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