New Design Methods for Offshore Piles

48 Pages·1996·2.554 MB·48\48

by  Jardine, R.J., Chow, F.C.

NEW DESIGN METHODS
FOR
OFFSHORE PILES
R J Jardine and F C Chow
Department of Civil Engineering
Imperial College
London S W2 BU
Publication 96/103
Published by MTD
The Marine Technology Directorate Ltd
Rwstered in England No 2022686
Rwstered ORice:
I9 Buckingham Street
London WC2N 6EF
UK
Rqstered as a charity under the Charities Act 1960
Rqstered Charity No 295576
OMTD 1996
ISBN 1 870553 31 4
Printed by Expedite Graphic Ltd, London SWI
CONTENTS Page
1 SUMMARY 7
2 INTRODUCTION
2.1 Rationale for Developing New Design Approaches 7
2.2 liiiperial College Research Programme 8
3 DESIGN METHODS FOR PILES IN SAND
3.1 lntroduction 11
3.2 Shaft Friction 11
3.3 Base Resistance 15
3.4 Group Effects 16
3.5 Effects of Time 17
3.6 Cyclic Loading 18
4 VALIDATION OF THE PROPOSED NEW DESIGN METHOD FOR
PILES IN SAND
4.1 Shaft Friction 19
4.2 Base Resistance 24
5 DESIGN METHODS FOR PILES IN CLAY
5.1 Introduction 29
5.2 Shaft Friction 29
5.3 Base Resistance 33
5.4 Group Effects 34
5.5 Effects of Time 34
5.6 Cyclic Loading 35
6 VALIDATION OF THE PROPOSED NEW DESIGN METHODS FOR
PILES IN CLAY
6.1 Shaft Friction 37
6.2 Base Resistance 41
6.3 Layered Soil Profiles 42
4 New Design Methods for mshore Piles (MTD 96/103)
7 CONCLUSION
7.I Main Points 43
7.2 Check List for Sands 43
7.3 Check List for Clays 43
APPENDIX I - BtBLIOGRAPHY OF SUPPORTLNG RESEARCH 44
APPENDIX 11 - KEY PAPERS ON IC RESEARCH 45
APPENDIX TI1 - OTHER CITED REFERENCES 47
APPENDIX IV - LIST OF NOTATfON 48
New Design Methods for Offshore Piles (MTD 96/103) 5
I SUMMARY
An extended progranuiie of research by a group from Imperial College, London has led to new methods for
assessing the axial capacity of offshore piles. Following the introductory conuiients offered below, th~s
document summarises the new procedures and provides:
A description of the new methods sufficient to allow pramsing engmeers to apply them
0
A demonstration that they offer considerable theoretical and practical advantages over existing
0
methods
Full validation of the methods for offshore conditions, showing that they are far more reliable and
0
accurate than existing methods
Commaits on how pile age, group action and other factors could influence field performance
0
The report sunuiiarises and puts in context the research findings that led to the new methods. For detailed
descriptions of the experimental work, background theory and validation of results the reader is referred to
Appendix 1 which lists the key PhD theses, OTH reports and papers relating to the research. Appendices U
and IU contain the 111 references for cited publications by others, while Appendix W defines the notation and
symbols used in the document.
2 INTRODUCTION
2.1 Rationale for Developing New Design Approaches
Predictions from current empirical methods for assessing the axial capacities, Q, of displacement piles are
relatively unreliable. As detailed later, the Coefficient of Variance (COW' in (QJQllflo)r
ecalctllaia~Jelllzast~~l
test piles analysed according to the API RP2A 20th edition (1 993) offshore reconuiiendations falls between
0.5 - 0.7.
These hgh COV values are not compatible with the relatively low safety margins (typically 1.5) adopted for
most offshore pile designs.
At the same time, many practitioners consider the existmg methods to be over-conservative in certain
circumstances. For example, it is recogused that in sand the MI RP2A approach leads to strong and
systematic skewing of QJQll, with respect to pile slenderness and sand relative density, leading to under-
predicted capacities for short piles in dense strata.
Foundation problems are relatively rare among the existing population of piled offshore structures. However,
a clear need exists to improve predictive methods to obtain economies in some cases and enhance
performance, and safety, in others. The implenientation of new niethods needs to be co-ordinated with any
parallel developments in the characterisation of environmental loading, with account being taken of any trends
towards structural designs which impose different patterns of loading on their foundations.
I The Coefficient of Variation (COW is defined as the standard deviation, s, divided by the mean value, p.
New Design Methods for Offshore Piles (MTD 961103) 7
I SUMMARY
An extended progranuiie of research by a group from Imperial College, London has led to new methods for
assessing the axial capacity of offshore piles. Following the introductory conuiients offered below, th~s
document summarises the new procedures and provides:
A description of the new methods sufficient to allow pramsing engmeers to apply them
0
A demonstration that they offer considerable theoretical and practical advantages over existing
0
methods
Full validation of the methods for offshore conditions, showing that they are far more reliable and
0
accurate than existing methods
Commaits on how pile age, group action and other factors could influence field performance
0
The report sunuiiarises and puts in context the research findings that led to the new methods. For detailed
descriptions of the experimental work, background theory and validation of results the reader is referred to
Appendix 1 which lists the key PhD theses, OTH reports and papers relating to the research. Appendices U
and IU contain the 111 references for cited publications by others, while Appendix W defines the notation and
symbols used in the document.
2 INTRODUCTION
2.1 Rationale for Developing New Design Approaches
Predictions from current empirical methods for assessing the axial capacities, Q, of displacement piles are
relatively unreliable. As detailed later, the Coefficient of Variance (COW' in (QJQllflo)r
ecalctllaia~Jelllzast~~l
test piles analysed according to the API RP2A 20th edition (1 993) offshore reconuiiendations falls between
0.5 - 0.7.
These hgh COV values are not compatible with the relatively low safety margins (typically 1.5) adopted for
most offshore pile designs.
At the same time, many practitioners consider the existmg methods to be over-conservative in certain
circumstances. For example, it is recogused that in sand the MI RP2A approach leads to strong and
systematic skewing of QJQll, with respect to pile slenderness and sand relative density, leading to under-
predicted capacities for short piles in dense strata.
Foundation problems are relatively rare among the existing population of piled offshore structures. However,
a clear need exists to improve predictive methods to obtain economies in some cases and enhance
performance, and safety, in others. The implenientation of new niethods needs to be co-ordinated with any
parallel developments in the characterisation of environmental loading, with account being taken of any trends
towards structural designs which impose different patterns of loading on their foundations.
I The Coefficient of Variation (COW is defined as the standard deviation, s, divided by the mean value, p.
New Design Methods for Offshore Piles (MTD 961103) 7
2.2 Imperial College Research Programmes
2.2.1 Research Aims
The lmperial College research has been aimed towards achieving: (i) a more fundamental and thorough
understanding of pile behaviour, and (ii) practical simple design methods that capture the basic mechanics of
driven piles. The main tasks were to identify:
How piles behave in different soils and layering sequences
0
The scaling laws that relate the behaviour of models to that of full-scale piles
0
0 The effects on capacity of pile properties (dimensions, wall th~ckness,e nd conditions, surface
roughness, material hardness, etc.) and installation methods
Any changes in capacity and stiffuess associated with time after pile installation
0
The response to different loading types, including group effects and cyclic loading
0
The controlling soil parameters that should be measured in site investigations.
0
2.2.2 Field Tests with Instrumented Piles
Until recently, the stress conditions surrounding driven piles have been open to conjecture. A central feature
of the lmperial College research has been the development of accurate and reliable on-pile instrumentation to
study the pore pressures, radial total stresses, local shear stresses and temperatures developed on pile shafts.
The gauges were mounted on 6 - 20 m long, 102 mm diameter, closed-ended2 steel pipe piles (termed ICPs)
and used in intensive test programmes involving a wide range of geomaterials between 1986 - 1994 at the six
sites identified in Table 1 and Figure 1.
The lCPs were installed by fast jaclung, allowing comprehensive measurements of the effective stress
conditions developed close to the shafts to be made at multiple levels during installation, long-tenn
equalisation and load testing to failure.
Detailed site investigations were also performed, involving in-situ tests and advanced laboratory experiments.
“Strain Path Method” numerical simulations of the ICP tests performed at Canons Park and Bothkennar were
also carried out in conjunction with Professor A. Whittle from MIT as described by Bond (1 988) and Lehane
(1 992).
The Pentre piles (Figure 1) were installed close to the large-scale driven piles (LDP) described by Clarke
(1993); tests on less intensively instrumented open-ended driven piles were conducted at Canons Park,
Cowden and Dunlurk.
’The use of closed-ended piles allowed more accurate and robust instnimentation to be deployed.
8 New Design Methods for Offshore Piles (MTD 96/103)
TABLE 1. SUMMARY OF RECENT IMPERIAL COLLEGE PILE RESEARCB SITES
Site Soil conditions
~~ ~ ~~~~ ~ ~~~~ ~~
I. Canons Park London clay: stiff to very stiff, high plasticity, Eocene niarine clay;
hgh YSR
2. Cowden Cowden till: stiff to very stiff, lean, glacial lodgement till; high YSR
3. Bothkennar Carse clay: soft, hgh plasticity, moderately orgamc, Holocene
II
marineestuarine clay-silt, lightly ceniented; moderate YSR
1
I
Labenne Dune sand: loose to niediuni dense, medium-sized, Holocene; low
YSR
4.
5. Pentre Glacio-lacustrine clay-silt and laminated clays: very soft to fimi, low
plasticity; low YSR
6. Dunkirk Marine sand: dense to very dense, shelly medium-sized sand,
Flandrian; low YSR
2.2.3 Results from Phases 1,2 and 3
The research has taken place in three phases. The first involved developing the ICP instruments and
experimental procedures, and perfomling multiple ICP tests and other experiments at the Building Research
Establishment’s @RE) Canons Park test site. The research was sununarised by Bond (1 988) and Bond and
Jardine (1 990).
The scope was broadened in Phase 2 to cover tests in sand at the French Ponts et Chausseks test site at
Labenne, the BRE’s stiff till site at Cowden, and the Enpeering and Physical Sciences Research Council’s
(EPSRC) national soft clay test site at Bothkmar. At each location an advanced site investigation was
performed, a field pile testing facility established, and a programme of multiple (closed-ended) ICP tests
carried out, Clear and strikmg results emerged from the experiments whch allowed new design approaches to
be proposed for closed-ended piles. The Phase 2 work was reported by Lehane (1992) and Jardine and
Lehane (1 994).
The third phase, whch has recently been completed, involved:
I. Establishing test facilities and performing advanced site investigations and multiple ICP tests at the
Pentre (clay-silts/Iaminated clays) LDP research site and at the Dunlurk “CLAROM” dense sand
research site.
2. Interpreting and performing tests on hll-scale driven open-ended piles (with diameters up to
760 nun) at the ICP sites to assess the efFects of scale, installation methods and pileend conditions.
New Design Methods for Offshore Piles (MTD 96/103) 9
3. Experiments to assess pile group and ageing effects in dense sand.
4. Using the above to refine the new approaches for closed-ended piles and extend the design methods
to cover open-ended driven piles.
5. Collating an up-to-date and critically approved database of full-scale pile tests that met rigorous
quality criteria.
6. Using the above to calibrate and validate the new methods for a wide range of practical applications.
The work is reported by Chow (1 996) and Chow and Jardine (1 996).
Figure I. Locations of ICP test sites.
10 New Design Methods for Offshore Piles (MTD 96/103)
3 DESIGN METHODS FOR PILES IN SAND
3.1 Introduction
The following sections describe the new design methods for piles in sand. Shaft friction and end bearing are
covered under separate headings, followed by brief sections discussing group action and the effects of time.
3.2 Shaft Friction
3.2.1 Basic Mechmisms
The ICP experiments at Labeme and Dunkirk showed that at failure the local shear stresses acting on the pile
shaft, zt; follow the simple Coulomb failure criterion:
s,
71‘ = Q’d. tan
The radial effective stress a&g on the shaft at failure, depends on drCth,e value acting after installation
and full pore pressure and radial stress equalisation, combined with any changes developed during pile
loading. The tan &-tern1r epresents the critical-state sand-interface angle of friction, which is developed when
the soil at the interFace has ceased dilating or contracting. The external shaft capacity is obtained by
integrating zl.over the external pile area.
The experiments at Dunlurk showed that for a given set of conditions, zI‘a nd CF’J increased when: (i) other
displacement piles were installed nearby, and (ii) the piles were allowed to age in situ. The design method
outlined below covers the medium-tenn shaft capacity of single piles, as developed in first-time tests
performed 50 days3 after driving. Separate conunents are given later on the additional effects of age and
group action.
3.2.2 Evaluating Medium-Term Shafi Capacity of Single Piles
Table 2 sets out the key steps required to evaluate shaft capacity, Qs,at a nominal 50 days after driving. The
inain points to note are:
I. Steps A2 and A6 gve the key equations for calculating zf. Both are crucially dependent on Q’,~,
which is evaluated at gven points along the shaft by following Steps A3 or B3, depending on the
pileend conditions‘.
2. In contrast to the 1993 APT reconuiiendations, drCva:r ies strongly with sand relative density, as
reflected by the local CPT tip resistance, q,. It is also highly sensitive to the relative pile tip depth, h,
as defined in Figure 2. The radial effective stresses developed at any gven depth decline sharply as h
increases. This feature leads to the method reproducing .the well-known tendency for average q to
reniain quasi-constant once a “critical depth” has been exceeded, without specifjmg any arbitrary
limits to zf.
‘50 days was the typical “set-up” time allowed in the test database collated by Chow ( 1996)
3T he equation for ofrcis not identical to that proposed provisionally by Lehane and Jardine (1994). It is
more sensitive to hlR and less affected by d,,~.
New Design Methods for Offshore Rles (MTD 96/103) 11