Specifiers' Guide to Steel Piling

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SCI PUBLICATION P308

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Specifiers’ Guide to Steel Piling

A R Biddle

BSc, CEng, MICE

E Yandzio BSc, MEng, CEng, MIMarE

Published by: The Steel Construction Institute Silwood Park Ascot Berkshire SL5 7QN Tel: Fax:

01344 623345 01344 622944

P308: Specifiers' Guide to Steel Piling

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 2002 The Steel Construction Institute Apart from any fair dealing for the purposes of research or private study or criticism or review, as permitted under the Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the UK Copyright Licensing Agency, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organisation outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publishers, The Steel Construction Institute, at the address given on the title page. Although care has been taken to ensure, to the best of our knowledge, that all data and information contained herein are accurate to the extent that they relate to either matters of fact or accepted practice or matters of opinion at the time of publication, The Steel Construction Institute, the authors and the reviewers assume no responsibility for any errors in or misinterpretations of such data and/or information or any loss or damage arising from or related to their use. Publications supplied to the Members of the Institute at a discount are not for resale by them. Publication Number: SCI P308 ISBN 1 85942 132 6 British Library Cataloguing-in-Publication Data. A catalogue record for this book is available from the British Library.

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FOREWORD The general knowledge base in the civil engineering industry has been improved by the publication of the Steel Bearing Piles Guide by SCI in 1997 (jointly funded by Corus and the DETR) and the 7th Edition of the Piling Handbook produced by Corus, together with steel piling seminars. Steel piling is the first choice for offshore deep foundations and on shore in several countries of the world such as the USA, Japan, Scandinavia, Holland and the Baltic States. The use of steel piling in the UK is still held back by a lack of knowledge of the technology amongst specifiers and designers or by lack of experience with steel pile installation. The lack of experience can hamper the selection of steel piling, particularly when important technical issues such as noise and vibration prediction need to be resolved at the concept design stage of a construction project. There is therefore a need to add to the published information to cover recent experience in dealing with these issues.

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This publication presents up to date advice based on recent research and on new steel piling products that have been developed to achieve better value solutions for various construction projects. It is hoped that this will give the specifier more confidence in the selection of steel piling.

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CONTENTS

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Page No. FOREWORD

iii

SUMMARY

vii

1

INTRODUCTION 1.1 Objective 1.2 Steel piling today 1.3 Advantages of steel piling 1.4 Scope of this document 1.5 Value engineering

1 1 1 2 2 2

2

SITE CONDITIONS 2.1 Site access 2.2 Environmental constraints 2.3 Ground Conditions 2.4 Suitability in Contaminated Land

4 4 5 5 6

3

STEEL PILE INSTALLATION 3.1 Sheet pile installation methods 3.2 Selecting installation equipment for sheet piles 3.3 Aids to sheet pile installation 3.4 Positional accuracy in sheet pile installation 3.5 Drivability prediction for sheet piles 3.6 Bearing pile installation methods 3.7 Selection of installation equipment for bearing piles 3.8 Positional accuracy in bearing pile installation 3.9 Dynamic testing of bearing piles

8 8 12 13 13 14 15 17 18 18

4

NOISE AND VIBRATION 4.1 Noise 4.2 Vibration

19 19 21

5

DURABILITY 5.1 Corrosion rates and expected lives in natural environments 5.2 Corrosion rates and effective life in contaminated land

27 27 28

6

WATER-RESISTANT SHEET PILE CONSTRUCTION 6.1 Prevention of water seepage through sheet pile walls 6.2 Sheet pile wall to reinforced concrete slab connections 6.3 Permeability rates in contaminated environments

30 30 34 36

7

FIRE SAFETY 7.1 Fire engineering solutions: structural fire design codes 7.2 Protecting structural steel

37 37 38

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8

STRUCTURAL DESIGN 8.1 Steel pile sections 8.2 Design soil parameters 8.3 Effect of corrosion on section 8.4 Design of retaining walls 8.5 Design for vertical loads 8.6 Connection of piles to superstructures

41 41 41 41 41 42 43

9

REFERENCES

46

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SUMMARY This publication offers guidance to specifiers and designers involved in the selection of piling at the concept stage of construction projects. It explains that steel piling offers benefits in terms of speed of construction, quality, economics and sustainability. Advances in construction expertise have resulted in solutions that can be installed with confidence in most types of ground in ways that meet modern requirements for health and safety and within environmental constraints. The publication discusses aspects of site conditions that can affect the selection of steel piles and describes the principal methods of pile installation. It explains that although noise and vibration are inevitable in construction, modern piling equipment need be no noisier than other methods of foundation construction. References are given to standards and reports that demonstrate this.

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The corrosion performance of piles in the ground is well understood; guidance is given on corrosion rates that demonstrate the long service life offered by steel piling. Fire safety performance and the water resistant performance of sheet piling are also described. An overview of structural design is presented, with references to guidance documents and standards.

Guide pour les spécifications des pieux en acier Résumé Le but de cette publication est de guider les personnes en charge de la rédaction des spécifications et du dimensionnement lors de la sélection de pieux pour des projets des constructions, dès le stade conceptuel. Il expose les avantages des pieux en acier en termes de qualité, économie, rapidité de construction et d’entretien. Les progrès dans le domaine permettent aujourd’hui de proposer des solutions pour pratiquement tous les types de sols, solutions qui rencontrent tant les contraintes environnementales que celles liées à la sécurité et la santé. Cette publication discute des aspects liés au site qui peuvent influencer le choix de pieux en acier et décrit les principales méthodes de mise en œuvre. Elle explique que, bien que le bruit et les vibrations sont inévitables, les équipements modernes ne sont pas plus bruyants que d’autres méthodes de réalisation de fondations. Référence est faite à des rapports techniques qui démontrent ces faits. La résistance à la corrosion dans le sol est actuellement bien maîtrisée ; des indications sont données montrant que les vitesses de corrosion sont suffisamment faibles pour permettre une longue durée de vie aux pieux en acier. La résistance au feu ainsi qu’à l’eau sont également abordées. Le dimensionnement structural est également présenté avec référence à divers documents techniques.

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Leitfaden für Stahlpfähle Zusammenfassung Diese Publikation bietet eine Anleitung für Fachleute und Tragwerksplaner, die mit der Auswahl von Pfählen in der Planungsphase eines Bauprojekts befaßt sind. Sie erklärt, daß Stahlpfähle Vorteile in Bezug auf Schnelligkeit, Qualität, Wirtschaftlichkeit und Dauerhaftigkeit bieten. Fortschritte in der Bautechnik haben zu Lösungen geführt, die bei den meisten Bodenarten so eingesetzt werden können, daß moderne Anforderungen an Gesundheit und Sicherheit und innerhalb von Umweltzwängen erfüllt werden. Die Publikation behandelt Aspekte der Baustellenbedingungen die die Auswahl von Stahlpfählen beeinflußen können und beschreibt die wichtigsten Methoden der Pfahlmontage. Sie erklärt daß, obwohl Lärm und Erschütterungen beim Bauen unvermeidlich sind, moderne Pfahlausrüstung nicht lauter sein muß als andere Methoden der Gründungserstellung. Auf Vorschriften und Berichte, die dies aufzeigen, wird Bezug genommen.

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Das Korrosionsverhalten von Pfählen im Baugrund ist gut bekannt; eine Anleitung zu Korrosionsraten wird gegeben, die die lange Lebensdauer von Stahlpfählen aufzeigen. Das Brandsicherheitsverhalten von Spundwänden und deren Widerstandsfähigkeit gegen Wasser werden ebenfalls beschrieben. Ein Überblick zur Bemessung wird vorgestellt, mit Bezug auf Leitfäden und Vorschriften.

Guía para autores de pliegos de condiciones sobre pilotajes de acero Resumen Esta publicación intenta servir de guía a autores de pliegos de condiciones y proyectistas involucrados en la selección de pilotajes en la etapa conceptual de proyectos constructivos. Explica los beneficios que ofrecen los pilotajes de acero en cuando a la velocidad de construcción, calidad, economía y sostenibilidad. Los adelantos en la experiencia constructiva han conducido a soluciones que pueden instalarse con confianza en la mayor parte de tipos de suelos cumpliendo los modernos requisitos de salud y seguridad respecto a las condiciones medioambientales. Se analizan aspectos de los emplazamientos que pueden afectar la selección de pilotes de acero y se describen los métodos principales para su puesta en obra. Se explica que aunque ruidos y vibraciones son inevitables, los equipos modernos de pilotaje no son mas ruidosos que otros procedimientos de cimentación y se hace referencia a normas e informes que lo demuestran. Las prestaciones frente a corrosión están bien comprendidas y se aconsejan valores de velocidad de corrosión que demuestran la larga vida de servicio que ofrecen los pilotajes de acero. También se describe el comportamiento de la resistencia de tablestacas frente al fuego y el agua. Finalmente se presenta una panorámica del proyecto global con referencias a documentos y normas de apoyo.

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1

INTRODUCTION

1.1

Objective

The use of steel piling in foundations and retaining walls can confer many benefits in terms of speed of construction, quality, economics and sustainability, but these benefits are not always appreciated by those specifiers and designers involved in the selection of piling. The purpose of this guide is to explain those aspects of steel piling that can give better value in construction. This publication is not a textbook but a practical ready-reference to assist in arriving at an effective steel piling solution that suits the soil conditions, satisfies specification requirements, minimises any installation risks, and complies with the Construction Regulations.

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1.2

Steel piling today

Steel sheet piling has always been the traditional solution for quay walls and marine structures in the UK and has a growing application to permanent walls for basements. It is widely used for temporary works. Steel pile construction expertise has progressed over the last 50 years thanks to increased usage worldwide, particularly in the USA, Japan and in European countries such as Scandinavia, the Benelux countries and the Baltic States. In those countries, steel sheet piling is widely used in permanent works for retaining walls and basements. In the USA, Japan, Finland and Norway, steel bearing piles are the first choice solution for most buildings. The development of sheet piling products has produced watertight wall construction and improved driving equipment that now offers quiet installation. Sheet piling is now used for permanent applications ranging from bridge abutments to basement walls. Greater understanding of the behaviour of steel bearing piles has led to the opportunity to use shorter and fewer piles than concrete alternatives. Publications produced by Corus (formerly British Steel) and The Steel Construction Institute, together with seminars and piling courses, have contributed greatly towards a wider dissemination of the technology and a wider range of applications of steel piling in the UK civil engineering industry. The publications on steel piling that are available include the 7th Edition of the Piling handbook[1], and the following SCI publications: Steel bearing piles guide[2] Design guide for steel sheet pile bridge abutments[3] Environmental assessment of steel piling[4] Integral steel bridges: Design guidance[5] Integral steel bridges: Design of a single-span bridge-Worked example[6] Integral steel bridges: Design of a multi-span bridge-Worked example[7] Steel intensive basements[8]

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1.3

Advantages of steel piling

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The advantages of using steel piling include: •

Installation is quick, reducing overall construction time and site occupation.

•

Bearing capacity can be confirmed during driving of each pile.

•

Steel piles cause only low displacement of adjacent soils during driving.

•

The driven sections have reliable section properties, avoiding the need for in situ integrity checking.

•

Steel piles are sufficiently ductile to adapt to the presence of obstructions in the ground with minimal effect.

•

Steel piles offer high ductility that can reduce earth pressures and bending stresses.

•

Higher end bearing resistance can be mobilised in granular soils and rocks by pile driving as compared to boring.

•

Temporary piles can easily be removed for site reinstatement.

•

Permanent piles can be extracted and reused or recycled after long periods in the ground.

•

Driving equipment is compact and requires minimal access space.

Steel piles have clear-cut advantages on projects such as river or estuary crossings where soils are typically granular and waterlogged and unsuitable for satisfactory pile boring or where soft recent low bearing strength alluvium overlies bedrock.

1.4

Scope of this document

This document offers advice to the specifier on the following technical aspects of piling selection and explains how steel piling can comply with requirements and legislation in those respects: •

Site condition (Section 2).

•

Steel pile installation (Section 3).

•

Noise and vibration (Section 4).

•

Durability (Section 5).

•

Water resistant sheet pile construction (Section 6).

•

Fire protection (Section 7).

•

Design methods (Section 8).

1.5

Value engineering

Economic comparisons between steel piling and concrete alternatives (such as contiguous bored piles and diaphragm walling) are often made simply on the basis of material and installation cost of steel versus that of concrete but this narrow basis can be misleading. The potential to save costs with steel piling requires a more thorough approach to appraise the effects of construction

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methods that maximise the benefit of the shorter construction period and parallel working of other activities. It should be noted that the differential in material/installation cost between concrete and steel piling has decreased dramatically in recent years when estimating the basic cost of construction. Any decisions at concept stage should take account of current cost rates from reliable specialist steel pile installation contractors. Other major construction benefits are that excavation can be minimised, the substructure is smaller and groundwater can be shut out more quickly. The recognition that reduction in construction time is vitally important to the client has caused a rethink of many traditional construction practices. As well as the direct savings from shorter construction programmes, earlier completion can bring economic benefits from reduced loan interest charges and earlier revenue payback from letting the building facility. For publicly funded projects, there can also be reduced traffic disruption and reduced lane rental charges or earlier use of hospital buildings, prisons or social housing.

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On urban high value sites, permanent sheet pile basement walls permit more use of site space for development, right up to the site boundary, avoiding the clearance space required for concrete embedded walling. Traditionally, design of a piled foundation was considered to be a separate activity that was left to the contractor and this often precluded the use of steel piling because of the lead time between design and supply was difficult to accommodate at a late stage. The advent of ‘Design and Build’ contracts for civil engineering work has created greater incentive for innovative design and encourages cheaper overall construction through incorporating the piled foundation in the structural concept from the outset. Steel foundation piles are ductile and in many ground engineering applications e.g. combined retaining wall and axial bearing duty, it is well known that earth pressure forces and bending moments are reduced from the ‘rigid wall’ assumption by deflection of the steel wall, thereby producing a saving in structural section. Soil-structure interaction computer programs are well established and permit the flexible wall design approach. The size and extent of propping can be optimised, offering further savings. Construction cost savings in steel intensive basements of up to 40% have been reported by designers; savings in integral bridge construction of 15% have been reported.

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2

SITE CONDITIONS

There are many factors related to site conditions that will influence a designer’s choice of piling type, such as: •

Site access for construction plant.

•

Environmental constraints at the site.

•

Compatibility with the chosen construction method.

•

The type of soil.

•

Soil disturbance and the spread of contamination on brownfield sites.

2.1

Site access

The designer needs to be sure that the equipment needed for the chosen type of piling can access the site and operate on it with minimum difficulty.

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Steel piling needs few delivery lorries that can be timed to be off-peak whereas bored concrete and flight auger piles need many daily deliveries of ready-mixed concrete often adding to road congestion in urban areas. Steel piling can be installed right up to the boundary of the site (see cover picture and Figure 2.1 whereas concrete piling rigs and diaphragm wall diggers need a clearance corridor. This permits maximisation of use of the land.

Figure 2.1

Panel driving sheet pile wall at Walton, Morecambe

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2.2

Environmental constraints

Environmental constraints from Government and Local Authorities can include control of site pollution, control of noise and vibration, restrictions on construction deliveries over local roads and control of the times of construction working on site. Steel piling produces no in-situ contamination and minimal on-site waste from pile cutoffs. Steel piles produce no arisings (spoil) at the surface and this eliminates waste disposal and the associated landfill tax costs, and avoids disruption to local traffic.

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All construction plants emit noise and vibration but prediction and control are possible. It should be recognised, when comparing steel piling with on-site concrete piling, that diesel power packs are involved in both and, with modern driving equipment, are the most noisy elements, and not the hammers! See Section 4 for further advice. The requirements for environmental protection associated with alternatives to steel piling should be considered when making a comparison. For example, where bentonite slurry is to be used on-site on large projects i.e. for construction of concrete piling and diaphragm walls, the circulating process for bentonite requires a large site area for the machinery and tanks involved in mixing, pumping, collection and screening. This can cause congestion on-site and result in increased plant emissions, in terms of noise, vibration and in exhaust pollution. Surface site pollution is also likely and there will be costly clearing up afterwards.

2.3

Ground Conditions

The type of subsoil beneath the site will affect the quality of installation of different types of piling and also their subsequent performance under load. Steel piles are suitable for all soil types but there are a number of potential problems with the following soil types: •

Chalk

•

Soils containing boulders

•

Very soft clays

Chalk Driving steel piles in chalk destroys the cemented structure of the soil and creates fine silt-sized residual soil particles in contact with the pile that have much less volume and are therefore very loose. Although this can facilitate penetration of steel sheet piles, for bearing piles the shaft skin friction is reduced to a low level that does not recover. Guidance is given in CIRIA Report PG6[9] and CIRIA Project Report 11[10]. Bored in-situ concrete piles are generally more suitable in all types of chalk. Where driven steel piles are selected for reasons of practicality and economy of installation, reference should be made to the guidance given in the SCI Guide Steel bearing piles when estimating the contribution of skin friction in chalk to the vertical bearing capacity. Pile load tests should be carried out to determine the resultant load resistance of such piles because empirical methods of estimation are dependent upon chalk type and may be misleading. 5

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Soils containing boulders Some soils of glacial origin or derived from the in situ weathering of rock layers often contain intact rock boulders, cobble sized fragments or slabs. If these are of significant size they can obstruct pile driving, causing premature refusal and leave the pile short of its design penetration. Typical types of soil in which this is a risk include boulder clays, Keuper Marl, and Lias clays. Such soils are of course also a problem to concrete piles. In the case of steel sheet piles, the risk of premature refusal may be overcome by using the ‘panel driving’ method rather than ‘pitch and drive method. Panel driving allows for continuation of piling whilst the obstruction is removed. For bearing piles, the risk can be mitigated by including some extra piles in the delivery to site in order to permit replacements or additional piles to be driven to make up any deficiency with any short obstructed piles. In many cases where an obstruction is met during driving, the steel pile may push the boulder to one side, but this may not be possible if the boulder clay is too strong. Where the boulder is a relatively soft sandstone or siltstone, it may be possible to drive through and split the boulder.

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Very soft clays and silts. The lack of strength of wet soft clays and silts creates a problem with any type of piling. Attempting to bore a hole creates instability and wall collapse or slumping and therefore driven piling may be the best suited. Where the soil shear strength is less than 15 kPa the steel pile section should be checked for the risk of buckling. On some sites a ‘running sand’ condition can be created by boring and this has caused undermining of adjacent property on a recent project in Glasgow city centre.

2.4

Suitability in Contaminated Land

The use of piling in contaminated land should follow the guidelines that have been set out by the Environment Agency document: Piling and penetrative ground improvement methods on land affected by contamination: Interim guidance on pollution prevention – 2000[11]. Considerable experienced geotechnical judgement is required to appraise the soil conditions encountered on contaminated land sites in relation to the ‘environmental risks’ that are identified in this document and to select an appropriate pile type that can minimise them to a level that conforms to the guidance. The potential ‘environmental risks’ at contaminated sites are stated as: •

The creation of preferential flow paths, allowing contaminated groundwater and leachates to move downwards through aquitard layers into underlying groundwater or between permeable horizons in a multi-layered aquifer.

•

The breaching of impermeable covers by piling or penetrative ground improvement, allowing surface water infiltration into contaminated ground (thus creating leachate) or allowing the escape of landfill or ground gases.

•

Contaminated material being brought to the surface by piling work (arisings), with the risks of subsequent exposure of site workers and residents to contaminants, and the need for appropriate handling.

•

The effects of aggressive ground conditions on materials used in piles. 6

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•

Driving contaminated materials downwards into an aquifer during construction / installation.

•

Concrete or bentonite contamination of ground or groundwater and any nearby surface waters.

The so-called ‘risks’ of creating flow paths and breaching aquiclude cover are common to all types of pile. Creation of preferential flow paths is thought to be a ‘risk’ created along a steel pile wall when driven through cohesive soils that are judged to provide an aquiclude or aquitard. Research on steel bearing piles[12] shows that the soil disturbance adjacent to steel pile walls heals as pore water pressures dissipate with time after driving and clay adhesion is created as intimate contact with the wall returns. Excavation of research piles after testing shows that clays chemically bond to the steel wall over time to eliminate any ‘gaps’ and give a high skin frictional resistance to applied pile load. There are risks of spreading and introducing contamination also with concrete piles. The arisings of contaminated materials from bored pile holes can spread contamination and the use of in-situ placed concrete and bentonite can cause new contamination of the groundwater.

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The selection of the most suitable pile type to use for a barrier wall or foundation in contaminated land sites is therefore an exacting judgement requiring much geotechnical knowledge and experience.

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3

STEEL PILE INSTALLATION

The designer needs to have confidence that piling can be installed (driven) to the design penetration and within specified tolerances. Confidence in pile driveability prediction grows with experience and therefore is sometimes perceived as a risk by less experienced designers. This Section summarises some of the experience with steel pile driving, to improve the confidence of those considering the choice of steel piling. More detailed advice can be obtained from expert contractors or from the UK manufacturer, Corus (www.corusconstruction.com).

3.1

Sheet pile installation methods

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Sheet piles can be installed by a variety of methods and equipment. Each method has particular advantages and disadvantages, and the final choice is, in most cases, a compromise between speed, accuracy, and economy of installation. High standards of alignment can be achieved by the use of a panel driving system (see cover picture and Figure 2.1 and 3.1), a matched hammer and above all a heavy enough sheet pile section to limit deviation during installation and to prevent declutching. A further consideration is the control of noise and vibration and this is covered fully in Section 4. There are three distinct methods of driving based on impact, vibration or jacking used for sheet pile installation.

3.1.1 Impact driving methods Impact pile driving installation methods involve either the use of a falling weight (drop hammers) or an explosive force to accelerate the hammer (air hammers and diesel hammers) and the relative suitability of each is covered in Tables 3.1 and 3.2. The hammer impacts onto a cushion sitting on the top of the pile, thereby causing the pile to penetrate the soil. Drop hammers are the most popular form of impact driving because they are light and reliable (Figures 3.1 and 3.2). They are more efficient and quieter in operation than other types of impact hammer especially when the body is enclosed in an outer casing (see similar hammers for bearing piles, Figures 3.6 and 3.7). They are also compact and adaptable, and may be used underwater with only slight modification. Drop hammers differ in the manner in which the weight or ‘hammer’ is raised. The most popular type is the hydraulic hammer in which the drop weight is lifted using a hydraulic ram then allowed to fall under gravity. In double acting hydraulic hammers, the direction of pressure is switched after lifting so that the hydraulic pressure adds to the gravity force in accelerating the hammer during its fall. There is a wide range of hydraulic hammers available worldwide. This competition and their popularity has encouraged development in both technology and size. Hydraulic hammers can tackle the largest sizes of piles; highly sophisticated instrumentation is available to measure energy output reliably which permits better control during driving.

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Figure 3.1

Unenclosed BSP 357 hydraulic drop hammer panel-driving high modulus sheet piles at A41 Stone Bridge, Aylesbury

Figure 3.2

BSP 357 hydraulic drop hammer

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3.1.2 Vibration-based driving methods Vibratory hammers are usually electrically powered (but can be hydraulically powered). The hammer consists of contra-rotating eccentric masses within a housing attached to the pile head. See Figure 3.3. During the driving process, the induced vibration effectively fluidises the ground adjacent to the pile, reducing its bearing capacity and shaft friction. This allows the pile to move downwards under the weight of the pile and the vibrodriver.

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Vibrations introduced to the ground during driving will be of uniform amplitude and in the range 20 to 40 Hz. This is above the natural frequency of most buildings. However, there has been concern that the vibrations caused during the start up and run down phases pass through the natural frequencies of the ground and adjacent buildings and, although of short duration, the vibration levels could increase dramatically. The introduction of variable frequency and amplitude machines has successfully addressed this issue and further developments have seen the introduction of high frequency vibrators which cause the pile to resonate, resulting in extremely high rates of installation in appropriate soils. These high frequency vibrators are also very appropriate for urban areas where adjacent structures are close to the piling line. Where mixed soils are present it is sometimes difficult to be sure of the vibration level prediction and it is becoming common practice to use a vibration level control system. This consists of detecting vibration levels in the ground with geophones and then changing the hammer amplitude by switches in the event that certain predetermined threshold values of vibration are exceeded.

Figure 3.3

Vibratory hammer on sheet piling

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3.1.3 Jacking and pressing methods

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Lengths of pile, either in short units or in continuous lengths, can be forced into the ground by jacking (usually hydraulically) against a reaction. The reaction system can be provided by tension piles, soil friction on adjacent piling or dead loads. Jacking methods are exceptionally silent and vibration-free in use, and by monitoring the pressure in the hydraulic system a good understanding of soil resistance can be obtained during installation. Two common systems available are the ‘Tosa’ and ‘Giken’. Tosa machines are shown in Figures 3.4 and 3.5.

Figure 3.4

Tosa jacking machine on sheet piles against site boundary

Figure 3.5

Tosa jacking machine on sheet piles

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3.2

Selecting installation equipment for sheet piles

A wide variety of installation equipment types are available to facilitate the economical installation (and extraction for any temporary piles) of all types of conventional piling. The choice of type of installation equipment must be considered well in advance of actual driving operations to ensure the best results from the outset. Tables 3.1 and 3.2 below may assist in selection of the type of installation equipment. These tables give an indication of the types of equipment that are available, which ground characteristics they are suited to, and other considerations relevant to their use. Table 3.1

Selection of driver for sheet pile installation in Granular Soils - Piling handbook[1]

Type of installation equipment

Density (Expressed as SPT ‘N’ Value) Loose 0-10

Med. dense 11 – 30

Dense 31 – 50

Very dense 51 and over

Small Drop/Hyd Drop

A

A

B

C

Large Drop/Hyd Drop

C

B

A

A

Air hammers

A

A

C

D

Diesel hammers

C

B

A

A

Small Vibro’s

A

B

B

D

Large Vibro’s

B

A

B

C

Jacking

B

B

C

D

Vibro Pushing

A

B

B

C

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Impact drivers

Vibratory drivers

Reactive drivers

Table 3.2

Selection of driver for sheet pile installation in Cohesive Soils - Piling handbook

Type of installation equipment

Cohesion (Expressed as cu kN/m2) Soft 0-45

Firm 46 – 80

Stiff 81 – 150

Very stiff >151

Small Drop/Hyd Drop

A

B

B

C

Large Drop/Hyd Drop

C

A

A

A

Air hammers

A

B

C

D

Diesel hammers

A

A

A

B

Small Vibro’s

C

D

D

D

Large Vibro’s

B

C

D

D

Jacking

A

A

A

B

Vibro Pushing

B

B

C

D

Impact drivers

Vibratory drivers

Reactive drivers

A = Most suitable, B = Suitable, C = Not ideal, D = Not suited

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3.3

Aids to sheet pile installation

3.3.1 Water jetting Sheet pile driving in sand, silty sand, fine sandy gravel, and similar non-cohesive soils can usually be assisted by jets of water at high pressure directed into the ground alongside and below the pile tip. In some cases jetting is so effective that ordinary driving can almost be dispensed with, but, as a minimum, the last metre of driving should be carried out without jetting, so that the lowest part of the pile can form a seal in undisturbed soil. The potential concern about water jetting is that the disturbance caused to the soil in the vicinity of the pile might affect the performance of the wall or of adjacent existing structures. Water jetting should not be employed where the designer feels that it may critically affect the soil behaviour on highly loaded piles or risk undermining adjacent building foundations.

3.3.2 Ground treatment

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Where the choice of section size is dominated by driving considerations, rather than strength capacity, it may be cost effective to consider pre-treatment of the ground to permit a lighter section to be used. This can take a number of forms, depending on the ground conditions and other site restrictions. Typical treatments include: •

Pre-auguring along the proposed sheet pile line This can be carried out either in advance of the piling works or alongside part driven piles and allows hard ground to be broken up or extracted and replaced with a material more suitable for driving.

•

Driving of a heavier section along the proposed sheet pile line Installation and subsequent extraction of a driveable pile section along the piling line can be used to break up the ground prior to driving the sheet pile wall.

3.4

Positional accuracy in sheet pile installation

The accuracy of pile installation is of the utmost importance to facilitate the connection to the superstructure. Tolerances must be accepted in the positioning of driven steel piles, as they are in all types of piling, and it will be necessary to consider lack of fit forces and local moments in the design of the connection details. The specification issued by the Federation of Piling Specialists[13], the BSI standard BS EN12699 Execution of special geotechnical works[14], and the TESPA publication Installation of steel sheet piles[15] may be used for reference. Only the least onerous levels of tolerance are given in those publications. Improvements on these can be achieved by most experienced contractors using careful procedures and guidance devices so the designer should discuss his requirements before finalising the specification. SCI publication Design guide for steel sheet pile bridge abutments[3] presents a summary (reproduced below in Table 3.3) of the most accurate or best alignments that can be expected for sheet piling using specialist equipment, skilled personnel, and careful planning of procedures in respect of site conditions.

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Table 3.3

Typical positional accuracy that can be achieved For pitch and drive method or over water

For panel drive method

± 50 mm

± 50 mm

Dependent on equipment used

Dependent on equipment used

± 20 mm

± 20 mm

± 120 mm

± 120 mm

Vertical alignment

± 1%

± 1%

Alignment in plan

± 1%

± 0.5%

Location In plan Centre line at pile head Level Level of pile head, after trim Level of pile toe

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Angular alignment (measured over the top 1m of wall)

The achievable tolerances are strongly related to factors such as the accuracy in setting up the piling equipment, the guidance frame, the accuracy or ‘repeatability’ of the measurement system, the fixity in machine parts, the presence of obstructions in the ground, variation in properties of the soil (especially near the point of entry of the pile into the ground surface), the inclination of the strata and operator error. The pile design should allow for bending stresses caused by a specified acceptable inaccuracy in the installed position of a pile. It is difficult to achieve an acceptable tolerance level at the top of the pile after driving that will avoid any need for on-site cutting. This is mainly due to differences in driving resistance. However, precision mechanical cutters, flame cutters and hydraulic grit blast cutters are available that can achieve a level cut within 1-2 mm accuracy.

3.5

Drivability prediction for sheet piles

The selection of a section suitable for installation requires experience, site information, soil conditions, and knowledge of the chosen method of installation. If all worst cases are taken into account the heaviest pile section will invariably result, which could be uneconomic. Risk assessment of the installation method is advantageous in order to arrive at a contingency plan that can be implemented on site to overcome any difficulties. Situations can arise where anticipated driving forces govern the selection of pile section in strong soils and structural requirements govern the selection of section in weak soils. Thus it may appear that the selection of a heavy section for driving will upset the economics of the sheet piling option but extra section strength can be used to offset the durability needs very effectively and at no extra cost. Whilst it is recognised that driving forces vary with driving method, BS 8002[16] provides a guide for minimum pile dimensions based on driving forces for cohesive soils when no other information or experience is available (see Table 3.3). The criteria used in this table are that the pile head should not be

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damaged by the hammer impact, the pile should sustain the driving force without buckling and the pile toe should not be damaged by the soil resistance. Table 3.4

Pile size as a function of driving conditions in cohesive soils – extracted from Table 6 of BS 8002: 199416. Minimum wall modulus – cm3/m

Clay description

Grade S270P mild steel to BS EN 10025: 1990

Grade S355 GP high yield steel to BS EN 10025: 1990

Soft to firm

450

400

Firm

600 to 700

450 to 600

Firm to stiff

700 to 1600

600 to 1300

Stiff

2000 to 2600

1300 to 2000

Very stiff

2600 to 3000

2000 to 2500

Hard (cu > 200 kN/m2)

Not recommended

4200 to 5000

Note: The ability of piles to penetrate any type of ground depends upon attention being given to good pile driving

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The maximum recommended length for each pile section depends upon the type of strata encountered, penetration required and type of construction for which the piling is to be used. Vibratory driving is the most effective means of installing piles in granular soils but this method may not be efficient when Standard Penetration Test N values exceed approximately 50 blows. In these conditions it will be necessary either to treat the ground with pre-boring or water jetting or to adopt impact driving. Care must be taken in the design as changes to the soil properties adjacent to the pile wall may result from ground treatment measures.

3.6

Bearing pile installation methods

Most bearing piles are pitched and driven singly using hanging leader equipment suspended from a suitable crane (see Figures 3.6 and 3.7). As for sheet piles, the common driving hammers are either hydraulic impact hammers or vibratory hammers. Impact hammers for bearing piles are larger than used for sheet piles because of the higher vertical load capacity. Driving often commences using a vibratory type driver (see Figure 3.8) and then is finished off using an impact hammer because the pile can be lifted and stabbed in position whilst gripped by the vibratory hammer jaws. Completion of the drive is best completed using an impact hammer because a dynamic load test can be obtained to check its capacity. Jacking methods have proved very successful in micro-piling (piles less than 250 mm in diameter); the reaction loads are provided by the structure being underpinned.

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P308: Specifiers' Guide to Steel Piling

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Discuss me ...

Figure 3.6

Enclosed drop hammer driving spiral welded tubular piles

Figure 3.7

A fully enclosed hydraulic drop hammer driving minipiles for housing (Courtesy of Roger Bullivant Piling Ltd)

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Figure 3.8

3.7

Vibratory hammer driving H-piles on Railtrack signal gantry

Selection of installation equipment for bearing piles

3.7.1 Drivability prediction Traditional methods of determining the size and type of hammer required, or of estimating the driving resistance that can be overcome, have involved the use of some form of pile driving ‘formula’. These methods include the Engineering News formula, the Hiley formula and Janbu’s formula and should only be used for approximate assessments of the sufficiency of a given hammer to drive a given pile to a required penetration. The pile driving formulae do not accurately represent the physics of energy transmission down the pile and therefore are not a reliable basis for deducing the driving resistance. A more rigorous investigation of the driving behaviour of a pile to determine driving resistance may be achieved by means of ‘wave equation’ dynamic analysis of the pile-soil system. At the design stage the main objective of dynamic analysis of pile driving is to assess the ‘drivability’ of a pile in given soil conditions and to determine a pile-hammer combination that can satisfactorily achieve the required design pile penetration for the predicted static soil resistance with acceptable driving stresses in the pile. This is adequately achieved using a one dimensional wave equation program. One such program is GRLWEAP marketed by Pile Dynamics Inc. (www.pile.com). For further information on the prediction of drivability for steel bearing piles the reader should refer to SCI publication Steel bearing piles guide[2].

3.7.2 Risks of damage during installation Steel piling is much more tolerant to heavy driving forces than pre-cast concrete piles. This is especially useful when driving steel piles to achieve a high end bearing into rock, or when there is a risk of obstructions in the sub-soil.

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3.8

Positional accuracy in bearing pile installation

Information on tolerances that can easily be met using commonly available pile driving equipment and methods is quoted in the Institution of Civil Engineers publication Specification for piling[17]. These are given in Table 3.4. Table 3.5

Bearing pile driving tolerances that can easily be achieved

Vertical bearing pile Deviation in plan

± 75 mm

Deviation from the vertical

1 in 75

Raked bearing pile Deviation in plan

± 75 mm

Rake up to 1 in 6

1 in 25

Rake greater than 1 in 6

1 in 15

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However, these are ‘least onerous’ tolerances that can be significantly improved by applying more control on site, using templates and guides. For vertical bearing piles, deviation in plan has been known to be as low as ± 25 mm and for deviation from the vertical as low as 1 in 100. If tolerances better than those given in Table 3.5 are needed then the specifier should not hesitate to ask for them in the contract.

3.9

Dynamic testing of bearing piles

There is often need to confirm that the required pile capacity is achieved at a given penetration. Traditionally, it has been a requirement of piling contracts to carry out load tests to ascertain the capacity of a pile or group of piles. This involves the use of a large amount of heavy concrete kentledge or anchor piles to create the necessary reaction, which is a very time consuming process. A strong loading frame is required so that the load can be reacted safely during the test without risk of instability developing. Static load testing is now recognised as being outmoded and dynamic testing is endorsed by an ICE Publication[17]. Static testing can be dispensed with where correlations exist between static and dynamic testing for the soil types found on the site. An explanation is given in the Steel bearing piles guide. Dynamic testing consists of recording and analysing the impact stress wave in the pile at several levels during driving using a strain gauge and accelerometer attached to the pile. The stress wave is analysed using the CAPWAP computer program to determine the soil resistance distribution down the pile and provide a dynamic load test capacity. Several pile testing companies provide an onsite service with real-time results so that a judgment can be made as to the acceptability of the pile being driven. Remounting the hammer and redriving on a test pile will check whether there is any reduction in capacity with time and appropriate allowances can then be made to the design.

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4

NOISE AND VIBRATION

In the past, steel pile installation was associated with high noise and vibration because piles were driven using high impact air or diesel hammers. Such hammers were not enclosed and were therefore very noisy. New enclosed hammers and sound-proofed power plant have significantly reduced the noise levels to a tolerable level that complies with the Control of Pollution Act 1974. Steel piling can now be installed with no more noise and vibration than for bored concrete piles in most situations and there is no longer any limitation to its application to sites in urban areas. In environmental planning for pile installation, simple and accurate methods are needed for the prediction of sound and vibration levels, both near the source of the installation and at nearby residential buildings in order to obtain the Consent to Work Agreement with specified noise and vibration limits. The specifier can then be sufficiently well informed to understand the reliability of a contractors proposals for installation and can easily check for compliance on site with geophones and noise meter measurement.

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4.1

Noise

In recent years, increasing attention has been given to the effects on the local environment of the noise and vibration associated with construction work, also the public are much more aware of their rights. Although the duration of a piling contract may be short in comparison with the whole contract period, the effects of noise and vibration may be more acute during the piling phase. Human response is very intolerant of noise and vibration or shock transmitted through the ground, although greater tolerance can be achieved by careful prior education of the public. Efforts made to advise the public and to plan the precise times of driving can reassure those likely to be affected in the vicinity of pile installation. In the UK, the Control of Pollution Act (1974) provides a legislative framework for, amongst other things, the control of construction site noise. A Code of Practice, BS 5228-4[18] deals specifically with piling noise and vibration, including tables of measured data for reference. In addition, two documents which discuss vibration in detail are the TRL Report 429 Groundborne vibration caused by mechanised construction works[19], and the British Steel publication Control of vibration and noise during piling[20]. BS 6472[21] deals specifically with evaluation of human exposure to vibration in buildings.

4.1.1 Noise from piling operations Data on typical noise levels produced by piling operations are given in BS 5228-4. Other data were published in 1977 in CIRIA Report No. 64 Noise from construction and demolition sites - Measured levels and their prediction[22]. The data are discussed and interpreted in the 1980 CIRIA Report PG9 Noise and vibrations from piling operations[23].

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In areas where severe restrictions are placed on noise levels, vibratory or jacking equipment can be used for pile driving. Such machines emit a different frequency and lower level of noise, which may prove acceptable.

4.1.2 Sound level predictions for piling BS 5228-4:1992[18] gives a table of sound levels that have been measured for a variety of hammers during pile installation. Table 4.2 below presents the typical noise levels from that code in a summarised form. Table 4.1

Typical noise levels during piling operations Noise level LAeq (dB) at 10 m

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Piling operation Sheet piling, diesel hammer

107-112

Sheet piling, air hammer

103-106

Sheet piling, enclosed drop hammer

66-70

Sheet piling, enclosed hydraulic hammer

85-93

Tubular casing, diesel hammer

94-104

Tubular casing/cast in place pile, drop hammer

97-100

Tubular casing/cast in place pile, enclosed drop hammer

91-97

Tubular casing/cast in place pile, internal drop hammer

84-86

Impact bored/cast in place pile, tripod winch

83-88

H section steel piling, diesel hammer

94-99

H section steel piling, hydraulic drop hammer

96

H section steel piling, enclosed hydraulic hammer

85-88

Precast concrete piles, drop hammer

79-86

Precast concrete piles, hydraulic hammer

91

Precast concrete piles, enclosed hydraulic hammer

78-83

Bored piling/Pile cast in place, auger with powerpack

82-87

Continuous flight auger injected piling

77-80

Diaphragm walling

84-86

Vibro replacement/ Vibro displacement

82-86

Note LAeq is the activity equivalent continuous sound pressure level at 10 m (1 cycle)

As can be seen from the table, the sound levels for driving steel piling where the hammer is enclosed to minimise noise are similar to those for diaphragm wall construction and bored piling. This is because the diesel powerpacks used to power pile boring and auger machines are just as noisy as driven piling plant and it is only when using unenclosed impact driving that the noise is greater. It is important to be aware that technology has also improved since 1992 and reduced noise in steel piling operations. Now vibro-drivers and hydraulic impact hammers are commonly used or piles are jacked into the ground. The relatively more noisy diesel and air hammers have now been largely phased out.

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4.1.3 Minimising noise hazards when driving steel piles It is a hazard to health when noise levels are excessive. Hazards occur when: •

Noise levels are excessive in adjacent residential areas (the public are exposed)

•

Noise levels are excessive in the works (the construction personnel are exposed).

Prevention and mitigation measures Noise due to pile installation can either be reduced at source (prevention measures) or at nearby buildings (mitigation measures). While it is always preferable to carry out prevention rather than mitigation measures, this may not always be feasible.

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Possible prevention measures include: •

Introduction of a non-metallic dolly between hammer and driving helmet

•

Enclosing the driving system in an acoustic shroud

•

Using a low noise driving hammer

•

Enclosing the hammer and the complete length of pile being driven within an acoustic enclosure. This method may be appropriate on very sensitive sites where a drop hammer is essential to achieve design penetration.

Possible mitigation measures include: •

Screening by barriers or hoardings, which is especially effective if used close to either the source of the noise or to the location of interest.

•

Reduction in working hours. It should be recognised that this method is much less effective than screening.

The speed of installation is important as it determines the number of hours that the public is subjected to the noise. Steel piling offers very quick installation and thus short exposure trimes.

4.2

Vibration

Groundborne vibrations caused by piling induce dynamic strains within nearby buildings. These vibrations can occasionally be of sufficient magnitude to lead to building damage but more commonly the problem is disturbance to occupants. The threshold of human perception to vibration is typically two orders of magnitude lower than that for the onset of building damage. Vibration is often quantified in terms of the velocity attained by particles in the ground as they are disturbed from their at-rest position during the passage of the wave. The peak particle velocity (ppv) is the most common parameter used to quantify vibration magnitude. Guidance on the levels of groundborne vibration that may cause damage is given in three British Standards and also in Eurocode 3. These are: •

BS 7385-1, Evaluation and measurement for vibration in buildings. Guide for measurement of vibrations and evaluation of their effects on buildings[24]

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•

BS 7385-2, Evaluation and measurement for vibration in buildings. Guide to damage levels from ground borne vibration[25]

•

BS 5228-4, Noise control on construction and open sites. Code of practice for noise and vibration control applicable to piling operations[18]

•

ENV 1993-5: Design of Steel Structures – Part 5: Piling[26]

BS 7385-1 provides information on the evaluation and measurement of vibrations in buildings. It defines three categories of vibration-induced damage: cosmetic, minor and major. BS 7385-2 presents thresholds for these categories of damage for transient vibrations in the range of frequencies usually encountered in buildings. BS 7385-2 suggests that the damage thresholds specified for transient events may be 50% lower for continuous vibration.

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BS 5228-4 considers vibration from piling, and recommends that a conservative threshold for non-structural damage to residential property should be taken as a peak particle velocity (ppv) of 10 mm/s for intermittent vibrations and a ppv of 5 mm/s for continuous vibrations. Guidance on human response to exposure to vibration in buildings is given in BS 6472[21]. Base curves are presented that define minimum perception levels. BS 6472provides weightings that allow for occupants’ varying tolerance of vibration, dependent upon circumstance and activity, and also considers the different sensitivities to vertical (head to toe) and horizontal (side to side) vibrations.

4.2.1 Types of vibration Vibrations originating from construction sites are of two types: •

Existing ambient vibration levels, in particular at sites adjacent to roads with heavy traffic, railway tracks and industrial factories

•

Vibration due to pile driving.

Vibration due to pile driving may be subdivided into the following categories: •

Continuous vibration produced by a vibrating pile driver

•

Intermittent vibration produced by isolated hammer blows

4.2.2 Estimation of vibration levels During the driving of a steel pile, hammer blows will transmit an impulse from the head to the toe of the pile, the pile will move and thereby transmit free vibrations into the soil medium. Vibro-drivers will create a continuous wave and similarly will transmit vibration into the soil medium. Empirical formulae relating vibration (ppv) with source energy and distance from the source of disturbance are usually expressed in the form:

v

=

C W R

where:

v

is the estimated vertical component peak particle velocity (ppv)

C is a factor that accounts for soil and driving conditions 22

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W is the hammer energy per blow or per cycle

R is the distance from the location of interest to the source of disturbance Values of the factor C, to give v in mm/sec for W in Nm and R in m are given in ENV 1993-5 and these are summarised in Table 4.2. Table 4.2

Recommended values for C (ENV 1993-5)

Driving Method Impact Vibratory

Ground Conditions

C

Very stiff cohesive soils, dense granular media

1.0

Stiff cohesive soils, medium dense granular media

0.75

Soft cohesive soils, loose granular media

0.50

All soil conditions

0.70

Values of the energy per blow W can be obtained from the hammer manufacturer or the contractor proposing the use of the hammer.

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4.2.3 Human response to vibration BS 6472 recognises that humans are known to be very sensitive to vibration and considers the threshold ppv to be in the range of 0.15 mm/s to 0.3 mm/s, at frequencies between 8 Hz to 80 Hz. The human body detects and registers vibration at a far lower level than is required to cause damage to a structure. BS 6472 sets out tables for vibrations in various types of accommodation for vibrations in the range 1 to 80 Hz. The vast majority of piling operations currently in use give rise to vibration energy within this range.

4.2.4 Structural and equipment response to vibration BS 5228-4 states that structural damage to sound buildings or components is not a phenomenon usually attributed to vibration from well controlled piling operations. Proof of actual damage to structures or their finishes from pile driving vibrations has never been documented. However, in some circumstances it is possible that vibrations could be sufficiently intense to cause minor cosmetic damage in the form of cracks to plasterwork and damage to roof tiles. In addition, it is stated that vibration could trigger an incipient failure of some component that hitherto had been in a meta-stable state e.g. frost-shattered brickwork or very old mortar. Table 4.4 summarises the threshold vibration exposure levels for various types of buildings, but these are generalised and not specifically defined according to type of building construction. Table 4.3

Threshold vibration exposure levels above which minor damage of buildings may occur (BS 5228-4)

Building

ppv (mm/s) continuous

ppv (mm/s) intermittent

Ancient / sensitive

2

4

Residential

5

10

Light commercial

10

20

Heavy industrial

15

30

The values in Table 4.3 are conservative as compared to the later BS 7385-2. The research reported in TRL 429 looked at prediction of levels of vibration, carefully calibrated by measurements. Case histories for residential buildings in

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a good state of repair show that no damage has occurred under ppvs of 10-20 mm/sec and this points to a need for the Codes to be revised.

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The assessment of the vulnerability of buildings to vibration due to piling should be carried out on an individual basis for each adjacent building under consideration. The following general points for consideration are identified by BS 5228-4: •

The design of the adjacent structure.

•

Foundation and soil conditions.

•

Age of the structure.

•

Method and quality of construction.

•

Existing defects, especially cracks.

•

Information regarding major alterations and repair.

•

Location and level of the structure relative to piling source of vibration.

•

Natural frequencies of the structural elements and components.

•

Duration of piling operations.

The assessment of vulnerability is a specialist activity best carried out by noise and vibration experts who have experience of vibration work and of previous measurements. Otherwise, it is very easy to specify an unrealistically low ppv limit that is not justified.

4.2.5 Vibration levels during steel pile driving A hazard to health may be claimed when vibration levels are excessive. Hazards occur when: •

Vibration levels exceed the threshold for humans.

•

Vibration levels exceed the threshold for equipment and/or processes in laboratory and other industrial facilities.

•

Vibration levels exceed the threshold that might lead to minor damage to wall finishes and/or components in buildings and/or other structures.

•

Vibration levels exceed the threshold levels that might trigger some component failures in nearby buildings.

•

Vibration levels lead to settlement in loose, wet and cohesionless soils.

4.2.6 Prevention and mitigation of vibration Vibration due to pile installation is best reduced at source (prevention measures). Possible prevention measures include: •

Use of alternative installation methods.

•

Good driving practice, including controlling frequency and amplitude of the vibrodriver.

•

Removal of below-ground obstructions to pile driving.

•

Reduction of resistance to penetration by pre-treatment of ground, e.g. pre-drilling; pre-augering or water jetting whilst driving.

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Possible mitigation measures include: •

Reduction of energy per unit blow (or cycle).

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Monitoring of ground vibrations is now a routine service provided by specialist companies. Vibratory hammers can be controlled by reference to vibration measurement with installed geophones so as to avoid critical ppvs that have been set to protect adjacent buildings. A case history from a site at Walton, near Preston, Manchester is presented in Figures 4.1 to 4.3 below.

Figure 4.1

Vibratory hammer working in close proximity to housing at Walton, Preston

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P308: Specifiers' Guide to Steel Piling

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Discuss me ...

Figure 4.2

Site plan showing proximity of sheet pile wall to adjacent houses at Walton, Morecambe

Figure 4.3

Measured ppv ground vibrations from geophones adjacent to houses showing effect of varying vibrodriver frequency

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5

DURABILITY

The rate of corrosion of steel piling and the consequent expected life, depends upon the environment that is present around the piles during service. For most environments, reliable corrosion rates are now available and the expected life of steel piling can be predicted confidently. A big advantage for steel is that it is not affected by sulphates and chlorides in the ground. The subject of corrosion and steel protection has received substantial attention both in the UK and abroad over the last 30 years. There is now adequate knowledge on the subjects of corrosion rates, coatings selection and specifications to permit the designer to make a reasoned judgement on the provision for corrosion protection on steel piling in natural environments.

5.1

Corrosion rates and expected lives in natural environments

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Corrosion rates for steel piling are given in the following documents: •

BS 8002 Code of practice for earth retaining structures[16].

•

BS 8004 Code of practice for foundations[27].

•

BS 6349-1 Maritime structures code of practice for general criteria[28].

•

Piling handbook, 7th Edition[1].

•

Corus brochure on Durability and protection of steel piling in temperate climates[29]

Apart from minor variations, probably due to the rounding of numbers to suit the specific requirements of a particular document, the information given in all these publications is in agreement and can be adopted for steel construction in the environments listed. Table 5.1 presents the most up to date information (taken from the Corus brochure relating to corrosion rates of unprotected carbon steel in typical environments). Reduced (corroded) section properties can either be obtained by calculation or from Corus in the form of data presented in the Piling handbook and the brochure on Durability or from the website www.corusconstruction.com. For steel piling, the mean values of corrosion rates measured on research test sites are considered to be the most relevant to the design and performance in most environments. These are the values quoted in the table. Where there is concern regarding the potential for greater corrosion rates occurring, the steel manufacturer should be consulted for advice. Further research on corrosion rates in contaminated land and brownfield sites is continuing by the BRE under a joint industry/Dti funded project (see Section 5.3)

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Table 5.1

Corrosion rates for unprotected carbon steel in various environments Mean corrosion rate (Total for both sides) (mm/year)

Side 1

Side 2

Atmosphere

Atmosphere

0.07

Atmosphere

Soil

0.05

Soil

Soil

0.03 max

Splash zone

Soil

0.09

Tidal zone

Soil

0.05

Low water zone

Soil

0.09

Immersion zone

Soil

0.05

Splash zone

Splash zone

0.15

Tidal zone

Tidal zone

0.07

Low water zone

Low water zone

0.15

Immersion zone

Immersion zone

0.07

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5.1.2 Effective section The effective section of a corroded steel pile section can be predicted by determining the corrosion loss at a given time as the product of the corrosion rate from Table 5.1 and the duration of exposure. Effective section properties can then be calculated by deducting the loss from the gross section. As the cross sectional area and section modulus reduces per year, the stress in the steel under the design loading will increase. The time to reach a limiting condition (Typically, the yield stress of the steel) will determine the expected life of that section.

5.1.3 Methods for increasing effective life Methods for increasing the effective life for steel piling in natural environments are presented and discussed in Section 4.4.4.4.3.5 of BS 8002[16] and Section 3.3 of the Piling Handbook. These methods are listed below, while a detailed discussion may be found in the references. •

Use of a heavier section than required for design stresses.

•

Use of a higher yield steel.

•

Cathodic protection.

5.2

Corrosion rates and effective life in contaminated land

Steel sheet piling may be used in contaminated land for containment barriers and in some cases, for both containment and load bearing applications. In barrier walls, the parameter of interest is the rate of permeability through the sheet pile wall and a prediction to first perforation from corrosion is required. Section 10.3.5 of BS 8004[27] provides a method for determining whether soils are ‘aggressive’ or ‘non-aggressive’, based on three soil parameters: resistivity (ohm–cm), Redox potential at pH=7 and mass of water in the soil. 28

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It is stated that special consideration should be given to cases where aggressive chemicals are present in the environment, as one would expect in contaminated land, although no specific recommendations are made.

5.2.1 Corrosion rates BRE Comparison between concrete and steel piling In general, it is difficult to find information regarding the corrosion rates for piling of any type in contaminated soils. A BRE research project is currently underway and a recent report[30] has compared the effect of corrosion strength performance of steel piles, pre-cast concrete piles and cast in-situ concrete piles in contaminated land. All pile types were found to perform well, and to have low rates of corrosion or degradation. However, when subjected to more aggressive corrosive agents in the laboratory, test results demonstrated the superior performance of steel piles (in the form of a lower rate of corrosion) in comparison to concrete piles. For the most acidic solution (the worse case) the mass of uncoated steel piles dropped by a maximum of 3.4%. On the other hand, for cast in-situ concrete pipes, large reductions in compressive strength were recorded (up to 84%). Table 5.2 summarises the laboratory results for the strong acidic solution.

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Table 5.2

Reduction in strength due to contaminated environment Reduction in mass %

Reduction in compressive strength %

3.4

N/A

Coated steel

0.07

N/A

In-situ concrete

N/A

83

Uncoated steel

The annual rate of corrosion for uncoated steel piles was found to be equal to be 0.262 mm/face per year for the worst case (a highly acidic condition). It was stated in the study report that the use of epoxy pitch coating on steel piles succeeded in reducing the rate of corrosion to an insignificant figure, although values for corrosion rates were not presented. The research is continuing.

5.2.2 Recommendations Results based on the strength parameter so far indicate that the use of steel sheet piling in contaminated land for load bearing applications may be more advantageous than the use of cast or in-situ concrete piles. However, steel sheet piling is rarely used in contaminated land for load bearing applications alone. Indeed, it is more commonly used for containment applications and embedded barrier walls where the wall permeability is the most important parameter.

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6

WATER-RESISTANT SHEET PILE CONSTRUCTION

The ability to prevent passage of water through a sheet pile wall is important in temporary cofferdams to reduce pumping costs but it is really essential for permanent applications such as containment barriers and basement structures. Without precautions, water will leak from the surrounding ground through any joints or cracks in walls and floors into a basement by a combination of flow under pressure and capillary action. Water vapour transfer may also occur from ventilated cavities in internally lined walls. Intrinsically permeable materials such as concrete or masonry will allow seepage of groundwater through the wall construction into the basement throughout its life. Concrete wall construction for basements generally brings the long-term maintenance problem of drainage pumping to remove this leakage water.

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Steel sheet pile walls are intrinsically impermeable and will have no such seepage problems, provided that the clutches are sealed. The use of permanent sheet piles in the basement perimeter wall is therefore increasing, as designers get to know of the advantages. Unfortunately, the available CIRIA publications and British Standards guidance on water-proofing of basements relate only to concrete wall construction and do not cover the most effective solution of incorporating permanent steel sheet piled walls to achieve a water-resistant construction. However, guidance is now given in the recent SCI publication Steel intensive basements[8]. The following guidance is extracted from that publication.

6.1

Prevention of water seepage through sheet pile walls

Steel sheet pile walls comprise a sequence of interconnected sheet pile sections that are joined together at the interlocks. Water seepage can only occur at the interlocks. However, if the gaps in the interlocks are filled, the wall can be made fully water-tight. To provide an adequate measure of water resistance to a sheet pile wall, vertical and horizontal sealing systems have to be applied. Vertical sealing systems are applied to the interlocks, whilst horizontal sealing systems prevent water ingress at the junction between the wall and the base slab. The following Sections provide information and guidance relating to the construction of water-resistant steel sheet pile walls, on the generic products and techniques that can be used, and on the performance that can be expected.

6.1.1 Vertical sealant systems Numerous sheet pile clutch interlock sealing systems are available. include: •

Non-swelling sealants

•

Hydrophilic (water-swelling) sealants 30

These

P308: Specifiers' Guide to Steel Piling Discuss me ...

•

Combination sealant systems

•

Welded interlocks

The applicability of each of the above types of systems is governed by a number of different parameters: •

driving conditions (both the nature of the soils and the method of installation)

•

water pressure requirements

•

permeability requirements

•

durability requirements.

It is important to note that for the systems that rely on a pre-applied sealant, the integrity of vertical joint seals are very much dependent on the driving conditions and the installation techniques used. Advice on particular applications should be sought from piling manufacturers or specialist piling contractors.

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A guide to the relative performance of each of the sealant types is given in Table 6.1. This is, of course, dependent on the effective application of the sealants and appropriate installation techniques. Table 6.1

Relative performance of sealant types

Description

Permeability class1

Applications

Installation technique2 Percussive

Vibro

Silent

Water swelling paste

C

Temporary Works

A

A

A

Hot casting compound

B

Temporary Works

A

G

G

Flexible epoxy resin

A

Permanent or Temporary Works

A

NR

NR

Protected system

B

Permanent or Temporary Works

A

G

G

Site welding

I

Permanent Works

P

P

P

1. Permeability class gives an indication of the achievable levels of permeability the system can achieve. Class I

= Impervious.

Class A = less than 10–8 m/s, Class B = 10–6 to 10–8 m/s, Class C = greater than 10–6 m/s. The level of permeability achieved will depend upon the soil conditions, pile section, water head and the quality of the installation. 2. Installation technique refers to the most suitable method of installing the piles containing the sealant. P

= Post installation sealant system,

A

= Acceptable method,

G

= Guidance should be sought regarding the best practice,

NR = Not recommended.

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Non-swelling sealants There are a number of non-swelling sealants available for sealing sheet pile clutches. These include hot-cast bituminous and vegetable oil based compounds and a range of flexible resins (which are epoxy based). Hot-cast compounds become viscous once heated, having a treacle-like consistency, and can be poured into pile interlocks. Flexible epoxy resins have a rubbery consistency and are pumped into the interlocks. Water-swelling products Water-swelling or hydrophilic sealants are urethane polymer products, which once in contact with water, expand to many times their original volume. They generally come in paste form and are brushed into the pile interlocks a number of hours prior to the pile being pitched and driven in order to allow curing to take place. Over a given period of time after contact with water these products swell to effect a seal in the interlocks. It is generally necessary to cure the treated piles over night and consequently after treatment, the piles will need to be covered to ensure that rain or accidental contact with water does not prematurely start the swelling process.

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Application of pre-applied sealants It is recommended that application of pre-applied sealants is made under controlled conditions by approved suppliers. Prior to application, the steel must be dry, clean and free from corrosion; this is usually best achieved under factory conditions immediately prior to delivery to site. If the piles are to be supplied in pairs, the intermediate interlocks can either be welded or treated with any one of the pre-applied sealants before being crimped. Installation of pre-applied sealants When installing piles containing an interlock sealant system, special attention is required, as sealants can cause an increase in the friction in the interlocks during pitching and driving. This means that the piles would experience some additional resistance during pitching and in extreme circumstances could cause draw down of the adjacent pile (if not held in place). The heat generated by vibro driving may cause sealants to burn or decompose. Hence if refusal is encountered it is recommended that vibro driving is stopped and the pile is driven to level with an impact hammer. Alternatively, the area of the interlocks can be cooled down with water. To ensure good joint integrity, it is important to control the alignment of the piles in the horizontal and vertical planes, so that the sealant remains intact as the piles are driven. Any procedure which aids good alignment of the piles is recommended e.g. panel driving and driving in pairs. Welding One way of achieving a water tight seal is to weld the interlocks of SSP walls. The majority of electric arc welding processes are acceptable to achieve a seal to the interlocks of steel sheet piling threaded in the workshop or on-site. Obviously welding in the workshop will be of more reliable quality. If the piles are to be driven individually, each of the interlocks will have to be welded on-site. However, if the soil conditions are such that piles can be driven in pairs or triples, welding can be shared between the workshop and site. Intermediate piles interlocks can be welded in the workshop, whilst the remaining alternate

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pile interlocks can be welded on site once all the piles have been driven and the soil excavated. Various welding procedures can be used, a number of which are described below. There are three distinct disadvantages in using site welding when compared to pre-applied sealants: (i) it is much more time consuming; (ii) on site welding can only seal the exposed portion of the piles above soil level (pre-inserted sealants achieve a seal over the entire length of pile); and (iii) any leakage of groundwater will interfere with the quality of the weld. The quality of welding is dependent on weld surface preparation and how dry the joint is. Also, an appropriate pile section size should be used that is stiff during driving, otherwise pile deviation can take place which can lead to opening of the clutches and more difficult welding conditions. Combination sealant systems

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Combination sealant systems use both mechanical seals and sealing compounds. One such system is the CS4200 protected system (formerly known as Haltlock) from Corus, which has been successfully used on numerous projects. See Figure 6.1. The CS4200 system comprises a standard Corus Larssen or LX sheet pile, factory fitted on one side with a special steel angle section. The steel angle is welded to the clutch of one pile and an impermeable sealant compound is pumped into the remaining void thus filling the pile clutch. When an interlocking pile is introduced, the driving action causes this angle to displace and effectively ‘bite’ the incoming flange whilst, at the same time causing the sealant to diffuse throughout the joint. Excavated side

Void filled with sealant prior to installation

Larssen or LX12 Paired and welded together

Earth side

On interlocking, the sealant is squeezed thoughout the joint

Figure 6.1

Driving direction

Angle bears directly onto adjacent pile when installed

The Corus CS4200 system showing the welded angle and the bituminous sealant

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6.2

Sheet pile wall to reinforced concrete slab connections

Sealing to form a water-tight connection between two types of dissimilar construction materials is often required at the interface between a steel sheet pile wall and the reinforced concrete base floor slab. Both ‘passive’ and ‘active’ waterproofing systems are available, with the former gaining more market share owing to its distinct advantages.

Steel pile

Concrete base slab

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Concrete protection Sealing membrane fixed: either mechanically or with adhesive Steel sheet

Figure 6.2

Concrete subbase

Passive waterproofing system

Passive waterproofing system To provide an adequate barrier to water ingress through the reinforced concrete slab, a membrane sheet is laid on top of the concrete blinding sub-base. See Figure 6.2. This membrane is fixed mechanically (or alternatively with adhesive) to a steel plate (‘puddle’ plate), at the elevation of the proposed membrane. The ‘puddle’ plate is cut to the profile of the sheet pile and welded to the sheet pile, hence the membrane can be terminated at the edge of the plate to provide an effective waterproof seal. The steel plate itself increases the length of the potential water seepage path significantly at the steel-concrete interface. To prevent any water seepage through any potentially poorly welded connection, a hydrophilic sealant can be applied to the top of the steel plate at the sheet pile-steel plate interface. A concrete protective layer is applied to protect the membrane during subsequent work. Active waterproofing system Due to recent technological advances in waterproofing products, active systems have greatly improved the water tightness of steel to concrete connections and are becoming the preferred option. Unlike the conventional passive method shown in the Figure 6.2, an active system includes a tube system which allows injection of a sealant if leaks are detected once the base has been concreted and has cured. See Figure 6.3.

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The injection system provides a conduit to the inside faces of the connection without drilling or damage to the concrete itself. This conduit allows the watertight integrity of the connection to be proven by pressurised water injection and provides for the sealing of any identified leaks. The penetrating resin is injected under pressure to seal the joint and any emanating cracks, to prevent water ingress and tracking along the joint. The integrity of the joint is proven once pressure can be maintained. The injection resin cures to form an elastic non-shrink filler for lifetime protection.

Junction box Shear studs site welded to sheet piling

1

Drainage channel U bar site welded to sheet piling

2

Bending and tension bars site welded to sheet piling

3 4

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5 6

75 mm concrete blinding

7

Polythene slip membrane and grout check Low permeability granular material

1 10 mm wide x 10 mm deep chase formed in top of slab with pourable sealant 2 100 mm wide adhesive waterproofing tape membrane with permanent mechanical bond to concrete, bonded to sheet pile at construction joint location 3 Hose injection waterproofing system clipped to face of sheet piles 4 20 mm x 5 mm hydrophilic waterstop bonded to sheet piling 5 Double sided self adhesive rubber/bitumen waterproofing membrane securely bonded to sheet pile, water bar and hydrophilic strip 6 P.V.C. waterbar returned 125 mm 7 P.V.C. waterbar with co-extruded hydrophylic elements at construction joints

Figure 6.3

Waterproofing with an active injection system

Sealing of concrete slabs In parallel with the advances made in injection systems, advances have also been made in the waterproofing of the concrete slab. High density polyethylene (HDPE) sheeting is now available which physically isolates the slab structure from the surrounding ground. The sheet comprises three layers: the HDPE layer which prevents the ingress of water, vapour and gas; a layer of special conformable adhesive; and a white protective coating. These layers work together to form a microscopic integral seal to wet concrete poured against it, hence it is unaffected by settlement of the substrate. This permanent bond prevents migration of water between the membrane and the concrete, 35

P308: Specifiers' Guide to Steel Piling Discuss me ...

eliminating a common cause of leaks. Importantly, it does not require protection and can be trafficked immediately after laying. As it is durable and easy to handle, liquid membrane products can be used to seal lap joints and seal the sheet membrane to the steel sheet pile. As there is no requirement for the ‘puddle’ plate (as shown in Figure 6.2), the costly and time consuming welding operation can be omitted. In basements where a controlled amount of water seepage can be tolerated (for example in an underground car park), the waterproof membrane may be omitted.

6.3

Permeability rates in contaminated environments

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A methodology has been developed by Delft Geotechnics for the consistent estimation of the seepage that takes place through unsealed sheet pile interlocks. This is given in BS EN 12063 Execution of special geotechnical work – Sheet pile walls [31]. The results that are obtained using this method should be used with caution as it does not take into account the effects of frequency and tightness of the clutches, the driving tolerance or clutch geometry. To obtain permeability rates with some degree of confidence, calibration of the model should be undertaken using specific sheet pile products installed under controlled site conditions.

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7

FIRE SAFETY

Fire Safety needs to be carefully considered where permanent sheet pile retaining walls are proposed to be used in basements. This is covered in detail in the SCI Publication Steel intensive basements[8]. The term fire resistance is used to characterise the performance of a structural component in fire, where fire resistance is defined in BS 476-20[32] as ‘the ability of an element of building construction to withstand exposure to a standard temperature/time and pressure regime without loss of its fire separating function or load bearing function or both for a given time’. The life safety requirement of The Building Regulations must be complied with and this may be achieved by satisfying one of the following: Approved Document B provisions Local regulations e.g. Local Building Acts (Amendment) Act 139

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Meeting defined objectives set out as part of a Fire Safety strategy Where collapse of a sheet pile retaining wall in a basement in fire will not result in additional structural collapse nor allow the passage of hot gases into other compartments, it would be reasonable to assume that element of the structure does not require fire resistance. Where the collapse of the steel sheet pile retaining wall in a basement will result in additional structural collapse and thus create a risk to life and to the integrity of the building’s compartmentation, its fire resistance should be considered. This situation will probably occur where the sheet piling has a function other than simple earthwork retention, e.g. it also has to take vertical load from the structure above.

7.1

Fire engineering solutions: structural fire design codes

Fire safety engineering can provide an alternative approach to fire safety. (See Section 12.4). It can be seen as an integrated package of measures designed to achieve the maximum benefit from the available methods for preventing controlling or limiting the consequences of fire. In terms of structural stability, fire safety engineering is aimed at adopting a rational scientific approach which ensures that fire resistance/protection is provided where it is needed and expense is not incurred needlessly to provide an ‘illusion’ of safety. Fire safety engineering is most effective where it can be demonstrated that the formal recommendations of Approved Document B or the requirements of the London District Surveyors Association Fire Safety Guide, No.1: Fire Safety in Section 20 Buildings, are not appropriate to the risk in the basement and can be reduced. A fire safety engineering assessment should, wherever possible, be carried out alongside the initial design. The development of fire engineering solutions will be supported in 2002 by the publication of BS 7974, the new British Standard for fire safety engineering.

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7.2

Protecting structural steel

Various generic and proprietary fire protection systems are used to protect structural steelwork. Manufacturers and/or specialist contractors offer comprehensive information on characteristics of materials, test results, advice about suitability for particular applications and installation procedures. A list of manufacturers and installers is available from the UK trade association, The Association for Specialist Fire Protection (ASFP). However, most but not all manufacturers and applicators in the UK are members of the ASFP and this list is therefore not exhaustive. The thickness of fire protection material required to satisfy a specific fire resistance period can be selected from authoritative material performance data sheets published by the manufacturers or from the 'Yellow Book'[33]. This is very much out of date now, although it does still contain some good information on the theory behind fire protection. The new third edition is currently being drafted and will be published during 2002.

7.2.1 Generic fire protection materials

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Guidance on the fire performance and use of generic materials such as concrete etc., is presented in the Building Research Establishment publication Guidelines for the construction of fire resisting structural elements[34] and in Appendix 2 of the ‘Yellow Book’.

7.2.2 Proprietary Fire Protection Materials The most common types of fire protection systems available can be classed into three main product groups •

boards and blankets

•

sprayed coatings

•

intumescent coatings

Boards, blankets and sprays are non-reactive whilst intumescent coatings are reactive. Most proprietary fire protection materials are designed for use in dry internal environments, i.e. Category C1 environments as defined in ISO 12944 Part 2. For other environments, for example underground car parks, advice should be taken from the fire protection manufacturers as to their product's suitability. Boards and blankets Blankets, semi-rigid and rigid boards are used as dry forms of fire protection installed in situ as either profile or boxed protection. Base materials include ceramic fibres, calcium silicate, rock fibre, gypsum and vermiculite. The principal advantages of using boards are: •

Rigid boards offer a clean, boxed appearance which may be pre-finished or suitable for further decoration.

•

Application is dry and usually does not have a significant effect on other trades.

•

Boards are factory manufactured hence thickness can be guaranteed.

•

Boards can be applied on unpainted steelwork. 38

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However, fitting around complex details may be difficult and board systems may be slower to apply than other methods. Blanket or flexible fire protection systems have been developed as a response to the need for a cheap alternative to sprays but without the adverse effects on construction program often associated with wet application. Board systems are the most popular type of fire protection in the UK. They are widely used where the protection system is in full view. Specifiers should be aware that board systems that can provide an aesthetic finish will be more expensive than purely functional systems. Up to 240 minutes fire resistance can be provided. Sprayed coatings

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Sprayed coatings are cement or gypsum based materials containing mineral fibre, expanded vermiculite, expanded perlite and/or other lightweight aggregates or fillers are also commonly used. The principal advantages of sprayed coatings are: •

Spray protection can usually be applied for less than the cost of the cheapest board. As the cost of spray material is low compared to that of getting labour and equipment on site, costs do not increase in proportion to resistance times.

•

It is easy to cover complex details.

•

Some materials may be used in external or corrosive environments.

•

Some materials may be applied on unpainted steelwork.

However, sprays are not visually appealing and, as it is a wet trade, significant knock-on effects on the construction program can result in an increase in the overall cost construction or prolonged construction duration. Difficulty may also be encountered when reinstating the protection if it is necessary after installation of services. Sprays are generally the least expensive form of fire protection. materials can provide up to 240 minutes fire resistance.

These

Intumescent coatings Intumescent coatings can be classified as either ‘thin film’ or ‘thick film’ (mastics). Thin film intumescents account for the majority of systems used in general construction, whilst thick film intumescents are commonly used for the heavy industrial petrochemical and offshore oil and gas industries. Thin film intumescent coating systems are similar in appearance to conventional paints and are applied either by airless spray, brush or roller, and either on-site or off-site. General guidance on the selection and use of intumescent coatings can be found in BS 8202–2[35], whilst guidance on the use of offsite application of intumescent coatings is given in SCI publication Structural fire design: Off-site applied thin film intumescent coatings[36].

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The principal advantages of thin film intumescent coatings are: •

The shape of the underlying steel can be expressed

•

Attractive and decorative finishes are possible

•

Complex details are easily covered

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However, typical application costs are higher than sprays, it is a wet trade and most intumescent coatings are economic only up to 60 minutes fire resistance. It is possible to achieve up to 120 minutes fire resistance but this is likely to be an expensive alternative. Coatings that can economically achieve more than 60 minutes fire resistance are under development and may soon be available. Potential problems can occur in corrosive environments (i.e. anything other than Category C1 in ISO 12944) and a maintenance programme and guarantees may be required.

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8

STRUCTURAL DESIGN

The structural design of sheet and bearing piles should make reference to the most up to date guidance that is available. Reference is recommended to the ICE Specification for Piling and Embedded Retaining Walls, 1997, to the SCI Steel bearing pile guide[8] 1997, and to the Piling handbook[1]. In many cases the minimum steel pile section size is governed by driveability requirements rather than that required for resisting static loads.

8.1

Steel pile sections

Reference to up to date piling section data is essential. A listing of currently manufactured steel pile sections with dimensions and properties can be found in Corus literature and on their website www.corusgroup.com.

8.2

Design soil parameters

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For steel bearing piles, guidance on he selection of soil parameters from soils data is given in the SCI Steel bearing piles guide. For sheet pile retaining wall design, guidance on the selection of soil parameters from soils data at the site is given in BS 8002[16] and in CIRIA Report 104[37]. Unfortunately, the nomenclature differs between these two documents but this is clarified by the new CIRIA Report 629[38].

8.3

Effect of corrosion on section

The reduction in section thickness due to corrosion during the design life of the structure has been the subject of much research in recent decades. The accepted rates of corrosion are to be found in the BS 8002, BS 6349[28] and in the Piling handbook and this has been summarised in Section 5.1. For sheet pile retaining walls, care should be taken to use the section properties that are relevant at the location of the maximum bending moment as it is possible that the corrosion rates may differ along the length of the wall. Reduced section properties (section modulus, etc.) allowing for corrosion can also be obtained from the Piling handbook. The reduced section properties and stiffness lead also to a further redistribution of earth pressures and a reduction in maximum bending moment.

8.4

Design of retaining walls

Guidance on the design of retaining walls is given in BS 8002, CIRIA 104[37] and in the new CIRIA Report 629[38]. Most of the methods presented are simple enough to permit hand calculation but for modern economic design, it is common to use soil-structure interaction software programmes such as WALLAP or FREW. These have the advantage that they already incorporate steel sheet pile serviceability design by enabling wall deflection to be calculated. This is essential so as to permit the ability of steel piling to flex and redistribute earth pressures, thereby lowering bending moments induced in the wall.

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Steel sheet pile walls can accommodate without overstress a much greater horizontal deflection than concrete walls. As the steel wall is significantly more flexible than the concrete wall, the interaction between the wall and the soil is very different. The inherent ductility of the steel wall causes a redistribution of earth pressure within the soil, which often permits a smaller wall section to be used. Soil-structure interaction software programmes are used to calculate the redistributed earth pressures for steel sheet pile walls.

8.5

Design for vertical loads

8.5.1 Vertical load resistance of sheet pile walls Where vertical loads act directly on the retaining wall or where inclined props or ground anchors are present, it is necessary to check that the depth of embedment of the retaining wall has adequate capacity to resist the vertical loads with the required factor of safety.

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To allow for uncertainties in the transfer of vertical load in skin friction along the surface of the retaining wall, it is recommended that transfer of vertical load in skin friction is restricted to the buried depth on the excavated side of the wall only (refer to SCI Steel bearing piles guide). Where significant resistance to vertical load from end bearing can be used e.g. when driving into buried rock there should be no need to use any frictional resistance.

8.5.2 Vertical load resistance of bearing piles The vertical load bearing resistance for driven steel piling is not covered correctly in BS 8004. Research carried out in recent decades for the offshore piling industry has led to a greater understanding of both frictional and end-bearing resistances in soils and rocks. This is covered in the Steel bearing piles guide. In particular, much higher end-bearing values are developed when driving steel piles into bedrock than are available to concrete piles (ref. Norwegian Geotechnical Institute research). Attention to this will prevent over design of steel piling and permit a more economic solution with much fewer piles for a given foundation load. Where the soil is cohesive and the steel pile is resisting the load predominantly by shaft friction, the resistance capacity offered is proportional to surface area. A steel H-pile of size D × D has a surface area per metre length of pile of approximately 6 × D whilst a concrete bored pile of diameter D only has a surface area per metre length of pile of B × D. It is known that the unit soil friction is the same for both steel and concrete piles in clay and hence the steel bearing pile of similar external dimension has approximately twice the frictional shaft capacity of a concrete pile. Where the soil is granular, the shaft resistance is enhanced for a steel pile because the act of driving increases the frictional resistance. Calibrated methods for estimating this pile load resistance are contained in the Steel bearing piles guide. In addition, the end-bearing resistance generated by a driven steel pile is much higher in granular soils than can be mobilised by a concrete bored pile (see Steel bearing piles guide).

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8.6

Connection of piles to superstructures

Either bearing piles or sheet piling can be used to support a superstructure. For a multi-storey building where no basement is required, steel bearing piles with reinforced concrete pile caps can be used to support the columns of the building. For a building with a basement, steel sheet piling is an attractive alternative for permanent use to form the external walls of the basement and steel bearing piles can be used for the foundations to the internal columns. In each case, reinforced concrete pile caps for bearing piles and pile capping beams for sheet pile walls are typically used to make the structural connection. For each situation, the most appropriate connection detail should be developed; these connections can be fully moment-resisting or just simple non-moment resisting connections.

8.6.1 Bearing piles

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Where steel bearing piles are being considered, the piling configuration (size, number and spacing of the piles in a group) should utilise their full capability. Economy in material and cost will not be achieved if steel piles are just substituted for a piling layout initially designed for concrete bored piles. Considering steel bearing piles at the onset will make it possible to reduce the number of piles, because the support capacity of a steel pile is much greater than that of a similar sized concrete bearing pile. Reducing the number of piles using steel will therefore also enable the size of the pile cap to be reduced. A range of methods for connection of steel bearing piles to superstructures have been established through construction experience and some are illustrated in BS 6349[28] for marine structures. Transfer of vertical load is either through bearing (onto a capping plate that is welded after driving) or, for thin-walled tubular piles, through shear connectors around the perimeter of the pile head. Either method is well able to transmit moment as well as vertical load.

8.6.2 Sheet piling Connections between steel sheet pile walls and a building superstructure can easily be achieved through a reinforced concrete capping beam. Shear connectors may be needed to transfer the vertical load to the pile but these can be accommodated within the width of the capping beam. Moment transfer is likely to be relatively small, because the pile wall is flexible. In addition to providing adequate and safe details to resist the forces acting on the connections from the superstructure, it is necessary to satisfy all other requirements. This may include making the steel-concrete connection resistant to water penetration, for example at a ground floor slab or base slab connection in basements and tunnels. In the case of a basement structure it is important to consider not only forces created by the soil retaining action of the wall but also those resulting from uplift due to ground movements and ground water. Typical connections for ground floor slabs and base slabs where water penetration resistance is required are presented in Section 6.2.

8.6.3 Connections for integral bridges Different types of connection between steel piles and composite deck superstructures are shown in the three SCI publications on the subject of integral bridges[5] [6] [7]. Those details demonstrate full moment connection as well as transfer of vertical loads, such as shown in Figure 8.1. 43

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8 No. T32 at 200 crs. 7 Rows 2 No. Studs/Flange at 200 crs

150 150 7 Rows 2 No. Studs/Flange at 200 crs

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T40 at 150 crs

Asphaltic plug joint

Concrete deck

Deck beam

Reinforced concrete pile capping beam/wall

Construction joint Note: Similar arrangement for reinforced concrete deck beam

High Modulus Pile Sheet pile

Figure 8.1

UB

Integral bridge abutment High Modulus Pile moment connection to an r.c. capping beam

Some practical means of providing composite connections to tubular steel piles are shown in Figures 8.2 and 8.3. Further details are available in the SCI publication Integral steel bridges: Design of a multi-span bridge - Worked Example[7]. 44

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Asphaltic plug joint

Road construction

Deck beam

Construction joint

Concrete cross head beam /endscreen wall Select granular fill

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Tubular pile Embankment fill

Figure 8.2

External shear stud composite connections for tubular piles to integral bridge abutment cross head beam

Deck beam/ structural beam

Waterproof membrane Holding-down bolts set in grouted pockets Studs for load transfer

Concrete infill Steel tubular column

Figure 8.3

Internal shear stud or weld bead load transfer connection to tubular column-pile support

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REFERENCES

1

BRITISH STEEL, SECTIONS PLATES & COMMERCIAL STEELS Piling handbook British Steel, SP&CS, 1997

2

BIDDLE, A.R. Steel bearing piles guide (P156) The Steel Construction Institute, 1997.

3

YANDZIO, E. Design Guide for steel sheet pile bridge abutments (P187) The Steel Construction Institute, 1998

4

GORGOLEWSKI, M. Environmental assessment of steel piling (P199) The Steel Construction Institute, 1999

5

BIDDLE, A.R., ILES, D.C. and YANDZIO, E. Integral steel bridges: design guidance (P163) The Steel Construction Institute, 1997

6

WAY, J.A. and YANDZIO, E. Integral steel bridges: design of a single span bridge – Worked example (P180) The Steel Construction Institute, 1997

7

WAY, J.A. and BIDDLE, A.R. Integral steel bridges: design of a multi-span bridge – Worked example (P250) The Steel Construction Institute, 1998

8

YANDZIO, E. and BIDDLE, A.R. Steel intensive basements (P275) The Steel Construction Institute, 2001

9

HOBBS, N.B. and HEALY, P.R. Piling in chalk CIRIA report PG6, 1979

10

LORD, J.A., TWINE, D. and YEOW, H. Foundations in chalk CIRIA Project Report 11, 1994

11

WESTCOTT, F.J., LEAN, C.M.B. and CUNNINGHAM, M.L. Piling and penetrative ground improvement methods on land affected by contamination: Interim guidance on pollution prevention Environment Agency, National Groundwater & Contaminated Land Centre, 2000

12

TOMLINSON, M.J. Adhesion of piles in stiff clays CIRIA Report No.26 Construction Industry Research and Information Association, 1970.

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P308: Specifiers' Guide to Steel Piling

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FEDERATION OF PILING SPECIALISTS Specification for steel sheet piling FPS, London, 1991

14

BRITISH STANDARDS INSTITUTION BS EN 12699: 2001 Execution of special geotechnical work. piles

Displacement

15

TECHNICAL EUROPEAN SHEET PILING ASSOCIATION Installation of steel sheet piles TESPA, L-2930 Luxembourg, 1993

16

BRITISH STANDARD INSTITUTION BS 8002: 1994 Code of practice for earth retaining structures

17

INSTITUTION OF CIVIL ENGINEERS Specification for piling and embedded retaining walls ICE, 1996

18

BRITISH STANDARDS INSTITUTION BS 5228-4: 1992 Noise and vibration control on construction and open sites. Code of practice for noise and vibration control applicable to piling operations

19

TRANSPORT RESEARCH LABORATORY Groundborne vibration caused by mechanised construction works TRL Report 429, Crowthorne, 2000

20

BRITISH STEEL Control of vibration and noise during piling, 1997

21

BRITISH STANDARDS INSTITUTION BS 6472: 1992 Guide to evaluation of human exposure to vibration in buildings (1 Hz to 80 Hz)

22

CONSTRUCTION INDUSTRY RESEARCH AND INFORMATION ASSOCIATION Noise from construction and demolition sites - Measured levels and their prediction CIRIA Report 64, London, 1977

23

WELTMAN, A.J. Noise and vibrations from piling operations DOE and CIRIA Piling Development Group Construction Industry Research and Information Association Report PG9, 1980

24

BRITISH STANDARDS INSTITUTION BS 7385-1: 1990 Evaluation and measurement for vibration in buildings. Guide for measurement of vibrations and evaluation of their effects on buildings

25

BRITISH STANDARDS INSTITUTION BS 7385-2: 1993 Evaluation and measurement for vibration in buildings. Guide to damage levels from groundborne vibration

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P308: Specifiers' Guide to Steel Piling

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European Prestandard ENV 1993-5 Eurocode 3: Design of Steel structures – Part 5: Piling CEN (European Committee for Standardization), Jan 1998

27

BRITISH STANDARDS INSTITUTION BS 8004: 1986 Code of practice for foundations

28

BRITISH STANDARDS INSTITUTION BS 6349: Marine structures BS 6348-1:2000 Code of practice for general criteria BS 6349-2:1988 Design of quay walls, jetties and dolphins

29

CORUS CONSTRUCTION AND INDUSTRIAL Durability and protection of steel piling in temperate climates Publication 202, 2001

30

WILSON J. Building materials in contaminated land: The Shotton field trial DETR Environment, Transport, Regions, May 2000

31

BRITISH STANDARDS INSTITUTION BS EN 12063: 1999 Execution of special geotechnical work. Sheet pile walls

32

BRITISH STANDARDS INSTITUTION BS 476-20:1987: fire tests on building materials and structures. Method for determination of the fire resistance of elements of construction (general principles).

33

ASSOCIATION OF SPECIALIST FIRE PROTECTION CONTRACTORS AND MANUFACTURERS LTD (ASFPCM, THE STEEL CONSTRUCTION INSTITUTE AND THE FIRE TEST STUDY GROUP Fire protection for structural steel in buildings, Revised 2nd edition ASFPCM/SCI/FTSG, 1992

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MORRIS, W.A. Guidelines for the construction of fire resisting structural elements Building Research Establishment 1988

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BRITISH STANDARDS INSTITUTE BS 8202-2:1992 Coatings for fire protection of building elements. Code of practice for the use of intumescent coating systems to metallic substrates for providing fire resistance.

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YANDZIO, E., DOWLING, J. and NEWMAN, G. Structural fire design: Off-site applied thin film intumescent coatings. Part 1: Design guidance Part 2: Model specification The Steel Construction Institute In association with British Steel Plc, Association of Specialist Fire Protection Contractors and Manufacturers Ltd, The British Constructional Steelwork Association Ltd SCI, 1996

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P308: Specifiers' Guide to Steel Piling

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CONSTRUCTION INDUSTRY RESEARCH AND INFORMATION ASSOCIATION Design of retaining walls embedded in stiff clay CIRIA Report 104, London, 1984

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CONSTRUCTION INDUSTRY RESEARCH AND INFORMATION ASSOCIATION Embedded retaining walls: guidance for economic design CIRIA Report 629, London, to be published in 2002

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