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SCI PUBLICATION P335

H-Pile Design Guide

A R BIDDLE BSc, CEng, MICE

Published by: The Steel Construction Institute Silwood Park Ascot Berkshire SL5 7QN Tel: 01344 623345 Fax: 01344 622944

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2005 The Steel Construction Institute

Apart from any fair dealing for the purposes of research or private study or criticism or review, as permitted under theCopyright Designs and Patents Act, 1988, this publication may not be reproduced, stored or transmitted, in any form or byany means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only inaccordance with the terms of the licences issued by the UK Copyright Licensing Agency, or in accordance with the termsof 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 ConstructionInstitute, 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 areaccurate to the extent that they relate to either matters of fact or accepted practice or matters of opinion at the time ofpublication, The Steel Construction Institute, the authors and the reviewers assume no responsibility for any errors in ormisinterpretations 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 P335

ISBN 1 84942 164 4

British Library Cataloguing-in-Publication Data.

A catalogue record for this book is available from the British Library.

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FOREWORD

Current practice is to place the responsibility for pile design on the designer (whereas before it was often on the contractor), and there is therefore need for him to become more familiar with the behaviour and advantages of steel piles.

It is hoped that this design guide will provide the necessary confidence for practising engineers to use steel H-piling more extensively and become more innovative in the use of steel piling in structural and building foundations. It also covers the use of UC sections for plunge columns.

The guide is laid out in Sections which follow the steps involved in a well established design procedure. A new Section 9, Technical and Cost Benefits is based on case studies to demonstrate the practical benefits of using H-piles on various projects.

The SCI database of axial load tests on steel H-piles that was established in 1997 for Steel Bearing Piles Guide, has been used again to validate load capacity prediction methods together with more recent test data and case studies.

Partnerships were formed with SCI members who have provided soils data and load test results from steel H-pile tests on their construction sites. Major partners were: Pell Frischmann Group; Volker Stevin Ltd; Stent Foundations Ltd; Testing and Analysis Ltd. Their assistance and time is gratefully acknowledged. Particular thanks is also expressed to the members of the Steel Piling Group who reviewed and contributed to the draft documents or contributed information and photographs for the case studies:

Andrew Bond Geocentrix Ltd

Robin Dawson Dawson Construction Plant Ltd Marwan Ghannam Corus Construction & Industrial Simon Griffiths Pell Frischmann Group Mike Kightley Testing and Analysis Ltd Steven Lee Volker Stevin Ltd

Norman Mure Stent Foundations Ltd Ron Mure Stent Foundations Ltd John Powell BRE Colin Souch Pell Frischmann Group David Thompson Dew Group Piling Ltd

David Twine Ove Arup Geotechnics John Vincett Tony Gee & Partners Mike Webb Corus Construction & Industrial Cliff Wren Stent Foundations Ltd Grateful thanks is owed to Corus Construction & Industrial who funded the preparation of this publication.

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Contents Page No.

FOREWORD iii

SUMMARY vii

1 INTRODUCTION 1 1.1 Piled foundation choice 1 1.2 Why choose steel piling? 2 1.3 Scope of this publication 6

2 DESIGN BASIS 7 2.1 General 7 2.2 Design standards 7 2.3 Limit state design rules 8 2.4 Bearing pile structural design 9 2.5 Design methodology 10

3 GEOTECHNICAL DESIGN 13 3.1 Terminology 13 3.2 Design premise 13 3.3 Limit State Design 14 3.4 Geotechnical design methods 19 3.5 Soil resistance on driven steel piles 21 3.6 Load / settlement behaviour friction piles 21 3.7 Pile-soil load transfer friction piles 25 3.8 Load / settlement behaviour end-bearing piles 26 3.9 Pile-soil load transfer end bearing piles 26 3.10 Site investigation 27

4 SELECTION OF SECTION 29 4.1 Steel piles in bearing only 29 4.2 Design method examples 29 4.3 Selection of steel section 30 4.4 H-Piles 30 4.5 Plunge Columns 32

5 AXIAL LOAD RESISTANCE 36 5.1 Interpretation of soil parameters 36 5.2 Predictive methods general 36 5.3 Pile axial movement models 38 5.4 Axial resistance in non-cohesive, granular soils 41 5.5 Axial resistance in cohesive soils 46 5.6 Axial resistance in rock 48 5.7 Negative shaft friction 52 5.8 Measures to increase steel pile axial capacity 52

6 LATERAL LOAD RESISTANCE 54 6.1 Introduction 54 6.2 Methods of analysis 55 6.3 Assessment of soil properties 57 6.4 Combined lateral and vertical loading 58

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7 PILE GROUP EFFECTS 60 7.1 Conceptual design axial load resistance 60 7.2 Methods of lateral load resistance analysis 61 7.3 Practical pile group design 62

8 THE INSTALLATION AND TESTING OF STEEL BEARING PILES 65 8.1 Pile driving installation methods 66 8.2 Offshore experience of pile driving analysis 68 8.3 Driving formulae and dynamic driving resistance 69 8.4 Pile load testing 75 8.5 Steel pile installation tolerances 80 8.6 Environmental factors with driven piles 81

9 TECHNICAL AND COST BENEFITS 84 9.1 Steel pile economics 84 9.2 Soil conditions 84 9.3 Design configuration 85 9.4 Case Studies 86 9.5 Cost comparisons 92

10 STEEL PILES/STRUCTURE CONNECTIONS 94

11 CORROSION AND PROTECTION OF STEEL PILES 97 11.1 The need for corrosion protection 97 11.2 Standard corrosion allowances 98 11.3 Corrosion in soil 99 11.4 Corrosion in fills and brownfield sites 100 11.5 Atmospheric corrosion 101 11.6 Corrosion below water 101 11.7 Methods of increasing effective life 101

REFERENCES 103

APPENDIX A CONTACTS 113

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SUMMARY

This publication gives guidance on the selection, design and installation of steel H-piles and UC section plunge columns for foundations to all types of structure. Current practice and experience in this field are presented, discussed and recommendations given.

The characteristics and advantages of steel bearing piles in construction are described in order to assist in the primary process of selection of the correct pile type for any given site and soil conditions. Load transfer mechanisms are described and limit state design methods applied in line with the new Eurocodes. The sections on design include axial and lateral load resistance prediction methods, combined loading effects on retaining walls and pile group analysis. Up-to-date pile driving analysis is presented as a basis for planning efficient installation and as an aid to design. Practical aspects of test loading, installation tolerances and connection details are covered.

It is noted that excessive conservatism has been found in current practice and this results in unnecessary overdesign. Currently used specifications for load testing piles only up to 1.5 working load, are insufficient to reach the ultimate pile resistance and the whole object of the new limit state design (LSD) procedures has been denied. This problem has been compounded by making unrealistically low design assumptions on the soil parameters in pile resistance prediction methods. This publication adopts LSD using the new Eurocodes and suggests more reliance be placed on static and dynamic load test methods to establish ultimate capacity to permit more economic steel pile design.

Guide de dimensionnement des pieux de type H

Rsum

Cette publication est consacre au choix, au dimensionnement et la mise en place de pieux de fondations en acier, de type H et UC, pour tout type de structure. La pratique actuelle est discute et des recommandations sont donnes.

Les caractristiques et avantages des pieux en acier sont dcrits afin d'aider au choix d'un systme correct de pieux pour tout site et toutes conditions de sols. Les mcanismes de transfert des charges sont dcrits et les mthodes de dimensionnement aux tats limites, selon les nouveaux Eurocodes sont prsentes. Les chapitres consacrs au dimensionnement prennent en compte les charges axiales et latrales ainsi que l'effet des murs en retour et des groupes de pieux. Les mthodes les plus modernes de mise en place sont prsentes. Les essais de rsistance, les tolrances d'installation et les dtails d'assemblage sont galement abords.

Un conservatisme excessif est constat dans la pratique courante et dans les spcifications actuelles conduisant des hypothses non ralistes dans les mthodes de calcul. Ceci a conduit des diffrences considrables entre les calculs et les essais de pieux mtalliques in situ ; avec pour consquence une grande difficult, pour les praticiens, d'interprter les rsultats d'essais, et ainsi toute la base des nouvelles procdures de dimensionnement un tat limite tait nie. Cette publication adopte la mthode des tats limites, qui conduit un dimensionnement plus conomique des pieux en acier.

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Leitfaden fr Pfhle mit H-Querschnitt

Zusammenfassung

Dieser Leitfaden gibt eine Anleitung zu Auswahl, Berechnung und Einbau von Stahlpfhlen mit H-Querschnitt und Tauchpfhlen mit UC-Querschnitt fr die Grndungen aller Tragwerksarten. Die gegenwrtige Praxis und Erfahrung auf diesem Gebiet wird vorgestellt, diskutiert und es werden Empfehlungen gegeben.

Die Eigenschaften und Vorteile von Stahlpfhlen werden beschrieben, um die Auswahl des richtigen Pfahltyps fr jede Baustelle und jeden Baugrund zu erleichtern. Die Mechanismen der Lastbertragung werden beschrieben und der Grenzzustand der Tragfhigkeit gem den neuen Eurocodes wird in Relation zu gemessenen Pfahlkopfverschiebungen interpretiert. Die Abschnitte zur Berechnung beinhalten Methoden zur Vorhersage des Widerstands fr axiale und horizontale Lasten, Pfahlwnde bei kombinierter Belastung und die Berechnung von Pfahlgruppen. Neueste Berechnungen zum Rammen werden vorgestellt als Basis fr einen effizienten Einbau und als Berechnungshilfe. Praktische Aspekte aus Versuchsbelastungen, Einbautoleranzen und Verbindungsdetails werden behandelt.

bertriebener Konservatismus wurde in der gegenwrtigen Praxis vorgefunden, was zu unntiger berbemessung fhrt. Gegenwrtige Regelungen fr Pfahlversuche mit bis 1,5-fachen Gebrauchslasten sind unzureichend um die Grenztragfhigkeit der Pfhle zu erreichen und das Ziel der neuen Berechnungsmethoden der Grenztragfhigkeit wurde bestritten. Dieses Problem wurde bei der Vorhersage des Pfahlwiderstands verbunden mit unrealistisch geringen Berechnungsannahmen hinsichtlich der Bodenparameter. Diese Publikation beinhaltet die Nachweise fr den Grenzzustand der Tragfhigkeit nach den neuen Eurocodes und schlgt vor, den statischen und dynamischen Belastungsversuchen zur Ermittlung der Grenztragfhigkeit mehr Vertrauen entgegenzubringen um eine wirtschaftlichere Berechnung von Stahlpfhlen zu erlauben.

Gua para pilotes de acero con secciones H

Resumen

Esta publicacin gua la eleccin, proyecto e instalacin de pilotes de acero y compuestos de hormign y acero con secciones H y UC para cimientos de cualquier tipo de estructura. Se presentan tanto la prctica como la experiencia actuales con su discusin y pertinentes recomendaciones.

Se describen las propiedades y ventajas de los pilotes de acero en la construccin con lo que se facilita el anteproyecto del tipo adecuado de pilotes para cualquier tipo de suelo. Se describen tambin los mecanismos de transferencia de cargas y los mtodos de diseo basados en los estados lmites de proyecto interpretados en relacin a los movimientos medidos en cabeza de los pilotes en lnea con los nuevos Eurocdigos. Los apartados relativos al proyecto incluyen mtodos de prediccin de la resistencia a cargas longitudinales y transversales, efectos de carga combinada en muros de contencin y clculo de grupos de pilotes. Se presentan clculos de hinca, actualizados, para una planificacin efectiva de la instalacin y como ayuda de proyecto. Tambin se tratan aspectos prcticos de los ensayos de carga, tolerancias de instalacin y detalles de uniones.

Se toma nota de que un conservadurismo excesivo ha sido observado en la prctica habitual y en las normas o recomendaciones utilizadas en el ensayo de pilotes, lo que

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adems suele venir combinado con hiptesis de proyecto poco realistas sobre los parmetros del suelo que se utilizan en los mtodos de prediccin de la resistencia, obteniendo como resultado un sobredimensionamiento innecesario. Por todo ello, los proyectistas eran incapaces de interpretar la resistencia ltima de los pilotes a partir de sus ensayos de carga y ello hizo que el nuevo mtodo de clculo en estados lmites ltimos fuese rechazado radicalmente. Esta publicacin adopta el clculo en estados lmites ltimos que debera permitir proyectos ms econmicos de pilotes de acero

Guida all'uso di pile portanti in acciaio con sezione trasversale a H

Sommario

Questa pubblicazione fornisce una guida per la scelta, la progettazione e l'istallazione di pile portanti con sezione a H e UC in acciaio per fondazioni di differenti tipi di strutture. In particolare,viene presentato lo stato dellarte sia a livello di prassi progettuale sia considerando le conoscenze acquisite.

Sono illustrate le principali caratteristiche e i vantaggi delle pile portanti in acciaio in modo da fornire un importante aiuto nella scelta della corretta forma strutturale della pila in funzione del luogo e del tipo di terreno. Vengono poi descritti in dettaglio i pi significativi meccanismi di trasferimento del carico ed presentato il metodo progettuale agli stati limite applicato secondo i requisiti della versione aggiornata dellEurocodice. La parte dedicata alla progettazione propone i metodi per la determinazione della resistenza in presenza di carichi assiali e trasversali, per la valutazione degli effetti combinati sulle paratie e per lanalisi di gruppi di pile. Un aggiornato metodo per l'analisi delle pile e' presentato come base per un conveniente utilizzo e valido aiuto per la fase progettuale. Sono inoltre affrontati gli aspetti pratici delle prove di carico, tolleranze di istallazione e dettagli dei collegamenti.

La corrente prassi progettuale e le raccomandazioni attualmente in uso per l'esecuzione di prove di carico risultano eccessivamente penalizzanti e portano ad inutili sovradimensionamenti. Le raccomandazioni attualmente in vigore, che prevedono prove di carico con azioni applicate pari a 1,5 volte quelle di esercizio, risultano inadeguate per valutare la resistenza delle pile e ci in disaccordo con la filosofia progettaule legata al metodo semi-probabilistico agli stati limite (LSD). In aggiunta, si hanno ipotesi poco realistiche per quanto riguarda i parametri base del terreno che condizonano la capacit portante delle pile.

Questa guida adotta il metodo progettuale degli stati limite in accordo alla recente versione dellEurocodice e si basa su indicazioni pi appropriate relative alla sperimentazione, statica o dinamica, per valutare la capacit portante e quindi per avere una progettazione economica di pile portanti in acciaio.

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1 INTRODUCTION

1.1 Piled foundation choice The first decision in considering a foundation design is whether piles are required or not. In some cases there may be alternative solutions, for which the costs may be compared with those of a piled foundation. In other cases, the bearing capacity of the soil at the foundation level may be satisfactory but, owing to high loadings, piles are required to keep settlement within acceptable limits. It is important to be clear about the reasons for using bearing piles before weighing the relative merits of using steel or concrete types of driven pile, because there are some essential differences in behaviour that may favour one or the other pile type for a particular project.

Bearing piles are used mostly for supporting vertical loads and for this purpose the main requirements are to:

Restrict average settlement to a low value. Minimise differential settlement. Achieve an adequate factor of safety or load factor against foundation

failure.

Many technical and cost-benefit factors affect the selection of the most appropriate type of pile for a given structure. Very broadly, these factors can be divided into those related to:

Site location and operating conditions. Type of soil and ground water level during installation. Type and size of the loads to be supported by the foundation. Type of structure, e.g. land or marine. Effect of the pile type on overall construction programme and cost. In some circumstances there will be additional technical factors that affect the choice of pile, for instance when overturning moments due to wind forces on a tall building have to be resisted, or when severe scouring of a river bed may expose piles supporting a bridge pier.

Where piles have to resist tensile loading or absorb energy in bending, as in marine dolphins for ship impact, and in integral bridge piers for vehicle impact, there are special requirements to be considered which favour the selection of steel piles. In particular, the ductility of steel piles creates an elastic compliance with the superstructure to absorb the impact energy by deflection.

The cost-benefit factors which may favour the choice of steel piles include:

Total cost of the foundation, where it is important that the comparison between pile types is related to the total construction cost including installation and not just the cost of the pile material.

Total construction time, where use of driven steel piling can result in a shorter construction period and an earlier project completion date.

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Environmental constraints, where the noise and vibration caused during steel pile driving has now been reduced by developing new installation equipment to be within limits stated in UK legislation.

Sustainability issues, where steel bearing piles are easy to extract from the ground at the end of structure life and can be reused or recycled so reducing the whole life cost of the building.

Many of the above factors are interrelated, and all require consideration in arriving at the most suitable pile type for a given situation. Broad guidance only is possible in this publication, as each project requires individual examination. For specific technical advice or product information, the organisations listed in Appendix A of this publication should be contacted.

There is no single pile type that is both technically and economically appropriate for every structure, site or set of soil conditions. Owing to the many different types of project and construction situations, there will always be a need for a variety of pile types, so selection is an exercise of judgement.

1.2 Why choose steel piling? Knowledge about the installation and in-service performance of steel bearing piles has progressed over the last 40 years due to increased usage worldwide, particularly in the USA, Japan and in European countries particularly Norway, Finland, Holland, Belgium and Denmark. Research work for the offshore industry has been carried out and reported in the UK[1][2][3] and the transfer of this knowledge was considered beneficial for UK onshore application.

The trend towards increased foundation loads is well catered for by steel bearing piles. H-piles are capable of carrying loads of up to 4,400 kN.

Steel piles offer many advantages compared to other types including:

Reduced foundation construction time and site occupation. Reliable section properties without need for onsite pile integrity checking. Ductility also gives high resistance to lateral loads for marine structures and

compliance in integral bridge foundations.

Larger wall surface area giving better friction capacity than equivalent diameter concrete pile

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

Closer spacing possible and therefore smaller pile caps. Pile load capacity can be confirmed during driving by Dynamic Pile

Analysis (DPA) on every pile driven.

Low displacement of adjacent soil during driving. No arisings and therefore no spoil disposal offsite Easily extracted at end of working life. Reusable or recyclable following extraction to meet Government objectives

in sustainable construction

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Steel piles have clear-cut advantages on projects such as on 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. On cohesive soil sites, there is a wide selection of acceptable pile types and other construction aspects will govern.

Nowadays, steel piling is an attractive and competitive alternative for permanent foundations owing to the research and development in piling technology and changes in the construction industry supply chain. These can be described under three broad headings, durability, performance and economy.

Durability

The subject of corrosion and steel protection has received substantial attention both in the UK and abroad over the last 40 years. There is now adequate knowledge on corrosion rates, coatings selection and specifications to permit the designer to make a reasoned judgement on the provision for corrosion prevention. Such information is readily available from Corus publications[94][111]; general guidance is also repeated in Section 11 of this publication. In addition, the corrosion guidance sections of BS 8002[5], BS 6349[6], Eurocode 3: Part 5 (EN 1993-5)[7] and in document BD 42[8] (part of the Design manual for roads and bridges) have embodied earlier research, and further revisions are in progress.

Performance

Reliable load capacity and driveability predictions are essential for the confident design and installation of driven piling. These topics have been poorly covered in most foundation and piling design textbooks and this publication therefore provides practical advice for the guidance of designers.

It was deemed appropriate to examine piling technology used in the offshore construction sector, where there is a body of research and accepted practice, and to transfer relevant practices to the onshore sector. The offshore design methods are simple in concept and the principles involved can be readily understood. They have been used with success in minimising foundation installation costs and the steel tubular piles have performed well for decades on offshore fixed structures. These methods are presented in Sections 4, 5 and 6, and supporting references are given for further detail on usage and applications.

For economic pile design, the methods require knowledgeable judgement of soil parameters and this, in turn, requires high quality soils data. Such data is obtainable using routine site investigation techniques, but care must be exercised in the soil sampling and testing specifications, in order to ensure that data collected on soil properties is relevant to driven steel piles as well as to bored concrete piles. In particular, there must be more emphasis on in situ penetration testing (see the advice given in Section 3 on SPT and CPT soil tests).

Economy

The differential in cost between concrete and steel construction has decreased steadily over recent years; the costs of site labour and concreting materials have increased, whereas the cost of steel has decreased in real terms (see Corus publications[107]). In addition, with the advent of Design and Build contracts for civil engineering work, there is more incentive for innovative design to

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permit cheaper overall construction by incorporating the piled foundation into the structure concept rather than leaving it separate.

Constructing in steel permits prefabrication of larger, but still easily erectable, high quality structural elements that can save construction time; this is an increasingly attractive project consideration.

In foundations and basements, steel bearing piles are compatible and easily connectable to the steel frame of a building thereby permitting savings in overall construction costs. Progress has also been made in more effective connection between reinforced concrete superstructures and steel piling using welded-on shear studs or angles, hoop bar connectors and careful detailing in composite connections in bridge engineering. For steel intensive basement construction, cost savings of up to 40% have been reported by designers.

Steel foundation piles are ductile and can deflect to absorb energy in marine applications producing a saving in structural section.

Sustainability and environment

The worlds available supply of construction materials is becoming more scarce and consequently more difficult and expensive to source and supply. Western governments have agreed to encourage more recycling of construction material in order to reduce the impact of mining more ore and aggregate, and to reduce the volume of waste construction materials from demolition of old buildings.

Steel is the worlds most recycled material and is 100% recyclable. In 2003, 965 million tonnes of steel were produced worldwide and approximately 43% of that was from recycled scrap steel.

The use of scrap is also essential to the efficient production of the stronger higher grade steels and it therefore has a commercial value that makes recycling economically viable. The supply chain for scrap is well established (see the SCI publication Environmental assessment of steel piling[110]).

When assessing the environmental impact of construction, it is important to consider the practicality and cost of removal of the structure at the end of its useful life and the disposal of the demolished materials. The construction industry, in common with many other industries, is now being encouraged to develop new processes that will allow more materials to be recycled or reused, helping to conserve natural resources and reduce waste.

Steel piling benefits from being easy to extract from the ground during demolition of previous structures, or after its temporary use as part of the construction process. Extraction equipment includes vibration hammers working under a pullout force from cranes and special high load jacking frames that can pull out the longer bearing piles. This facility creates an additional environmental benefit from being able to easily restore a previous building site to a greenfield state without any remaining contamination below ground. The steel piles can either be reused or recycled.

Concrete piles on the other hand are difficult to demolish or extract and the process is therefore time consuming and expensive. On many sites the degree of contamination with concrete piles is so expensive to remove that developers have been deterred from using that brownfield site and have used a greenfield site instead. On some brownfield sites, the new piled foundation

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has been interwoven through the old concrete piles, creating more contamination and rendering the site much worse for any future redevelopment. The large diameter bored concrete under-reamed piles that have often been used on inner city sites such as in London and Manchester, are particularly difficult to remove.

Figure 1.1 shows steel piles extracted from some jetties in Hong Kong harbour and stacked on the quayside. The concrete piles in Figure 1.2 were also part of the same complex of jetties, which took more time to extract and were difficult to break up, illustrating the problems in removing concrete substructures.

Figure 1.1 Recovered steel H-piles from a site in Hong Kong harbour

Figure 1.2 Concrete piling at the same Hong Kong harbour presented

considerable demolition and extraction problems

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1.3 Scope of this publication This publication supplements the information given in existing textbooks with up-to-date guidance on those aspects of steel bearing piles that have not been covered elsewhere.

Section 2 presents a design basis for H-piles and Section 3 a treatment of Limit State Design (LSD) that is consistent with the new Eurocodes and uses the same notation as those standards. LSD is described in a way that relates pile design to the real performance that is observed in pile load tests, and thereby permits the designer to understand the small pile head settlement that occurs in generating pile load resistance with steel piles. The pile-soil load transfer mechanism is also explained.

Section 4 covers the selection of steel section for the intended purpose.

Sections 5, 6, and 7 cover the geotechnical aspects of steel pile design in the context of other design references and textbooks.

Section 8 deals with an up-to-date treatment of pile installation and the testing of steel bearing piles, especially the growing use of dynamic analysis of driving as a substitute for expensive static loading procedures. The environmental assessment of noise and vibration during driving is also explained.

Section 9 covers Economic Design to illustrate the technical and cost-benefit factors of steel piles by means of case studies.

Section 10 presents some typical connection details for the pile to structure interconnection, and Section 11 covers durability aspects, including a discussion of appropriate corrosion allowances.

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2 DESIGN BASIS

2.1 General This publication provides information and guidance to enable an experienced engineer to carry out the design of steel bearing piles. The designer should have sufficient knowledge of the basic physics of soil-pile interaction to be able to design a pile but may need the assistance of geotechnical engineers to interpret the soil parameters from the soils data given in a site investigation report in respect of steel piles. The geotechnical engineer needs detailed references on steel pile load tests in all types of soil to enable characterisation of behaviour that will serve as a basis for the empirical factors to use with generic prediction methods. The information is available from various references including: Clarke et al[1], Biddle and Wyld[22], Jardine et al[112], Harris and Sutherland[91], and Euripides[64]. These are referred to in later Sections.

To date, in the UK, design resistance of foundations has been evaluated on an allowable stress basis design (ASD) both for the soils and for the structural components such as piles. However, structural design in the UK has already largely converted to a limit state design (LSD) basis. The structural Eurocodes have all been formulated on an LSD basis whereby partial factors are applied to various elements of the design according to the reliability with which the parameters are known or can be calculated. Eurocode 7 Part 1:Geotechnical Design has already been published in the UK as BS EN 1997-1:2004[9] and presents rules and principles for foundation design on a LSD basis.

Limit State Design brings with it a change of emphasis which, when carefully considered, has many benefits for the economic design of piling. The Eurocode approach is particularly rigorous, and this publication adopts the partial factors presented in the Eurocodes.

This publication, therefore provides guidance expressed in LSD terminology using the notation given in Eurocode 7[9] where possible, and relates the guidance to previous ASD where it is helpful. However, it has to be recognised that the application of limit state design philosophy to geotechnical design is causing difficulty in a discipline where the Allowable Stress approach and terms such as the allowable bearing pressure, permissible steel stress, and allowable pile capacity are widely accepted and understood.

2.2 Design standards British Standards do not cover the geotechnical design of steel piles in any detail, although there is general guidance given in BS 8002[5], BS 6349[6], BS 8004[15]. This publication makes reference to the offshore industrys recommended practice for steel tubular piles, based on US and UK North Sea experience, which is contained in the American Petroleum Institute Code RP 2A[11] that has been adopted in the ISO Code 13819-2[12]. This has been verified as applicable to steel H-piles by Biddle and Wyld[22] and Jardine et al[112]. Other technical references are used, such as CIRIA Report 103 The design of laterally loaded piles[13], CIRIA Report 104 Design of embedded retaining walls in stiff clay[14], Offshore Technology Conference (OTC) papers and other research papers, and selected textbooks.

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BS EN 1997-1:2004[9] was published by BSI in 2004, and the UK National Annex will be available in 2006. It presents a more rigorous treatment of LSD than any of the British Standards relating to foundations so far (BS 8002[5], BS 6349[6], BS 8004[15], BS 8081[16], or BS 8006[17]) and is compatible with the other structural Eurocodes. It is planned that the Eurocodes will co-exist with the British Standards for a period of up to 5 years.

The application of LSD methods is only progressed in BS EN 1997-1:2004 to the Rules and Principles level but the SCI and Corus have participated in the drafting of Eurocode 3: Part 5 (as EN 1993-5) Design of steel structures - piling[7] to ensure there was technical input to that document derived from UK experience and practice. The essence of that work is presented here because it permits adoption of limit state design principles in a rational way for the geotechnical design of steel piles. Allowable Stress Design (ASD) is still permitted in BS 8002[5], BS 8004 and BS 6349[6], to be compatible with the approach taken in BS 449[19]. However, ASD will be phased out as the Eurocodes are adopted between now and 2008.

Comprehensive design guidance on all aspects of geotechnical design to Eurocode 7 has recently been published by Thomas Telford[113].

2.3 Limit state design rules 2.3.1 Ultimate limit state axial bearing design of piles Limit state design is a method that achieves a certain level of reliability of structural design against the range of possible adverse variances of presumed loading, strength and behaviour. It assigns partial factors to the presumed values and verifies that, at the ultimate limit state (ULS), the factored design value of resistance (strength) is at least equal to the factored effects (forces, moments, etc.) of the design loads (referred to in the Eurocodes as actions). Thus it achieves a level of safety against collapse or failure.

A serviceability limit state (SLS) is also considered, at which, typically there is to be no significant permanent deformation or settlement that would affect the use of the structure (or, in the case of a foundation, the structure or other facility that is founded upon it). The SLS is verified using the same presumed loading and strength but smaller partial factors (typically unity). This permits realistic modelling of soil-structure interaction using strains and stiffness to predict pile movements.

In BS EN 1997-1 there are three sets of partial factors, one applied to actions, or the effects of actions (denoted by A), one applied to soil parameters (denoted by M) and one to resistances (denoted by R). The values of the partial factors are given in the Eurocode itself but, since the level of safety required is a matter for national choice, the National Annexes are allowed to vary the values of the partial factors. At present there is no UK National Annex and so this publication adopts the partial factors given in the Eurocode (it is likely that the UK NA will also generally adopt those factors).

BS EN 1997-1[9] also sets out three possible Design Approaches and the National Annex may chose the method to be used. It is likely that the UK will adopt Design Approach 1 and that approach is described below.

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Design Approach 1

For axially loaded piles, adequacy at ULS has to be verified applying two possible combinations of partial factors. These are described as:

Combination 1: A1 + M1 + R1, or

Combination 2: A2 + M1 or M2 + R4.

Where A are action factors, M are material factors, and R are resistance factors. See the UK National Annex for the factor values to be used.

2.4 Bearing pile structural design Currently the structural design of bearing piles is outlined in Section 7 of BS 8004. There it refers to the use of BS 449 for ASD or BS 5950 for buildings or BS 5400 for bridges (the latter two being LSD Codes). Clearly, the designer must choose whether to design his piles to either an ASD or LSD basis. To be consistent with the design basis proposed in this publication, LSD should be used for the structural design but, for information, an overview of both is given below:

ASD design

Structural design using ASD principles is referred to in the Corus Piling Handbook[4], in BS 8004 and in BS 8002. These codes invoke the use of procedures that are outlined in BS 449 and in the old CP2: 1951[30]. Neither of these latter codes are in print any longer and only library copies are available. Many temporary works designers in contractors still have an affinity for ASD design because of the increased allowable stresses granted in that code for such temporary works.

The ASD basis generally involves use of conservatively assessed safety factors that were appropriate to the then limited knowledge of pile behaviour and the lack of research data using pile instrumentation. Consequently, the degree of utilisation of structural strength of steel piles was lower and the relative cost of the steel option was higher than that used now. This made steel H piles less competitive than concrete piles for many bearing pile applications.

The ASD procedures involved using permissible stress values of 0.3fy for axial loading, and 0.5fy for bending moment stresses. For temporary works loads an increase to 0.67fy was permitted.

LSD design

More economic use of steel piles is now possible as a result of research into pile behaviour using instrumentation to understand the physics of soil-pile interaction under applied static loads and under dynamic forces when driving (see Sections 3,5 and 8). It is now known that fully buried steel piles derive sufficient lateral support from the soil to prevent buckling and no special allowance needs to be made for this in safety factors. (This is covered in the new Eurocode 3: Part 5[7].

The partial factors have values that are related to the degree of confidence that can be attached to each part of the load and resistance equations. For instance, a material factor o = 1.0 is used for steel strength because of the high consistency in high quality steel that is produced in the UK and Western Europe. This factor is applied to the nominal yield strength for each grade of

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steel. On the other hand, the partial factors on load are 1.35 for dead load and 1.5 for imposed loads, as given in Eurocode 1. The overall factor between nominal load and nominal yield strength is thus between 0.74 and 0.67.

2.5 Design methodology Obtaining the soil data at the site and the loading data for the project are prerequisites for design.

In the first stage of the structural design of a steel bearing pile, the required cross-section is determined based on the design pile resistance needed for the design value of the axial loading that it is required to carry (note that in the structural analysis of the building there is a pinned joint at the connection with the pile and therefore the pile does not need to be designed to carry moment). The pile section shape and steel grade should be selected making an allowance for loss of section due to corrosion according to the required design life (see Section 11).

The second stage is to determine the length of that pile section that is required to provide a design compressive resistance at least equal to the design value of the axial load. Axial load is determined using an appropriate geotechnical prediction method and the soil tests at the site or using measured load resistance from pile load tests (see Section 5).

The third stage is to assess the practicality of installing such a pile to the depth required using available driving hammers by using a wave equation programme such as GRLWEAP[27] for a more precise analysis, or using a pile driving formula such as Hiley[28] or Janbu which is comprehensively presented in a paper by Flaate[68] (see Section 8.3) for an approximate check.

For friction piles, the desirability of selecting a different pile size or steel grade to adjust the required length can then be judged from sensitivity analyses of various situations to optimise the geometry in relation to driveability, availability from stock, road transport to site, site installation and connections considerations.

For end-bearing piles, the provision of a driving shoe might also be evaluated in order to achieve penetration into a sound rock stratum whilst avoiding local buckling of the pile base (see Figure 4.3 for typical examples). Dependent on the pile cross section, this can reduce the available skin friction on the remaining shaft above the tip by over-coring and should be allowed for in design or after pile load tests.

The fourth stage is to assess the possible bending stresses that can be induced in the pile, dependent on the type of connection to the structural foundation and the installation tolerances in pile position (see Sections 8 and 10). If bending stresses demand a larger pile size in a group, then a global analysis of the whole foundation (or at least the critical pile group) may be required (see Section 7), in order to apportion the moment between each pile in the group. Each pile will then need a lateral loading analysis, in order to check that the cross section is adequate to take the combined bending and axial force at all levels down the pile according to the pile lateral deflection and the stresses induced, (see Section 6). Fully corroded section properties should be used for the end of design life condition taking account of the corrosion allowance profile with depth.

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If a different pile size is required for the combined effects, then a new pile length will have to be determined from the geotechnical design and the driveability should be checked anew.

The fifth stage is to evaluate the environment and cost benefit factors (see Section 9) for different pile types and configurations and types of connection before selecting a solution and moving onto final detailed design of the connection between pile and structure.

As explained in Section 3, the generation of soil resistance requires pile movement. The new limit state design procedures involve estimation of pile axial and lateral movements in order to satisfy SLS criteria. The lateral deflection profile will obviously be affected by any change in steel section or pile length.

Figure 2.1 shows a flow diagram for the pile design procedure.

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Obtain input data:Soil propertiesLoad effects

Corrosion allowances

Select initial pile size:Pile type

Section sizeGroup configuration

Stage 1

Determine pile length:based on axial resistance

Stage 2

Check pile driveability:Required penetration

Driving stresses

Stage 3

Pile driveable?

Design for lateral loads:(if applicable)

Yes

Pile suitable?

Evaluate economic &programme aspects:

Construction programme,pile types & configurations

Yes

No

No

Change pile type, sectionor configuration

Stage 4

Design connections betweenpile & structure

Stage 5

Figure 2.1 Single pile design procedure flow diagram

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3 GEOTECHNICAL DESIGN

3.1 Terminology Historically, the terminology used by UK geotechnical engineers has been to refer to the ultimate load capacity of a pile as Q, hence Qt=Qs+Qb, where the suffix t is total, s is shaft and b is base. The new Eurocodes have rationalised the symbols used and Eurocode 7 uses R for pile resistance, F for the applied force, and the partial factors involved in the LSD design methods are termed or . A comprehensive glossary of definitions is given in BS EN 1997-1[9].

Suffixes to the Eurocode symbols distinguish, inter alia, between characteristic values (R-;k ) and design values (R-;d ). The Eurocode symbols are mainly used in this publication unless indicated otherwise.

3.2 Design premise The basis of design for any bearing pile is its ultimate axial resistance (capacity) in the particular soil conditions at the site where the structure is to be built. This ultimate resistance can be determined by either:

load tests on piles at the site, or the use of an empirical formula to predict resistance from soil properties

determined by testing.

The design value of the pile resistance is derived from the measured or calculated ultimate resistance by applying appropriate factors and the designer verifies this as adequate to carry the required design loads (actions) from the structure.

The ULS procedure is one that is used to ensure that a limit state of failure is avoided. Under ASD (allowable stress design) this used to be achieved by applying Factors of Safety but the factors also ensured that settlements were controlled to an acceptably low level. The latter is now ensured by checking the SLS or serviceability limit state separately and it is at this juncture that we must relate to real soil-structure interaction physics to understand what we must achieve in design.

Pile movement is needed to generate a soil resistance. The practical design of steel piles therefore involves an appreciation of axial pile strain, shaft wall slip and base movements, and these are obtained from research references and the analysis of pile load tests on site.

Reference to a pile head load-displacement (Fc-) diagram given in Figure 3.1 permits an understanding of the different pile head deflections that are appropriate to the serviceability limit state (SLS) and ultimate limit state (ULS) specifications for friction pile design. The SLS is generally governed by settlement or deflection, and a working limit of about 10 mm is suggested; and the ULS is generally governed by load to cause failure or near failure, and a practical limit for pile settlement is probably about 40 mm (see Appendix D, page 152 of the ICE Specification for piling and embedded retaining walls)[20].

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Reference to Figure 3.8 for end-bearing piles in hard rock shows that even settlements at ULS can be within the elastic compression shortening of a pile, i.e. a few mm (generally 5

Factor 1 on mean Rcm 1.40 1.30 1.20 1.10 1.00 1.00 Factor 2 on minimum Rcm 1.40 1.20 1.05 1.00 1.00 1.00

Different correlation factors 5 and 6 are used for dynamic pile load testing to allow for the confidence by which values can be determined from a small number of tests. The values of these factors are given in Table 3.2, based on Table A.11 of BS EN 1997-1.

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Table 3.2 Values of correlation factor for dynamic load tests Number of dynamic Load Tests

2 5 10 15 20

Factor 5 on mean Rcm 1.60 1.50 1.45 1.42 1.40 Factor 6 on minimum Rcm 1.50 1.350 1.30 1.25 1.25

Resistance factors

The design value of the compressive resistance (Rc;d), is then given by:

t

ckcd

RR =

The value of the partial resistance factor on total resistance, t is 1.3 for driven piles, as given in Table A.6 of BS EN 1997-1. The partial factors for bored and CFA concrete piles from Tables A.7 and A.8 are also given for comparison purposes in Table 2.2 below.

Table 3.3 Values of the resistance factors, b, s and t in BS EN 1997-1

Component factors b s t

Driven steel piles 1.3 1.3 1.3

Bored in situ concrete piles 1.6 1.3 1.5

CFA (continuous flight auger) in situ concrete piles 1.45 1.3 1.4

For example, comparison with traditional ASD basis for a single driven steel pile test, the relationship between the design value Rc;d and the measured value Rc;m (for Combination 2) is:

82.13.14.1

mc;mc;

1

mc;dc;

RRRR

t

=

==

Note that the t resistance factor for driven piles is lower than that for bored concrete piles owing to the greater confidence in driven pile capacity predictions that is due to more consistent behaviour after installation. Traditional practice in allowable stress design procedure (ASD) has been to use a lumped Factor of Safety of 2 for soil resistance on all types of pile, but this has been rationalised due to research that shows that bored concrete piles show more variable behaviour in load tests dependent on the degree of care taken during installation. The limit state design (LSD) procedure taken from BS EN 1997-1, using partial factors on single load tests, gives a total factor of 1.82 between measured and design resistance for steel piles, 2.1 for bored concrete piles and 1.96 for CFA bored piles. This difference is due to the greater reliance of bored concrete piles on end resistance where a lot of disturbance occurs to the soil in the boring process.

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It may also be noted that the design value of the load on the pile (against which the resistance has to be verified) is given by:

kc;Fdc; FF =

where F is the partial factor on actions and Fc;k is the characteristic value of the load. The value of F is given in Table A.3 of BS EN1997-1 as 1.35 and 1.5 on permanent and variable actions for Set A1, or 1.0 and 1.3 for Set A2. The overall effect of all the factors is thus approximately to ensure that a factor of 2.5 is maintained between Rc;k and Fc;k.

The effect of the application of the separate partial material factors is illustrated in Figure 3.1 using a load-displacement diagram.

It can be seen that application of the correlation factor , and resistance factor t, places the design working load on the pile at a level within the elastic range where very little pile head movement is required, thereby satisfying the SLS criterion for allowable settlement, if set at about 10 mm. (By comparison, the generally accepted limit for settlement for structural spread footings is 25 mm and therefore piled foundations give more control over structural movement).

3.3.2 ULS axial design resistance predicted from soil tests The design compressive resistance of a pile, determined from soil tests is given by:

b

kb;

s

ks;dc;

RRR +=

which combines equations 7.6 and 7.7 of BS EN 1997-1 (7.6.2.3) ,

where:

Rs;k is the characteristic value of shaft resistance of the pile

Rb;k is the characteristic value of base resistance of the pile

500 10 20 30 40

Rc;m

R

R

c;k

c;d

c;kFAxi

al c

ompr

essi

ve lo

ad

Measured compressive resistance

Characteristic compressive resistance

Characteristic axial load (working load)

Design value of compressive resistance

Pile head displacement (mm)

Figure 3.1 Load-displacement diagram, showing the effect of partial factors

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s is the partial factor on shaft resistance for driven piles = 1.0 for Set R1

= 1.3 for Set R4 (BS EN1997-1 Table A.6).

b is the partial factor on base resistance for driven piles = 1.0 for Set R1


The characteristic values of shaft resistance, Rs;k, and of base resistance, Rb;k, are representative minimum values determined from the relevant geotechnical prediction methods and appropriate soil parameters (see Section 4).

It should be noted that these semi-empirical geotechnical prediction methods are based on load test databases and have to contain conservatively assessed empirical factors, (as stated in the requirement in BS EN 1997-1), to ensure that Rc;k # Rc;m. The overall Factor of Safety applied in this limit state procedure therefore comprises a further partial factor, the model factor (to replace the empirical factor in the prediction method), and which allows for the scatter in soil properties and in the load test results.

Therefore, where Rs;k, and Rb;k are determined from characteristic values of unit shaft and base resistance, qs;k, and qb;k, the additional model factor should be applied to s and b. It is understood that the UK NA (UK National Annexe) will call this factor Rd and assign it a value of 1.4. The design compressive resistance of a pile, Rc;d determined from soil tests is then given by:

bRd

kb;

sRd

ks;dc;

RRR +=

where:

Rs;k is the characteristic value of shaft resistance of the pile

Rb;k is the characteristic value of base resistance of the pile

s is the partial factor on shaft resistance for driven piles = 1.0 for Set R1


b is the partial factor on base resistance for driven piles = 1.0 for Set R1


Rd is the model factor given in the UK NA that is due in 2005. 3.3.3 ULS axial design resistance for piles end bearing in rock

Where the ground conditions at the site include an underlying rock stratum within a driveable depth, the only reliable method for the designer to determine the ultimate load capacity is to carry out a pile load test.

The same framework of design rules apply as for soils, except that the endbearing resistance from the rock at the base will dominate. Even if there are

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overlying soils, the elastic compression due to applied load on the pile will be so small, due to the high stiffness of the base resistance, that some of the potential frictional resistance on the pile shaft cannot be mobilised.

The use of the nominal allowable rock bearing pressures that are given, for example, in BS 8004[15], page 11, or in the API Code RP2A[11], such as 10 MPa or 15 MPa, will greatly underestimate most rock resistances and lead to uneconomic designs for steel piles. This is because such limit judgements were made for spread footings, and the extension of that bearing capacity theory to deep bored cast in situ concrete piles in clay is not relevant to driven steel piles. In addition, there is the difficulty of drilling a clean rock socket without leaving a layer of soft compressible drill cuttings under the toe of the concrete pile. Further information is given in the CIRIA Guide R181: Piled foundations in weak rock[21].

Due to the high variability of rock types in the UK, site specific load testing is always required to ascertain capacity. The ultimate design resistance of steel piles driven into sound rock is often governed by the allowable stress in the pile section.

Steel bearing piles are ideally suited to piling in rock because no excavation is required as with bored concrete piles, and any variations in peak load or in the degree of weathering in the rock can be accommodated by varying the driven length. Their small displacement also ensures penetration to a sound layer (see CIRIA Guide R181).

SCIs database of steel pile load tests, reported in Validation of vertical load capacity prediction methods for steel bearing piles[29], includes tests with end bearing into rock, and those results together with current accepted practice from various sources are given in Section 5.6.

The basic recommended procedure is to plan the site investigation to include soil penetration testing (SPTs and CPTs)[26] which will help to differentiate the weathered rock layers from the intact rockhead levels. From offshore experience and European experience with the CPT, it is known from pile driving back-analysis and static load testing that the CPT qc value can be assumed as an ultimate unit resistance pressure beneath the steel pile wall tip area. Since the limiting pressure of the load cells within the CPT tool is about 70 MPa to 100 MPa, this should be adequate to cover most of the soft rocks found in the UK, e.g. mudstones, sandstones, chalk and their weathered derivatives.

In harder rocks, like granites, metamorphic types, carboniferous limestones and intact unweathered sedimentary types, the unit end bearing resistance is more likely to be of the order of 200 MPa to 400 MPa or more (see Section 5.6). In many cases of end-bearing into rock therefore, the ultimate load capacity of a steel pile is governed by the steel yield stress, and not the rock resistance limit.

3.3.4 SLS axial bearing design SLS axial bearing resistance will be the pile load resistance at a selected pile head settlement that is structurally acceptable to the designer. The design pile load is determined in the LSD procedure by setting all the partial factor values to 1.0.

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In practical terms, the designer has two choices, i.e.:

to specify the SLS criterion to be a pilehead settlement at which there is no permanent set of the pile (a s1 of 10 mm is suggested), or

to specify a pilehead settlement that is the maximum that the supported structure can sustain without affecting its serviceability (a s2 of 25 mm is suggested for buildings, or perhaps 10 mm for bridge foundations).

Obviously, the proportion of the potential maximum ultimate capacity of the pile achieved at either of these SLS criteria will vary dependent on the pile cross-section, pile length and the soil resistances in friction and end-bearing on a particular site. Also, the major proportion of pilehead settlement will be permanent under the dead load component. However, serviceability criteria, in practice, rarely govern steel pile design because movements are small.

3.3.5 ULS lateral load design resistance The design resistance of a transversely loaded pile is termed Rtr, according to BS EN 1997-1:2004[9], and it must be demonstrated that

Ftr;d # Rtr;d where:

Ftr;d is the design value of transverse load, and

Rtr;d is the design resistance to transverse loading taking into account any effect of coexisting axial pile loading

Useful guidance is given in Sections 7.3.2.4 and 7.7 of BS EN 1997-1[9] to assist the designer to judge the criteria applicable to the design of piles and pile groups for lateral loading.

3.3.6 SLS lateral load resistance The serviceability limit state for transverse loading of a pile can be defined as the pile head loading and the resulting soil resistance distribution that occurs at the maximum allowable in-service transverse pile head deflection of the supported structure that is permitted or is structurally imposed at the structure/pile connection. The design pile load is determined in the LSD procedures by setting all partial factor values to 1.0.

Advice on the geotechnical design and analysis of piles for lateral load resistance is given in Section 6 and where the contribution to lateral loading resistance of a vertical bearing pile is required, the designer is recommended to follow the guidance given in CIRIA Report 103 The design of laterally loaded piles[13] and the textbooks by Poulos and Davis[23], and Tomlinson[24]. Guidance on pile group effects is given in Section 7.

3.4 Geotechnical design methods Soils are characterised as either clays/cohesive or granular/non-cohesive types in order to separate their two fundamentally different behavioural responses to applied pile load. The generic formulae used to predict soil resistance to pile load include empirical modifying factors (see Section 5), which can be adjusted according to previous engineering experience of the

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influence on the accuracy of predictions by changes in soil type and other factors such as the time delay before load testing.

It will be shown in Section 3.7 that the mechanisms of axial load transfer involved in shaft friction Rs and base resistance Rb are completely different. The separate prediction of shaft friction and base resistance therefore forms the basis of all predictive calculations of pile load-carrying capacity. The basic equations to be used for this are written as:

Rc = Rs + Rb - Wp (3.1)

and,

Rt = Rs + Wp (3.2)

where:

Wp is the weight of the pile

The weight of the pile (Wp) should be included in the actions acting on the pile foundation and the increased end-bearing resistance due to overburden pressure included in the base resistance. Since these terms often cancel each other out, it is common to ignore them (although strictly they should be included in the calculation).

There is a move towards applying reliability criteria to evaluate structural design procedures in construction, but care should be taken in applying these to geotechnical methods. Many geotechnical design methods rely on averaging soil properties over the length of a pile, and practitioners have found that simple formulae can be used with confidence to represent soil response to applied load, provided that expert judgement is applied to the selection of the soil parameters involved.

The crucial skill involved is the knowledgeable judgement, because there is usually such a wide variation in soil strength and properties within a site that it defies use of a precise interpretive formula. Statistical analysis procedures for soil spatial variables are not relevant either, because many of the soil response parameters are also time-dependent.

Refinement of geotechnical design methods is difficult to justify because of the considerable scatter in all pile load test databases that compare Rm to Rc, and this indicates that our knowledge of soil-pile interaction and the ways in which we apply it are, as yet, imprecise (see Section 5.1). It is therefore preferable that each formula involves as few variables as possible, to permit designers to appreciate cause and effect during the analysis of problems and thereby to aid their judgement.

The design basis described in the following sections requires the use of either measured pile resistance from load tests or predicted pile resistance using soil tests at the site and empirical generic formulae. The prediction methods and formulae available and the various aspects of soil mechanics are explained in detail in Sections 5, 6, 7 and 8.

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3.5 Soil resistance on driven steel piles Soil resistance is mainly governed by the type of soil, the type of disturbance caused to the soil by the installation of the piles and the nature of the interface that is created between the surface of the pile and the adjacent soil. In the case of steel piles, there is no relaxation or softening of the soils, as there may be in concrete bored or CFA piles, but there is considerable remoulding of the soil surfaces in contact with the steel shaft and base caused by forcing the pile into the soil.

To complicate matters further, the soil resistance to any applied load on driven steel piles is time-dependent. During driving, the frictional resistance is lowered in the remoulded soil zones that are immediately adjacent to the pile wall. In fine granular soils, this remoulding is often a liquefaction that is caused by the high local porewater pressures that result from displacement of the soil structure to accommodate the steel pile volume. In clays, this is generally a plastic deformation of the clay structure that is accompanied by porewater pressure changes.

Set-up

The soil frictional resistance to applied pile axial load recovers within a finite time interval, and this time interval is dependent on the permeability of the soil and the structure of the soil fabric (i.e. the presence of discontinuities such as fissures or laminations or lenses within the soil mass can contain more permeable soils and provide local porewater pressure drainage paths). As an indication, full recovery of shaft resistance may take seconds in a coarse granular material; minutes in sand; hours in a silt or clayey silt; days in a sandy clay; and many months in a high plasticity clay. This phenomenon is referred to as set-up and appreciation of its time dependency is essential to understanding pile load test results and to planning a trouble-free installation. References Clarke et al[1], Fellenius[113] and Jardine et al[3][112], give data on set-up from research measurements on full-scale steel piles.

As a result of pile driving, the maximum shaft resistance can only be achieved in pile load tests if sufficient time is allowed between completion of driving and the commencement of loading for full set-up to occur. Where this is not practical, as in heavily overconsolidated very plastic clays, (e.g. London Clay, Oxford Clay, Weald Clay etc.) the effects of set-up must be allowed for in design by applying appropriate empirical factors that have been derived from a load test database for each particular type of soil and pile (see Section 5). Dynamic load testing of piles may also be used to investigate set-up, particularly on test piles (see Jardine et al[112], Fellenius[113], Komurka[115], and Section 8).

3.6 Load / settlement behaviour friction piles The settlement of a pile head resulting from progressively increasing compressive load in maintained load stages, i.e. effectively a series of static loadings on the pile, can be represented as a pile load-settlement curve, or an Fc - diagram, such as that shown in Figure 3.2.

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The load settlement response is composed of two separate components, the linear elastic shaft friction Rs and the highly non-linear base resistance Rb (see Equation 3.1). These are shown diagrammatically in Figure 3.3.

3.6.2 Linear elastic response of pile Initially, the pile-soil system behaves in a linear-elastic manner up to some point A on the Fc - diagram in Figure 3.2. Applying load to the head of the pile produces axial strain in the steel pile shaft wall and a corresponding downward movement with slippage at the pile wall /soil interface.

Load transfer occurs in the form of shaft friction that at any level on the pile has an elastic-perfectly plastic load-displacement relationship (see Figure 3.4 for load in pile due to shaft friction resistance).

B

A

0 CPile head vertical deflection ( )

DRc

c

Initial loading

Unloading

Reloading

Pile

hea

d ax

ial c

ompr

essi

ve lo

ad (F

)

Figure 3.2 Axial load-settlement for a friction pile (Fc ) curve

0Pile head vertical settlement ( )

R

Rb

s

Pile

hea

d ax

ial c

ompr

essi

ve lo

ad (

F )

Base resistance

Shaft resistance

c

Figure 3.3 Resistance Components in (Fc ) curve

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Hence the upper part of the piles shaft compresses and transfers load to the upper soils and if the load is released at any stage up to this point, the pile head will rebound elastically to its original level as the shaft steel relaxes (see Figures 3.5 and 3.6 for examples of pile head load-displacement relationships from load tests that demonstrate the repeatability of this phenomenon). Negligible end-bearing is mobilised up to this point A.

3.6.3 Elastic-plastic response of pile The onset of nonlinear behaviour at point A in Figure 3.2 is associated with the development of base or end bearing resistance Rb as the load strain in the shaft reaches the pile base level and the lower end of the pile starts to move downwards. Further movement will lead to the mobilisation of full shaft friction Rs by some point B. If the load is released at this stage, the pile head will rebound to some point C, the amount of permanent set being the distance OC, which is mostly the irrecoverable settlement of the pile base sustained in generating a proportion of the base resistance (Rb), the shaft friction movement being, as explained, an elastically recoverable component. The latter phenomenon is illustrated in Figures 3.5 and 3.6.

It should be noted that a small residual compression force may remain in the pile wall after unloading, as measured in pile load tests (see Clarke et al[1]), especially for long piles and where the proportion of friction is high. This residual load may cause a corresponding small contribution to the irrecoverable pile head settlement.

The pile head settlement required to mobilise the full shaft friction Rs is comparable to the elastic compression of the steel wall, i.e. only of the order of 7 mm to 10 mm for piles of typical length 15 m to 20 m.

1100 10 20 30 40 50 60 70 80 90 100

Movement (mm)

Load

tra

nsfe

r (k

Pa)

-50

300

0

50

100

150

200

250

Penetration m141516.35817.6

Figure 3.4 Load in steel pile wall at different levels due to shaft

friction resistance (as measured in LDP tests, Paper 13 pg 297, Clarke et al [1])

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The full base resistance of the pile Rb requires a greater settlement for its mobilisation, and the amount of movement is related to the size of the pile base area involved. For unplugged steel piles this will depend on the wall section thickness or in the case of fully plugged piles, on the diameter or full base width of the pile. For H-piles or sheet piles, the movement may be 2 to 3 times the steel pile wall thickness (i.e. 30 to 40 mm) to generate the wall tip bearing resistance (see page 152 of the ICE Specification for piling and embedded retaining walls[20]). For a fully plugged pile on the other hand, OC on Figure 3.2 may be of the order of 10% of the base diameter or width, depending on the soil type. See Section 5 for further discussion on when to assume plugging.

Note that if the pile base is in dense sand or rock, the end bearing may be developed with negligible base settlement and the compression of the pile shaft may be insufficient to mobilise the full potential shaft friction (see Section 3.9).

Figure 3.5 Example pile head load/displacement relationship for repetitive loading in a normally consolidated clay (as measured in LDP tests, Pentre site, Paper 13, pg 283, Clarke et al [1])

Figure 3.6 Example pile head load/displacement relationship for repetitive loading in an overconsolidated clay (as measured in LDP tests, Tilbrook Grange site, Paper 13, pg 283, Clarke et al [1])

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When the stage of full mobilisation of the base resistance or ultimate base resistance Rb is reached (i.e. at some point D in Figure 3.2), the pile will settle at an increasing rate under only very small further increases of load (near to the ultimate pile resistance asymptote). Extended loading periods during pile tests indicate that it is very difficult to achieve the ultimate axial compression resistance, because the curve becomes virtually flat, and to reach the asymptote requires very large settlements, (see Figure 3.1). However, a pile load test in soil should aim to achieve within about 5% of that value and accepted practice for friction piles is to use the load resistance reached at a tip movement of about 30 to 50 mm.

3.7 Pile-soil load transfer friction piles The process of driving a steel pile in clays and sands produces a thin layer of completely remoulded soil adjacent to the pile shaft wall that acts as a slip and load-transfer layer; its behaviour is now well understood as a result of research on trial piles (Reference Tomlinson[25]; and Clarke et al[1]). If strain gauges are installed at various points along the steel pile shaft, the compressive load remaining in the pile can be measured at each level; the distribution of load in the pile is found to be in the form of that shown in Figure 3.7 (which shows the transfer of load from the pile to the soil at each stage of loading identified in Figure 3.2).

Thus when loaded to point A in Figure 3.2, the whole of the load is carried by skin friction on the pile shaft and there is no transfer of load to the base of the pile (Figure 3.7(a)). When the load reaches point B, most of the pile shaft friction is mobilised and the pile base has started to feel load (Figure 3.7(b)). At point D, there has been no further increase in the load transferred in wall friction but the base load will have reached its maximum value (Figure 3.7(c)), i.e. the ultimate pile bearing capacity is reached, beyond which the pilehead will move down vertically under nearly constant load.

s

s sb

bb

Fcc c

'ULS deflectionfailure' loadon pile F =R c

c s (c) F =R =R + Rcc s b s bc

R R R

bBase of pile

Full baseresistance R

Base reaction R


3.8 Load / settlement behaviour end-bearing piles

If the pile base is in dense sand or rock, the base or end bearing resistance can be developed with very little base movement and the compression of the pile shaft is often insufficient to mobilise the full potential shaft friction resistance in the soils overlying the rock layer. Often the pile head moves only due to elastic compression of the steel wall up to the predicted ultimate pile resistance since there is negligible set at the base, see Figure 3.8. The total pile head movement will obviously be dependent on the pile length required to reach the rock or other dense bearing stratum and the design is governed by steel material strength and not by rock strength.

In many end-bearing piles, the pile base resistance will be controlled by structural design considerations to limit the stress in the steel wall so as to prevent local yield or buckling during driving and not governed by deformation or allowable bearing pressure in the rock at the pile tip.

A suggested approach for economic design of such piles is explained in Sections 2.5 and 5.6.

3.9 Pile-soil load transfer end bearing piles The pile-soil load transfer diagrams for end bearing piles are very different to those for friction piles and a typical generic diagram is shown in Figure 3.9. It shows the load transfer from the pile to the soil and rock, at points A, B, and C on the pile head load displacement curve shown in Figure 3.8.

At point A, proportions of the pile load are taken in both shaft friction and end bearing because the high stiffness at the tip causes reaction at very little pile head movement. By point B, more shaft friction may be developed, but a greater proportion will be carried by the pile base. And by point C, the full pile base resistance has been reached whilst the pile may start to deform plastically due to local yielding near to the pile tip which is the onset of structural failure. In the Figure3.9, Rb denotes a portion of the ultimate base resistance, and Rs denotes the portion of the ultimate shaft friction that is available.

0

Rc

c

A

B

C


3.10 Site investigation There are many good publications and references that can be used to decide the scope of soil tests in a site investigation, but very few address the specific requirements in respect of steel bearing piles. It has been found that in situ testing of soils is particularly relevant to all types of driven pile.

Soil testing should apply a method of loading to soils that resembles as closely as possible the type of loading that is to be applied by the pile to the soil. In this respect, soil tests are therefore required to provide the properties relevant to predicting the response of soil to the various phases of construction, namely:

Pile driving. Pile loading during construction. Pile static loading during working life. Pile live loading (transient) during working life.

3.10.1 Soil test data for design Granular soils

In situ soil testing should comprise the use of the following:

The Standard Penetration Test (SPT) as specified by BS 1377-9: Methods of test for soils for civil engineering purposes: Part 9: In situ tests[26], is a universal test applicable to all types of granular soil for which it has been extensively calibrated for the prediction of pile driving resistance, shaft friction and end-bearing correlations.

The Cone Penetration Test (CPT) also specified by BS 1377-9, has been extensively calibrated against steel pile design parameters in fine grained granular soils (sands, silts and clays). An explanation of the interpretation of CPT test results to derive soil design parameters is contained in Cone Penetration testing in geotechnical practice[108].

c

R b

c

R

R

s

b

c

Rb

c(b)c(a) c c(c)bs bs

R s Rs

F = R = R + R

F = R = R + R

F =R = R +Rs bc c

FF F

Figure 3.9 Compressive load transfer, tip in rock

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The Dilatometer Marchetti Test (DMT), used to determine the in situ earth pressure coefficients and confined modulus of soils for use in estimates of lateral soil resistance to applied displacement or force.

The pressuremeter, used extensively in France and increasingly being applied in the UK to derive in situ soil properties relevant to driven piles.

Laboratory testing should include:

Saturated and unsaturated bulk densities (unit weight). Shear box tests to determine the angle of internal friction (). Particle size distribution classification tests. Cohesive soils

For cohesive soils, the geotechnical pile design and resistance prediction methods for axial loading generally rely on correlations of pile behaviour with the undrained cohesive strength cu, but care should be taken to select the soil strength at a consistent strain to failure. This has been addressed in Norwegian and offshore specifications for triaxial soil testing and is taken as the strength at failure or at a strain of 4%, whichever occurs first.

For lateral loading and retaining wall design, the geotechnical methods for limit state design now require the following deformation and stiffness properties:

Youngs Modulus (E50 and initial tangent modulus). Poissons ratio. Coefficients of subgrade reaction and horizontal subgrade reaction. For earth pressure calculation the geotechnical methods for limit state design require the following properties:

Coefficient of earth pressure at rest, Ko. Coefficients of active and passive earth pressure (Ka and Kp). Con

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