Recent developments in axial design of driven … driven offshore piles DGF Copenhagen April 1st...

10/04/2014

1

© Imperial College London

Recent developments in axial design

of driven offshore piles

DGF Copenhagen

April 1st 2014

Richard Jardine

© Imperial College London Page 2

Themes:

• Changed API, ISO axial capacity recommendations

for sands, including ICP-05

• Research background leading to new methods

• Applications, case histories, some surprising results

• New research: ageing, cyclic and lateral loading

• Next set of issues: clay methods & improving load-

displacement predictions

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2


New rules for static axial capacity in silica

sand: 2011 API recommendations

• Before: had τ = K σ´vo tanδ for shaft, qb = Nq σ´vo for base. Limits to τ, qb and values for δ, Nq depend on D50 and Dr

• Now: modified to β = K tanδ, no loose sand case

• Recognises: API main text method’s significant bias and poor reliability:

Qcalculated /Qmeasured ratios = Qc/Qm subject to CoV ~ 65%

• Recommend: completely different ‘CPT’ based methods, including ICP. Note need for different SI & specialist staff


The axial capacity prize:

Feedback from one UK

wind-farm design team

Critical economies through

ICP-05 or UWA-05 sand

axial capacity methods

Also applied in onshore civil

engineering: Williams et al 1997

Derived from field ‘ICP’ research

Lehane et al 1993

Chow 1997

Jardine et al 2005

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Research background:

Critique of conventional approach &

how new ‘CPT’ methods were derived


MTD and ICP methods

First proposed in 1996,

extended in 2005 to cover:

Group action

Pile shape

Seismic effects

‘Special and problem’ soils

Factors of safety

Ring shear test methodology

Ageing

Cyclic loading

Databases: mini-to-mega piles

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Shaft capacities from 81 tests in sand; Jardine et al (2005)

0 10 20 30 40 50 60 70 80 90 100

Relative density, Dr (%)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Qc /Q

m

DK

EU

EU EUEU

H

H

BD

GDK

EU EU EUEU

H

SP

SP

SP

Pile ty pe & test direction

Steel, closed-ended, tension

Steel, closed-ended, compression

Concrete, closed-ended, tension

Concrete, closed-ended, compression

Steel, open-ended, tension

Steel, open-ended, compression

Concrete, open-ended, tension

0 20 40 60 80 100

L/D

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Qc /Q

m

DK

EU

EU EUEU

H

H

BD

DK

HOEU EU EUEU

H

SP

SP

SP

0 10 20 30 40 50 60 70 80 90 100

Relative density, Dr (%)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Qc /Q

m

DKEU

EU

EU

EUH

HBD

DK

EU

EU

EU

EUEUEU

DK

HSP

SP

SP

0 10 20 30 40 50 60 70 80 90 100

L/D

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Qc /Q

m

DKEU

EU

EU

EUH

HBDEU

EU

EU

EUEUEU

DK

HSP

SP

SP

API: skewed for relative density, pile length and tension loading

API API

ICP ICP

Pile failures at

Hound Point and

Sungai Perak Bridge

Williams et al (1997)

ICP shows no skewing (tension solid) Average pile age = 25 days


Critical review of ‘conventional theory’

• What controls shear (τ) and normal stress σ΄rf at failure?

• How does σ΄rf vary with σ΄v0 and local Dr?

• What controls the friction angle δ?

• Any missing key variables?

• Is tension loading different to compression?

• Does ‘shallow foundation’ Nq apply to end-bearing?

• Do any limits apply to τ and end bearing pressure qb?

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Background IC

research with

instrumented piles

Closed-end, 102mm

OD; up to 20m long

SSTs measure local

σr and τ

Bond, Jardine and

Dalton (1991)

Intensive testing at 2

sand and 4 clay sites

0

1.0

2.0

3.0

4.0

Dis

tan

ce fro

m p

ile t

ip, h

(m

)

surface stress transducer

pore pressure probe

axial load cell

leadinginstrument cluster, h/R=8

followinginstrument cluster, h/R=27

trailinginstrument cluster, h/R=50

lagginginstrument cluster, h/R=72

ICP Configuration for Labenne tests, SW France

Definition of stresses and tip parameter - h

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Geotechnical profile: Labenne, after Lehane (1992)

Loose dune sand, including thin organic layer

Labenne end bearing

Measured base resistance qb and CPT qc

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Local s′r during

penetration at

Labenne

K not constant but

varies with:

Sand state - qc

Pile tip depth (h/R)

Effect of h/R

Combined effects of qc and h/R on σ´r

ICP tests at Labenne: Lehane et al (1993)

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Page 15

Denser North Sea Marine sand Dunkerque France,

Chow (1997)

Borehole log

Very dense, light brown, uniform, fine to

medium, subrounded SAND with occasional shell fragments (Hydraulic fill)

GWL

Dense with shell fragments

(Flandrian Sand)

Organic layer

Dense, green-brown and grey-brown,uniform fine to medium, subrounded

SAND with some shell fragments(Flandrian Sand)

Becoming very dense

Dep

th (

m)

CPT q (MPa) CPT f (kPa)C C

0 10 20 30 40 0 100 200 300 4000

2

4

6

8

10

12

14

16

18

20

22

24


Influence of CPT qc and pile tip position on σ΄r

Dunkerque, after Chow (1997)

API

For fixed depth σ΄r falls with h/R

σ΄r varies with qc

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Possible

causes for h/R

influence on

local stresses

along pile

length;

Chow (1997)

After Chow (1997)

Heave

Whip

Extreme

driving

cycles

Relaxation as

tip stress

concentration

moves away

‘Friction fatigue’?

Page 18

Load cycles

degrade shaft

capacity

Can recover with time

Not true ‘fatigue’

See recent keynotes

on cyclic design:

Jardine et al (2012)

Andersen et al (2013)

-0.2 0 0.2 0.4 0.6 0.8 1

Qaverage / Qmax static

0.2

0.4

0.6

0.8

1

Qcyclic

/ Q

max s

tatic

First failureCyclic failure after previous cyclic or static failureAged pile, no previous failureAged pile, after previous failure

13

31

345

24

27221

>200

12

41

10

20

50

100

200

400

1

Nf

>1000

2069

Datapoint number = N f> indicates unfailed by cycling

Field tests on 457mm OD,

19m long steel pipe piles, Dunkerque

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Delft University photoelastic particulate test rig

Tip installation stress focus and bearing failure


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Dunkerque: loading response & ICP effective

stress paths, similar patterns to Labenne

Base qb≈ qc not related

linearly to σ΄v0

σ΄r varies under load

Tension ≠ compression

Δσ΄rd = 2G δr/R

Dr affects response

through G

δcv not affected by Dr

δcv angles: sand-on-steel interface ring-shear tests

Ho et al (2010) Steel interface

Crushed sand

Intact sand

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10/04/2014

Interface shear δcv: silt-to-fine gravel,

Interfaces with roughness of industrial piles

Direct & ring shear: Barmpopoulos et al (2009), Ho et al (2010)

Direct shear trend

Ring shear


‘Full’ ICP design principles: closed ended piles

• Radial shaft stresses σ΄r = A qc (σ΄v0)a (h/R)b

• Loading to failure alters σ΄r by factor that varies with 1/R

• Differences in tension and compression responses

• At failure τf /σ΄r = tan δcv with δcv from interface lab tests

• Base capacity qb linked to CPT qc – diameter dependent

• No upper limits to τf or qb - care needed in variable profiles

How to deal with open ended piles?

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Generalisation of ICP

shaft expressions to

open ended piles

Choices considered by

Chow (1997)

B2: R* = (R2o – R2

i)0.5


Assessment by Chow (1997)

• Five hypotheses checked

with instrumented open-

ended (325mm)

Dunkerque pile

• B2 choice considered

most practical

• Re-checked against full

scale data base, adopted

for MTD-96

• Later UWA-05 follow

alternative A2 route

-200 -150 -100 -50 0 50 100 150 200

Peak shear stress (kPa)

0

2

4

6

8

10

12

Dep

th (

m)

0

2

4

6

8

10

12

Pile CST'89aC'89aPrediction

-200 -150 -100 -50 0 50 100 150 200

Peak shear stress (kPa)

0

2

4

6

8

10

12

Dep

th (

m)

0

2

4

6

8

10

12

Pile CST'89aC'89aPrediction

A2 – Scalar reduction based on IFR

B2 – h/R term revised with R*

defined by solid area of pipe pile

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‘Full’ ICP: checks for possible wall thickness ratio bias

Shaft capacity of open piles in sand: IC data base

0 10 20 30 40 50 60 70 80 90 100

D/t

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5Q

c/Q

m

Pile type & test direction

Steel, open-ended, tension

Steel, open-ended, compression

Concrete, open-ended, tension


Pile plugs and open-end bearing: diameter dependence

Plug qb falls with

diameter D

ICP qb/qc = f(D)

IFR rises

sharply with D

Because of

interface shear

scale effect

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Debate over new API/ISO recommendations

• Agreement: ‘CPT based’ methods offer great potential

• Debate over how to scale up from closed ended

model piles to full scale

• API (2011) cites four approaches: ICP, Fugro, NGI

and UWA

• What are the differences? Practical evidence that the

‘full’ ICP and other methods work?


2011 API commentary methods: NGI-05

‘Sliding triangle’ approach to capture h/R effects, z = depth

τ = z/ztip Patmospheric FD Fs Ft Fl Fm

z/ztip term not normalised by D or affected by Length L

F factors depend on: Dr; σ΄v0 ; loading sense; pile type

Base qb does not vary with D, τ unaffected by L/D

Terms fitted from NGI data base, checked against 28 high

quality load tests

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2011 API commentary methods

Fugro-05, Kolk et al (2005)

ICP shaft expressions, coefficients revised to fit 37 steel pipe piles (some non-silica sand, some repeat tests) τ varies with L/D

Interface dilation neglected

Different expressions for tension and compression

δ = 29o for all cases

Base capacity not affected by D


API commentary methods

UWA-05; Lehane et al (2005)

Re-working of ICP, adopting Chow Set A2 approach

σ΄v0 term dropped and (h/D) term models ‘friction fatigue’

model, with modified exponenent

σ΄rc = a qc (Ar)b (h/D)c

‘Effective area’ Ar depends on Incremental Filling Ratio

(IFR) to define stresses near tip τ varies with L/D

‘Full’ and more conservative ‘offshore’ variants

Choices for a, b, c checked against large database

10/04/2014

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UWA-05 & ICP-05 end bearing for plugged piles vary with D

UWA qb/qc depends on Final IFR, which varies with D

Open ended piles in sand, taking D/t = 30

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 0.5 1 1.5 2 2.5 3

Outside diameter, m

qb

/qc

ICP-05

UWA-05


UWA data base study: Lehane et al (2005)

• 74 high quality tests after ≈ 30 days, silica sands, CPT profiles

• New CPT methods greatly reduce bias and scatter (CoVs)

• Overall: Mean Qc/Qm ± CoV

API-93 0.81 ± 0.67

NGI-05 1.11 ± 0.37

Fugro-04 1.11 ± 0.38

(full) ICP-05 0.95 ± 0.30

(full) UWA-05 0.97 ± 0.27 (measured IFRs)

• Similar data-base results to Jardine et al (2005)

New studies in hand: IC & ZJU, UWA and new NGI JIP

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Applications:

all with ‘full’ ICP approach


Sungai Perak, Western Malaysia; Williams et al 1997

Pile tests

1&2 3 4

Balanced cantilever bridge on 1.5m OD driven steel piles

API design 33m penetration had to be doubled after tests

Medium-dense

gravelly sands

Average results

API: Qc/Qm= 1.99

ICP: Qc/Qm= 1.10

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760mm OD heavily

instrumented steel

tubulars

Average results:

API: Qc/Qm= 0.58

ICP: Qc/Qm= 0.97

ICP predictions (thick

lines) fit tests at 3 L/D

values, no skew or bias

with L/D

Dense North Sea sand

EURIPIDES - Holland

Kolk et al (2005)

Page 38

Oil and gas: all N. Sea Shell’s installations since 1996

Now widespread wind energy applications

Piled tripods for Borkum West II

German N. Sea Merritt et al 2012

Overy 2007

10/04/2014

20


North Sea track record since 1996

• 13 installations of ICP designed foundations reported

Overy (2007)

• Encouraging correlations with driving SRDs and stress

wave matches in sand, clay and mixed profiles

• Substantial ± variations from conventional API, depending

on soil profile, pile details etc

• Engineering potential – facilitated new low-cost marginal

field options: Sayer & Overy (2007)

• Borkum West II wind-turbine tripods: see Merritt et al 2012


If conventional API is so unreliable, why are

offshore failures rarely reported?

• Almost no offshore static testing, limited driving monitoring

• Problems revealed by tests performed for ‘near-shore’ projects: Hound Point, Sungai Perak, Jamuna Bridge etc

• Systematic conservative bias in some conditions – such as very dense marine sands

• Unrecognised positive factors: shaft ageing characteristics in sand

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Page 41

Dunkerque

programme:

Dense marine sand

Eight steel pipe piles

457mm OD, 19m

Static & cyclic loading

9 days to 1 year after

driving

Jardine et al 2006

Jardine & Standing

2012

1st time tension tests at Dunkerque

235 days

81 days

9 days

Creep important at Q > 1MN

ICP capacity after ≈9 days

EoD shaft ≈ 0.63 ICP

Low driving base capacity

Ageing disrupted by pre-testing

Pile age after driving: a missing parameter

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New and ongoing research

Pile installation and stress system it creates

Ageing in laboratory and field

Cyclic axial loading

Layering and base capacity

Lateral loading response

Extending the field database


Long term calibration chamber tests in Grenoble

with new 36mm OD mini-ICP: Jardine et al (2009)

• 1.2 m ID, 1.5 m deep chamber

• On pile stress measurements

• Multiple soil stress cells installed in sand mass

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Experiments on NE34 Fontainebleu sand:

Yang et al (2010)

CPT cone resistance, qc

Critical depth

Shear zone

Crushing zone

Shear zone developed around the pile shaft,

Yang et al (2010)

Plan view Side aspect

Zone 1 material 0.5 to 1.5mm adheres to pile shaft

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Schematic

development of

Zones 1 to 3

Related to stress

regime in:

Crushing area

beneath tip

Degradation over

shaft length

1

2

3

Microscope images: progressive grain crushing

(a) Fresh sand (b) Zone 1 sand

(c) Zone 2 sand (d) Zone 3 sand

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Stresses in soil

mass during

penetration

(and at rest)

σ´r normalised

by CPT qc, %

Similar plots for

σ´θ and σ´z

Jardine et al (2013)

0.25

0.50

0.75

0.75

1.0

0.50

1.5

2.0

3.04.06.0

14

0.50

0.25

0 5 10 15 20-30

-20

-10

0

10

20

30

40

50

r / R

h /

R

0

4.0

8.0

12

16

20

0.500.75

1.0

1.5

2.0

2.0

1.5

3.0

1.0

1.0

4.06.0

1012

0.75

16

0 5 10-10

-5

0

5

10

r / Rh /

R

0

4.0

8.0

12

16

20

30

Local stress paths at Leading pile instrument

One cycle towards end of installation

0 50 100 150 200 250 300-150

-100

-50

0

50

100

150

o

2nd

P.T.

end point

She

ar

str

ess (

kP

a)

Radial stress (kPa)

start point

1st P.T.

o

peak load

(c)Peak load

Start of push

Unloading

End Point

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Interim conclusions from field & laboratory

• Driving analyses highly variable. Base capacity well below static estimates

• Shaft capacities build over time from low EoD values to far exceed ‘full’ ICP or UWA Qs estimates

• Installation stress regime promotes ageing, as may interface ‘crust’ and physiochemical effects

• Base resistance very sensitive to local variations – take lower bound CPT profile for qb design

• Limit qc to 100 MPa in North Sea sands, beware tip buckling

• Address cyclic loading in design & consider ICP clay method


ICP effective-stress clay approach:

• τ = σ΄rf tan δ, analogous to sand: σ΄rf/σ΄v0 = f (YSR, St, h/R*)

• Good predictions for ICP data base, reduces CoV and bias

• Applied since 1996, particularly in North Sea

• Needs different SI approach. Key issues, including low IP clays, debated at OSIG 2007

• Micro-fabric in shear zones is crucial, as in landslides. Altered by driving, promotes progressive failure

• Ring shear tests to measure δ; qb related directly to CPT qc

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Microscope thin section

through clay around piles at

Pentre; Chow (1997)

Residual shearing mode,

unusually low δ′ for given Ip

More common with plastic clays

Pile shaft

Principal displacement shears

Reidel shears


Interface friction angles for piles driven in clay

Measure δmax, δmin in ring shear interface tests, can be surprising!

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Load-displacement behaviour

Axial, lateral and moment monitoring of Magnus and Hutton

TLP foundations: errors of 400% in conventional T-z, P-y

predictive approaches

Far better fit with (Class A) non-linear “small strain” FE

predictions; Jardine and Potts (1988), (1993)

Central role in new DONG-led PISA lateral loading JIP

Widely used in onshore Civil Engineering: many conferences

and case histories


Advanced soil testing & non-linear modelling:

Six TC-29/101 conferences since 1994

Lyon 2003 Atlanta 2008 Seoul 2011

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200 x 100mm samples

Accurate cyclic loading

Longer term creep tests

Interactions with cycling

Higher resolution strain gauges

Multi-axial BE systems

Advanced stress-path

triaxial equipment

Jardine 2013

Page 58

Or, IC Resonant column HCA

Static mode

σ΄1 , σ΄2 , σ΄3 and αcontrol

Dynamic mode

Torsional resonant column

38/71 mm hollow cylinder

71/101 mm hollow cylinder

Static loading ram

torque system

and hydraulic

pressures

Oscillator (RC)

Specimen sizes

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Predictive tools

• FE code ICFEP, fully coupled includes range of possible

elastic-plastic soil models, can model progressive failure

• Simulate ‘small-strain’ behaviour by tangent stiffness

functions between (Y1) elastic and outer (Y3) yield surfaces

G/p΄ = f(εD)

K/p΄ = g(εvol)

• Fitted from lab tests, applied in 100s of projects

• Illustrate with simulations of tension tests on 19m long,

456mm OD Dunkerque steel piles driven in dense sand

Page 60

Dunkerque anisotropic stiffness profiles: lab and field

Anisotropic Y1 stiffness profiles: Dunkerque

0 100 200 300 400 500 600 700

Elastic stiffness, MPa

0

5

10

15

20

25

Dep

th, m

Legend:Eu from TXC testsE`v from TXC tests E`h from TX testsGvh from TX BE testsGhh from TX BE testsGvh from field seism. CPT tests

Elastic stiffness MPa

Depth

m

Field seismic & lab Gvh measurements agree within ≈ 10%

10/04/2014

31

Page 61 0.001 0.01 0.1 1

es, %

0

200

400

600

800

1000

1200

1400

G/p

'

Legend:Curve used for FE analysisTC test curve OCR=1TE test curve OCR=1TS test curve for OCR=1

Dense sand non-linear secant shear stiffness data

OCR=1, other tests at different OCRs

Non-linear ICFEP predictions for tension test

19m, 457mm OD, steel pipe pile at Dunkerque

0 5 10 15 20 25 30 35

Pile cap displacement, (mm)

0

500

1000

1500

2000

2500

Pil

e re

sist

an

ce, Q

(M

N)

Legend:

predicted - ICFEP

observed

Shaft σ′rc from ICP-05

CPT approach

Estimates for other

soil σ′ components

Non-linear stiffness

and interface shear

from lab tests

Jardine et al 2005b

Pile h

ead load, Q

(M

N)

Pile head displacements, δ (mm)

Good for capacity &

working load stiffness

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Conclusions

• Need for improved capacity methods highlighted

• Background instrumented pile & data base research reviewed

• New API/ISO ‘sand’ methods and practical application discussed

• Case histories demonstrate ‘full’ ICP is fit for purpose

• New factors highlighted, including strong time effects, base capacity in variable strata & cyclic loading


Conclusions

• Recent research outlined and interim conclusions noted

• Focus on stress regime and soil fabric around shaft

• Greatest practical impact with sands

• Key aspects of ICP clay effective stress approach outlined

• Way to improve pile-soil deformation analysis reviewed and illustrated

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Acknowledgments

Sponsors & partners: BP,

BRE, IFP, EPSRC, Exxon

HSE, Shell, INPG 3S-R

group, Total and others

Current and former co-

workers: Andrew Bond,

Fiona Chow, Reiko

Kuwano, Barry Lehane,

Siya Rimoy, Jamie

Standing, Zhongxuan

Yang, Bitang Zhu and

many others Pierre Foray

1949-2014

Recent developments in axial design of driven … driven offshore piles DGF Copenhagen April 1st...

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Transcript of Recent developments in axial design of driven … driven offshore piles DGF Copenhagen April 1st...