and the T-connection re-connected at the top of the borehole. When the stipulated pressure has been maintained and further grouting at that level is not possible without generating greater pressure, the Contractor will withdraw the drill string up to 3 m (10 feet). Further grouting will then proceed and further withdrawal up the shaft shall not be carried out at this or any subsequent stage or level until the stipulated pressure has been recorded and maintained at that stage or level. This procedure will continue until the shaft has been pressure grouted to the surface or near surface, to the satisfaction of the Engineer. 2.7.4
Grout mix
The grout mix will be of composition 10 parts pulverised fuel ash to 1 part cement by weight, or as otherwise specified by the Engineer. The Contractor may be required to include sand in the grout mix. The grain size and consistency of any materials used for grouting must be approved by the Engineer and only the minimum quantity of water shall be used so as to render the grout workable. The consistency and slump nature of the mix must be approved by the Engineer. The mixing and pumping plant used shall be such that constant agitation of the grout is possible and the Contractor must specify in writing the method of controlling, metering and pumping the amount of material to be used when submitting the completed tender documents. 2.7.5
Recovery of materials
The Contractor is expected to recover from each borehole, metal rods, connections and castings used in the operations, and it will be necessary for the Contractor to identify the Employer against any claims for loss of such items. 2.7.6
Drilling platform
The Contractor’s particular attention is drawn to the possibility of the collapse of fill material into the shaft, or of the shaft itself whilst drilling and/or grouting operation are being carried out. During the drilling and treatment of the shaft, the drilling equipment shall be supported on and secured to a suitable platform such that in the event of a collapse the drilling rig remains supported over the shaft. When assessing the platform requirements, the Contractor shall consider whether the platform itself could become dislodged by a collapse and in such a case shall make provision for the platform to be secured to suitable anchorages so as to prevent such an occurrence. The dimensions of the platform/staging/scaffolding and any anchoring devices must be approved by the Engineer, prior to its transport and erection at the site. Subsequent to the shaft being located no drilling is to take place at any suspected old mine shaft until the Engineer is satisfied that the appropriate platforming and anchoring systems have been established. It is recommended that all
Supplementary Document 4: Sample specification for mine infilling works
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personnel involved in the drilling and treatment of a shaft shall wear appropriate safety harnesses secured at a safe distance from the shaft. 2.7.7
Group caps
If the depth of fill materials at the suspected shaft location is greater than about 5.0 m a group cap will be formed at rockhead, rather than a conventional reinforced concrete cap. The grout cap shall consist of boreholes drilled on a 2 m grid within 6 m radius circle of the shaft centre, ie 29 number boreholes. Boreholes shall penetrate 4 m into rock. Grouting shall take place generally in stages back to surface. For this grouting the same procedures shall apply as for the general grouting of mineworkings. If the grout take exceeds 5 tonnes for any one stage, the Engineer shall be informed immediately. The addition of sand shall be at the Engineer’s discretion. A further five secondary holes shall be drilled and grouted not less than 12 hours after the initial grouting. 3
NOISE CONTROL
The Contractor shall comply with the general recommendations set out in NS5228: Part 1:1984 and Part: 1986, Noise Control on Construction and Open Sites, together with the specific requirements described in the following table:
Period
Hours
Noise level in Leq dB (A)
Period of hours over which Leq is applicable
Peak noise measured in dB(A)
Monday to Friday
0800 – 1800
75
12
85
Saturdays
1800 – 0800 0800 – 1300 1300 – 1800 1800 – 1600 0900 – 1600 1600 – 0800
50 75 60 50 60 50
4 5 8 4 4 4
55 85 65 55 65 55
