Recent developments in axial design of driven offshore piles. DGF Copenhagen April 1 st 2014

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1 Recent developments in axial design of driven offshore piles DGF Copenhagen April 1 st 2014 Richard Jardine Page 1 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- Page displacement 2 Imperial predictions College London 1

2 New rules for static axial capacity in silica sand: 2011 API recommendations Before: had τ = K σ vo tanδ for shaft, q b = N q σ vo for base. Limits to τ, q b and values for δ, N q depend on D 50 and D r Now: modified to β = K tanδ, no loose sand case Recognises: API main text method s significant bias and poor reliability: Q calculated /Q measured ratios = Q c /Q m subject to CoV ~ 65% Recommend: completely different CPT based methods, including ICP. Note need for different SI & specialist staff Page 3 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 Page 4 2

3 Research background: Critique of conventional approach & how new CPT methods were derived Page 5 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 Page 6 3

4 Shaft capacities from 81 tests in sand; Jardine et al (2005) Qc /Qm API: skewed for relative density, pile length and tension loading API SP SP Pile type & 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 H SP EU EU EU EU EU ICP Relative density, D r (%) H HBD DK DK G Qc /Qm API SP SP H DK H SP H DK EU HO EU EU EU EU EU L/D ICP BD Pile failures at Hound Point and Sungai Perak Bridge Williams et al (1997) ICP shows no skewing (tension solid) Average pile age = 25 days Qc /Qm SP EU EU SP H BD EU SPH EU EU EU DK DK Qc /Qm SP H SP SP H DK H EU EU DK EU EU BD EU EU Page Relative density, Dr (%) L/D 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 D r? What controls the friction angle δ? Any missing key variables? Is tension loading different to compression? Does shallow foundation N q apply to end-bearing? Do any limits apply to τ and end bearing pressure q b? Page 8 4

5 Background IC research with instrumented piles 4.0 lagging instrument cluster, h/r=72 Closed-end, 102mm OD; up to 20m long SSTs measure local σ r and τ Bond, Jardine and Dalton (1991) Distance from pile tip, h (m) trailing instrument cluster, h/r=50 following instrument cluster, h/r=27 Intensive testing at 2 sand and 4 clay sites 0 Page 9 surface stress transducer pore pressure probe axial load cell leading instrument cluster, h/r=8 ICP Configuration for Labenne tests, SW France Definition of stresses and tip parameter - h 5

6 Geotechnical profile: Labenne, after Lehane (1992) Loose dune sand, including thin organic layer Labenne end bearing Measured base resistance q b and CPT q c 6

7 Local s r during penetration at Labenne K not constant but varies with: Effect of h/r Sand state - q c Pile tip depth (h/r) Combined effects of q c and h/r on σ r ICP tests at Labenne: Lehane et al (1993) 7

8 Depth (m) 10/04/2014 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 CPT q (MPa) CPT f (kpa) C C Page Influence of CPT q c and pile tip position on σ r Dunkerque, after Chow (1997) API σ r varies with q c For fixed depth σ r falls with h/r Page 16 8

9 Possible causes for h/r influence on local stresses along pile length; Chow (1997) Heave Whip Relaxation as tip stress concentration moves away Extreme driving cycles Page 17 After Friction Chow (1997) fatigue? Load cycles degrade shaft capacity Can recover with time Not true fatigue Qcyclic / Qmax static First failure Cyclic failure after previous cyclic or static failure Aged pile, no previous failure Aged pile, after previous failure Field tests on 457mm OD, 19m long steel pipe piles, Dunkerque >200 Nf Datapoint number = N f > indicates unfailed by cycling >1000 See recent keynotes on cyclic design: -0.2 Jardine et al (2012) Andersen et al (2013) Qaverage / Qmax static Page 18 9

10 Delft University photoelastic particulate test rig Page 19 Tip installation stress focus and bearing failure Page 20 10

11 Dunkerque: loading response & ICP effective stress paths, similar patterns to Labenne Base q b q c not related linearly to σ v0 σ r varies under load Tension compression Δσ rd = 2G δr/r D r affects response through G δ cv not affected by D r δ cv angles: sand-on-steel interface ring-shear tests Intact sand Crushed sand Ho et al (2010) Steel interface 11

12 Interface shear δ cv : silt-to-fine gravel, Interfaces with roughness of industrial piles Ring shear Direct shear trend Direct & ring shear: 10/04/2014 Barmpopoulos et al (2009), Ho et al (2010) Full ICP design principles: closed ended piles Radial shaft stresses σ r = A q c (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 τ f /σ r = tan δ cv with δ cv from interface lab tests Base capacity q b linked to CPT q c diameter dependent No upper limits to τ f or q b - care needed in variable profiles How to deal with open ended piles? Page 24 12

13 Depth (m) Depth (m) 10/04/2014 Generalisation of ICP shaft expressions to open ended piles Choices considered by Chow (1997) B2: R* = (R 2 o R 2 i) 0.5 Page 25 Assessment by Chow (1997) Five hypotheses checked with instrumented openended (325mm) Dunkerque pile Peak shear stress (kpa) Pile CS T'89a 2 C'89a Prediction B2 choice considered most practical Re-checked against full scale data base, adopted for MTD-96 Later UWA-05 follow alternative A2 route Page A2 Scalar reduction based on IFR Peak shear stress (kpa) Pile CS T'89a C'89a Prediction B2 h/r term revised with R* defined by solid area of pipe pile

14 Full ICP: checks for possible wall thickness ratio bias Shaft capacity of open piles in sand: IC data base Pile type & test direction Steel, open-ended, tension Steel, open-ended, compression Concrete, open-ended, tension Qc/Qm Page D/t Pile plugs and open-end bearing: diameter dependence Plug q b falls with diameter D ICP q b /q c = f(d) IFR rises sharply with D Because of interface shear scale effect Page 28 14

15 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? Page API commentary methods: NGI-05 Sliding triangle approach to capture h/r effects, z = depth τ = z/z tip P atmospheric F D F s F t F l F m z/z tip term not normalised by D or affected by Length L F factors depend on: D r ; σ v0 ; loading sense; pile type Base q b does not vary with D, τ unaffected by L/D Terms fitted from NGI data base, checked against 28 high quality load tests Page 30 15

16 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 = 29 o for all cases Base capacity not affected by D Page 31 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 q c (A r ) b (h/d) c Effective area A r depends on Incremental Filling Ratio (IFR) to define stresses near tip τ varies with L/D Full and more conservative offshore variants Page 32 Choices for a, b, c checked against large database 16

17 qb/qc 10/04/2014 UWA-05 & ICP-05 end bearing for plugged piles vary with D UWA q b /q c depends on Final IFR, which varies with D Open ended piles in sand, taking D/t = ICP-05 UWA Page Imperial College 1London Outside diameter, m 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 Q c /Q m ± CoV API ± 0.67 NGI ± 0.37 Fugro ± 0.38 (full) ICP ± 0.30 (full) UWA ± 0.27 (measured IFRs) Similar data-base results to Jardine et al (2005) Page 34 New studies in hand: IC & ZJU, UWA and new NGI JIP 17

18 Applications: all with full ICP approach Page 35 Sungai Perak, Western Malaysia; Williams et al 1997 Balanced cantilever bridge on 1.5m OD driven steel piles API design 33m penetration had to be doubled after tests Pile tests 1&2 3 4 Average results Medium-dense gravelly sands API: Q c /Q m = 1.99 ICP: Q c /Q m = 1.10 Page 36 18

19 Dense North Sea sand EURIPIDES - Holland Kolk et al (2005) 760mm OD heavily instrumented steel tubulars Average results: API: Q c /Q m = 0.58 ICP: Q c /Q m = 0.97 ICP predictions (thick lines) fit tests at 3 L/D values, no skew or bias with L/D Page 37 Now widespread wind energy applications Overy 2007 Page 38 Piled tripods for Borkum West II German N. Sea Merritt et al

20 North Sea track record since 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 Page 39 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 Page 40 20

21 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 Page st time tension tests at Dunkerque Creep important at Q > 1MN 81 days 235 days 9 days 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 21

22 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 Page 43 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 Page 44 Multiple soil stress Imperial College cells London installed in sand mass 22

23 Experiments on NE34 Fontainebleu sand: Yang et al (2010) CPT cone resistance, q c 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 23

24 Schematic development of Zones 1 to 3 Related to stress regime in: Crushing area beneath tip Degradation over shaft length Microscope images: progressive grain crushing (a) Fresh sand (b) Zone 1 sand (c) Zone 2 sand (d) Zone 3 sand 24

25 Shear stress (kpa) h / R h / R 10/04/2014 Stresses in soil mass during penetration (and at rest) σ r normalised by CPT q c, % Similar plots for σ θ and σ z Jardine et al (2013) r / R r / R 1.0 Local stress paths at Leading pile instrument One cycle towards end of installation (c) 1 st P.T. o peak Peak load load 50 0 Unloading nd P.T. End end Point point o Radial stress (kpa) start Start point of push 25

26 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 Q s 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 q b design Limit q c to 100 MPa in North Sea sands, beware tip buckling Page Address 51 cyclic loading Imperial College London 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, S t, 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 I P 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; q b related directly to CPT q c Page 52 26

27 Microscope thin section through clay around piles at Pentre; Chow (1997) Pile shaft Principal displacement shears Reidel shears Residual shearing mode, unusually low δ for given I p More common with plastic clays Page 53 Interface friction angles for piles driven in clay Measure δ max, δ min in ring shear interface tests, can be surprising! Page 54 27

28 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 Page 56 Lyon 2003 Atlanta 2008 Seoul

29 Advanced stress-path triaxial equipment 200 x 100mm samples Accurate cyclic loading Longer term creep tests Interactions with cycling Higher resolution strain gauges Multi-axial BE systems Jardine 2013 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 Page 58 29

30 Depth m Depth, m 10/04/2014 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 (Y 1 ) elastic and outer (Y 3 ) yield surfaces G/p = f(ε D ) K/p = g(ε vol ) 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 59 Dunkerque anisotropic stiffness profiles: lab and field Elastic stiffness, MPa MPa Anisotropic Y1 stiffness profiles: Dunkerque 5 Legend: Eu from TXC tests E`v from TXC tests E`h from TX tests Gvh from TX BE tests Ghh from TX BE tests Gvh from field seism. CPT tests Page 60 Field seismic & lab G vh measurements agree within 10% 25 30

31 G/p' Pile head load, Q (MN) 10/04/2014 Dense sand non-linear secant shear stiffness data OCR=1, other tests at different OCRs Legend: Curve used for FE analysis TC test curve OCR=1 TE test curve OCR=1 TS test curve for OCR= Page es, % Non-linear ICFEP predictions for tension test 19m, 457mm OD, steel pipe pile at Dunkerque Shaft σ rc from ICP-05 CPT approach Estimates for other soil σ components Non-linear stiffness and interface shear from lab tests Pile resistance, Q (MN) Good for capacity & working load stiffness Legend: predicted - ICFEP observed Jardine et al 2005b Pile head Pile cap displacements, displacement, δ (mm) 31

32 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 Page 63 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 Page 64 32

33 Acknowledgments Sponsors & partners: BP, BRE, IFP, EPSRC, Exxon HSE, Shell, INPG 3S-R group, Total and others Current and former coworkers: Andrew Bond, Fiona Chow, Reiko Kuwano, Barry Lehane, Siya Rimoy, Jamie Standing, Zhongxuan Yang, Bitang Zhu and many others Page 65 Pierre Foray