offshore technology report - oto 2000 007 · settlement at end of test. final relative movement of...

OFFSHORE TECHNOLOGYREPORT - OTO 2000 007

Pile Load Testing Performed for HSE CyclicLoading Study at Dunkirk, France

Volume 2

Date of Issue: July 2000 Project: 3687

This report is made available by the Health and Safety Executive as part of a series of reports of work which has been supported byfunds provided by the Executive. Neither the Executive, nor the contractors concerned assume any liability for the reports nor do

they necessarily reflect the views or policy of the Executive.

Pile Load Testing Performed for HSE CyclicLoading Study at Dunkirk, France

Volume 2

Prepared by

R J Jardine & J R Standing for IconDept of Civil & Environmental EngineeringImperial College of Science & Technology

CONTENTS

60REFERENCES6.

58SUMMARY5.

53Comments on effects of cyclic loading4.6

52Comments on combined effects of age and pre-testing to failure4.5

44Allowing for variations in ground conditions4.4

43Comments on the GOPAL compression tests4.3

Allowing for variations in ground conditions4.242Introduction4.1

42PRELIMINARY INTERPRETATION4.

17Graphical results3.37Tabular summary of main results3.27Introduction3.17PILE LOAD TEST RESULTS3.

6Programme undertaken2.45Pile testing procedures2.3

4Pile loading, monitoring and instrumentation systems2.24The test site2.14FIELD LOADING EXPERIMENTS AT DUNKIRK2.

2Scope of the field work1.3

1Background of the project1.2

1Scope of the report1.1

1INTRODUCTION1.

PAGE

1

1. INTRODUCTION

1.1 Scope of the report

This report describes the additional (Phase III) pile testing carried out by ImperialCollege at Dunkirk, NW France between April and May 1999. The field testingcontributed to both the special study commissioned by the UK Health and SafetyExecutive (HSE) and the EU funded GOPAL research project. The GOPAL projectteam consists of D'Appolonia (Italy), Bachy Soletanche (France) and ImperialCollege, London. The HSE funded study involves only Imperial College.

This report supplements the report submitted to HSE in March 1999: "Final Reporton HSE Funded Cyclic Loading Study": Jardine and Standing. The March reportcovered all of the earlier review, modelling and field testing work commissioned byHSE. A short third report will be issued at the end of the collaborative work withWS Atkins that will complete the project's scope of work by summarising the keypoints and setting out proposals for further research.

Aspects of the research carried out by Imperial College into the ground conditions atDunkirk have been reported in a comprehensive Information Pack that has beenpublished on the Internet (Jardine and Standing 1999, www.geocentrix.co.uk/IC50).

1.2 Background to the project

The recent Dunkirk field pile tests researched three aspects of the behaviour of steel(open-ended tubular) piles driven in dense sand:

(i) The effects on compression behaviour of forming a large jet grouted bulb atthe pile base (GOPAL project only).

(ii) The time dependence of shaft capacity (GOPAL and HSE projects).

(iii) How capacity is affected by load cycling (HSE study only).

2

The field tests have been carried out in parallel with advanced numerical simulationsof the Dunkirk cyclic and static tests, and liaison with WS-Atkins to explore thequantitative effects of the above on the system reliability of a large North Seastructure.

1.3 Scope of field work

The GOPAL/HSE programme of static and cyclic pile tests was carried out in threephases, as itemised below.

Phase I - September 1998

1. The first Phase of testing consisted of a single tension test to failure onreaction pile R1. This provided a baseline assessment for the static shaftcapacity available 9 days after driving.

Phase II - October to November 1998

1. Phase II started with the pair of static compression tests to failure performedon the jet grouted pile (JP1) and on the control, ungrouted, driven pile (C1).Additional cyclic loading and tension tests were conducted subsequently onC1.

2. Ten static tension loading tests were then carried out on reaction piles ofmedium term age (typically testing 60 to 90 days after driving) to assess: (i)the effects of time on the shaft capacity of piles that had not experiencedcyclic loading, (ii) the possible variation of load carrying characteristicsbetween the piles and (iii) the static capacities available following cyclicfailure.

3. Ten main cyclic loading experiments were performed on C1 and fourreaction piles. The tests included a mix of seven 'one-way' tension and three'two-way' (tension-to-compression) cyclic loading experiments. some of thetest piles had not been loaded previously, while others had been pre-failed.

3

Phase III - April 1999

1. Phase III featured static tension tests on four reaction piles to assess theeffects of a further 150 days of ageing on the static capacities of: (i) anuntested pile (R2), (ii) a previously statically failed pile (R1) and (iii) twopiles that had pre-failed by high level cyclic loading (R3 and R5).

2. High level cycling was performed on an aged reaction pile (R2) just after ithad been taken to static failure for the first time.

3. Two cyclic tests were carried out on reaction piles that had previously failedunder cyclic action. One (on R6) involved high level cycling, the other (onR4) imposed an extended period of low level cycling.

4. Three further static tests were made to assess the tension capacities availableimmediately after the cyclic tests on R2, R4 and R6 had been completed.

The piles were extracted in June 1999, by Soletanche Bachy, working under thesupervision of D'Appolonia.

4

2. FIELD LOADING EXPERIMENTS AT DUNKIRK

2.1 The test site and experimental arrangements

All three phases of the field testing programme were carried out at the Zip des Huttessite, west of Dunkirk. Full details of the test site, its geotechnical profile and the piletesting arrangements were given in the March 1999 report. For completeness, Table2.1 re-summarises the eight test piles' make-up details and gives each pile'sinstallation history.

21/8/98As R1 aboveR6, 18.90m25/8/98As R1 aboveR5, 19.05m24/8/98As R1 aboveR4, 19.37m20/8/98As R1 aboveR3, 19.24m21/8/98As R1 aboveR2, 18.85m

24/8/98Top 2.5m: 20mm thick st 52 steel,

Lower18m: 13.5mm thick ST 44 steel

R1, 19.32m

25/8/98Top 2.5m: 20mm thick ST 52 steel,

Lower 10m: 13.5mm thick ST 44 steel

C1, 10.02m

25/8/98

Jet grouted: 25/9/98

Top 2.5m: 20mm thick ST 52 steel,

Lower 10m: 12mm thick ST 44 steel

JP1, 10.00m

Date drivenMake-up detailsPile & tipdepth

Table 2.1 Summary of GOPAL - HSE projects' 457mm OD steel piles driven atDunkirk

2.2 Pile loading, monitoring and instrumentation systems

The Phase III pile tests were again performed in association with a specialistsub-contractor, Precision Monitoring and Control (PMC) who provided the testingrigs, displacement transducers and load cell arrangements. Loading was appliedthrough an automatically controlled pneumatic system that activated hydraulic jacksin a load controlled testing mode.

Imperial College devised the testing programme, manufactured (and installed)extensometer instrumentation for piles JP1 and C1, wrote the pile testing

5

specifications, and supervised the field work. The extensometer systems consisted ofsleeved stainless steel rods that were anchored with mechanical expanding sectionsinto vertical angle sections welded onto the piles' exterior. Displacement transducerswere used to monitor the compression of the steel between the pile head and threeanchor levels. Another system monitored the relative movement of the jet groutedpile's internal concrete plug during load testing.

Minor delays were experienced over the three months of field work, with occasionalload cell malfunctions and other test system breakdowns. Many of the piles tested inPhase II developed higher capacities than had been anticipated and the testingarrangements were upgraded for the Phase III work. The equipment and testingspecifications generally worked very well. All of the tests were completedsuccessfully, with large volumes of high quality data being captured.

2.3 Pile testing procedures

The Phase III tests were carried out following the general specifications detailed inthe March 1999 report. As before, minor variations from these specifications wereoften agreed on site for operational reasons. Brief notes are given below on the threemain test types.

Slow static maintained load tests

These load controlled tests were typically conducted over a period of up to 12 hours.All of the Phase III tests were continued until failure had occurred.

Cyclic tests

The pile head loads were varied between target minimum and maximum values. Thecycles, which were applied at a rate of typically one per two minutes, were notprecisely sinusoidal and had a significantly non-symmetrical wave form. Some testslasted only a few minutes, others extended for almost two continuous days.

Rapid loading to failure tests

Separate loading tests were performed, over a period of around 15 minutes, after theend of cyclic loading experiments.

6

2.4 Programme undertaken

The programme of tests undertaken in Phases I, II and III achieved all of the primaryresearch objectives. The later parts of the testing programme were also re-directed toexplore some of the unexpected phenomena identified in the earlier phases of testing.

7

3. PILE LOAD TEST RESULTS

3.1 Introduction

This section sets out the results of the Dunkirk pile tests. The compression tests onpiles JP1 and C1 are mentioned only briefly; as more details of these tests are givenin the reports prepared by Imperial College for the GOPAL project. The pileexperiments are first summarised in Section 3.2, and by Tables 3.1 to 3.9, whichpresent of the results obtained in all three phases of field work. Note that alltabulated failure loads have been corrected to take account of the weights ofcomponents such as strong-back spacers, jacks, etc for all tests. Figures 3.1 to 3.24provide more details of some of the experiments.

3.2 Tabular summary of main results

Table 3.1 to 3.9 identify each test stage by a unique code. The tables give theinterpreted maximum loads and displacements for each static test. For cyclic teststhe load ranges, numbers of cycles and cyclic displacements are tabulated.Comments are also provided on particular feature of the tests. Any minor gaps in thedata sets caused by data logger breakdowns are noted, as are any disruptions causedby loading system problems.

8

Failure involvedsubstantialcracking of groutbulb, as proven bysubsequent rotarycoring.

Load continuingto climb atgradient of around25 kN per mmsettlement at endof test. Finalrelative movementof plug at pilehead = 1.44 mm.

Loading haltedwhen settlement>10% pilediameter

Break point inload displacementcurve at 4200 kNload, 12mmsettlement.

Creep becomessignificant(exceeding 1 mmduring pauseperiods) at loadsabove 4,000 kN

Loading on to5250 kN overeight hours.

Extensometerreadings indicatestrain distributionin steel pile wall.Plug monitoringby dial gaugeonly.

Only 0.3 mmsettlement over2400 kNmaintained loadperiod

Loading 0 to2400 kN oversix hours, thensix hoursmaintained loadpause

No clear peak incapacity

5250 kN appliedwith pile headdisplacement of46.90 mm

Compressionloading(2.JP1.C1)

29-30/10/98JP1CommentsOutcomeApplied loadingDatesPile

Table 3.1 Summary for compression test on pile JP1

9

Final load-settlementgradient around2kN/mm, andreducing.

PMC extensometersystem transducersfail to record.Back-up dial gaugereadings indicateambiguous pattern ofstraining.

Load-displacementcurve flattens at2300 kN.

Creep seenduring pauseperiods exceeds1mm at loadsabove 2100 kN,when settlements>10mm.

No break inloading sequence

Test halted to avoidoverloading theloading frame, whichhad started to tilt.Load-settlementcurve showed pile tohave practicallyfailed.

Test stopped at2819 kN, 33.92mm settlement.extra 30 kNprojected for45mm settlement(ie 10% ofdiameter)?

Compression(2.C1.C1)

01/11/98C1CommentsOutcomeApplied loadingDatesPile

Table 3.2 Summary for compression test on pile C1

10

Stick slipbehaviour. Loadsranging between400 and 500 kN.

Max load of496 kN.

Slow tension testafter a one hourrest period(2.C1.T6)

06/11/98

Fourteen cycles,last ten showinglargedisplacements butno brittle loss ofcapacity.

'Cyclic pull-outfailure' at N=12on basis ofdisplacements.Rapid test stableand unfailed at401kN.

Cyclic loading~440kN (C)~401kN (T)(2.C1.CY5)

05/11/98

Five largedisplacementcycles. No brittleloss of capacity.

'Cyclic pull-outfailure' at N=1 onbasis ofdisplacements

Cyclic loading~280 kN (C) and~611 kN(T)(2.C1.CY4)

04/11/98

'Cyclic pull-outfailure' at N=41on basis ofdisplacements.Total failureimminent.

Cyclic loading640kN (C) to561kN (T)(2.C1.CY3)

03/11/98

Stick slipbehaviour atfailure.Extensometerstransducersfunction correctlyin this andsubsequent tests.

Peak load 82 1kNTension loading(2.C1.T2)

02/11/98C1CommentsOutcomeApplied loadingDatesPile

Table 3.3 Summary of tests on C1 following first compression test

11

Clear peak load of1632 kN at 8mm.Ultimate =1556 kN.

Tension test 239days afterdriving(3.R1.T3)

26/04/99

Reaction forces.Tension andcomp loadingfrom tests on C1

01-06/11/98

Test halted andunloaded from1400 kN due tomalfunction.Reloaded tofailure next day. Slight indicationof damage due tofirst loading cycle.Brittle failure

Clear peak load of1500 kN at 8mm.Ultimate load =1400 kN.


27-28/10/98

Long durationtest: very smallfinal increments.No clear peak,interesting timeeffects.

Max load of1450 kN at 30mm


01-02/09/98R1CommentsOutcomeApplied loadingDatesPile

Table 3.4 Summary for pile R1

12

Flat stick-slipbehaviourresponse afterreaching 8mmmovement.

Rapid test failedat 1655 kN. After30mm.

Rapid tensiontest (3.R2.T3)

18/04/99

Evidence ofdamage caused byfirst static test.

'Cyclic pull-outfailure' at N=9 onbasis ofdisplacements.

Cyclic loading 0to 2000 kN (T)(3.R2.CY2)

18/04/99

No clear peak,considerable gainin capacity withtime.

Max load of3147 kN at 34mm

First tension testto failure(3.R1.T1)

18/04/99

Reaction forces.Tension andcomp. loadingfrom tests on C1

01-06/11/98

Reaction forces.Tension fromloading JP1.



13

Very brittle andstick-slipresponse,substantialrecovery over sixmonths

Clear peak load of1986 kN at 10mm

Tension test(3.R3.T5)

20/04/99

Unable to reach1900 kN.Capacity on rapidloading afterwards~1650 kN max(no plot given).

Failure (based onmeandisplacement,cyclicdisplacement andload) after N=12with a rapid dropof capacity to1700 kN on 13thcycle.

Cyclic loading 0to 1900kN(T)(2.R3.CY3)

15/11/98

Test stopped after200 cycleswithout failure.

N=200 with apermanentdisplacement of6.8mm


14/11/98

Loading stoppedbefore failure.

Max load of2000kN applied,pile headdisplacement of10.30mm


13/11/98

Reaction forcesTension fromloading FP1



14

Very brittleresponse, thenstick-slip.Capacityenhanced byprevious cycling.

Clear peak at2491 kN after12mm; ultimatecapacity = 1884kN


24/04/99

Pile headmovement stablebetween 0.3 and3mm after 500cycles.

No 'Cyclic failure'at N = 1000.Very stable.

Cyclic loading5- 805 kN(T)(3.R4.CY6)

23/04/99

Flat response after8mm withstick-slipbehaviour.

Capacity varying between 1400 &1850 kN (1625kN average), to40mmdisplacement


18/11/98

2000 kN reachedafter largedisplacements onfinal cycle.

'Cyclic failure' atN=3 on basis ofdisplacements;capacity is alsofalling

Cyclic loading500-2000kN(T)(2.R4.CY3)

18/11/98

Pile reloaded over15 min periodafter test to 2000kN. Failure notreached.

'Cyclic failure' onbasis ofdisplacements atN=221. No datafor first 35 cycles

Cyclic loading0 to 2000kN(T)(2.R4.CY2)

17/11/98

Loading stoppedbefore failure

Max load of2000kN applied,pile headdisplacement8.73mm


16/11/98

Reaction forcesLoading fromtests on C1



15

Brittle responsewith some loss ofcontrol post-peak.Substantialrecovery over sixmonths.

Clear peak load1794 kN at 9mm,ultimatecapacity-1636 kN


15/04/99

Flat response to8mm thenstick-slipbehaviour

Capacity varyingbetween 1150 to1450 kN (1300kN average) at40mm


21/11/98

Total failureimminent:capacity dropping

'Cyclic failure' atN=27 on basis ofdisplacements

Cyclic loading0 to 1400kN(T)(2.R5.CY3)

21/11/98

Two short pauseperiods related tomalfunctions

'Cyclic failure' atN=345 on basis ofdisplacements,total failureimminent


20/11/98

Loading stoppedbefore failure

Max load of2000kN applied,pile headdisplacement8.86mm


19/11/98

Reaction forcesTension andcomp. loadingfrom tests on C1

01-06/11/98

Reaction forcesTension fromloading JP1



16

Extreme stick-slipbehaviour.

Peak load of 1426kN at 40mm.


22/4/99

40mm movementafter 211 cycles.

'Cyclic failure' atN=206 on basis of20mmdisplacements.


22/04/99

Flat response after8mm, thenstick-slipbehaviour.

Capacity varyingbetween 1150 &1500 kN (1325average) at 30mm.


12/11/98

Capacity fallingover followingfive cycles.

'Cyclic failure' atN=24 on basis ofdisplacements


12/11/98

Not brittle.Max load of 1585kN, after pile headdisplacement9.57mm.


11/11/98

Unable to apply2000 kN displt.~35mm (datalogger breakdownduring day and fortest, no plot).

'Cyclic failure'(based on meandisplacementcyclicdisplacement andload) after N =1(?)


10/11/98

Loading stoppedjust before failure:risk of rig failure.No clear peak.

Max load 2400kNat 30mm. FailureQmax = 2450 kN?


09/11/98

Reaction forcesTension fromJP1



17

3.3 Graphical results

Plots to accompany Tables 3.1 to 3.7 are given as Figures 3.1 to 3.24. While thetables present all of the results obtained in all three phases of field work, the figuresdo not repeat the plots that were included in our March 1999 report. Attention isfocused on: (i) the static tests conducted on piles JP1 and C1 during Phase II (Figures3.1 to 3.8) and the all of the Phase III tests performed in April 1999 (Figures 3.9 to3.24). Note that all plotted loads have not been corrected to take account of theweights of components such as strong-back spacers, jacks, etc for all tests. Theseresults may differ slightly from those shown in Tables 3.1 to 3.7.

18

4. PRELIMINARY INTERPRETATION

4.1 Introduction

The following sections offer a preliminary interpretation of the pile test results. Afterdescribing how spatial variations in soil properties were accounted for, a briefdiscussion is given on the two key GOPAL compression tests. Attention is thenfocussed on the cyclic and ageing studies, considering the:

1. Positive influence of pile age on the shaft capacity of untested piles.

2. Damaging effects of pre-testing to failure.

3. Cyclic loading effects.

4. Recover of capacity with time after prior to re-testing to failure.

5. The net effect of the jet grouted bulb on compressive capacity.

4.2 Allowing for variations in ground conditions

It was noted in the site characterisation reports that variations exist between the CPTprofiles found at each pile test locations. To help allow for this, predictivecalculations were made (prior to all testing) employing the Jardine and Chow (1996)procedure in conjunction with the intensive local CPT information. The capacitiesmeasured in each test could be expressed as a multiple of the (nominally 50 day)capacity expected from the Jardine and Chow procedure, so normalising for changesin the CPT profiles. Table 4.1 summarises the range of predicted '50 day' capacities.

Estimates were also made of the base capacity of the jet grouted pile. Working fromthe grout bulb test data available prior to the JP1 load test, and making allowance forbulb shape and progressive failure, the combined compression capacity was predictedto be 4894kN.

43

964964C1 C1 - tension25922592C1C1 - compression

22694894

22694894

JP1BJP1B

JP1-compressionWithout jet bulbWith jet gr bulb

12701268-1288R5-R6; JP1BR6 - tension14161589-1244R4-R5; R5-R6R5 - tension17341698-1769R4-R5; C1R4 - tension13481432-1263R2-R3; JP1BR3 - tension13901385-1394R1-R2; R2-R3R2 - tension1500 1500-1779R1-R2; C1R1 - tension

Adopted prediction,kN

Capacity rangekN

CPT profiles usedPile

Table 4.1 Predicted pile capacities; nominally 50 day. Jardine and Chow procedure,plus simplified approach for jet grout bulb case.

4.3 Comments on the GOPAL compression tests

The results presented in Tables 3.1 and 3.2 and Figures 3.1 to 3.8 show that the jetgrouting procedure led to a marked improvement in the ability of a 10m long drivenpile to sustain compression loading.

Pile JP1 was able to carry 84% more load at a settlement of D/10 than the controlpile (C1), with a reserve of capacity that appeared to be available if settlementsexceeded D/10. JP1 was also able to carry 90% more load before undergoing 'creepyield' - defined here as the stage where the movements observed during maintainedload pause periods exceeded 1mm. A more precise assessment that incorporatedallowance for the spatial variations in soil properties indicated that jet groutingimproved the overall compressive capacity by a factor of 2.1.

44

4.4 Allowing for variations of capacity with time

Three first-time loading tests were incorporated into the pile test programme atDunkirk. Their results, combined with data from two prior Imperial College tests1

are plotted against time on a semi-logarithmic scale on Figure 4.1, along with themean trend for growth in shaft capacity indicated by Jardine and Chow's scatterdiagram. Four main points are clear:

1. The new field tests showed much stronger growth of capacity with time thanexpected.

2. The '50 day' capacities expected by the Jardine and Chow (1996) procedurewere in fact available around 10 days after driving.

3. The medium-term rate of gain with time is steeper than expected.

4. The rate of gain slows over the first 100 days; the process seems to stabiliseand halt after 200 to 300 days.

2.263147239GOPAL R21.92245081GOPAL R60.9714509GOPAL R1

2.273150(two pile tests)

2050CLAROM, 22m,327mm OD

Test capacity/50daycapacity predictedby Jardine & Chow

Tension capacity,kN

Age after driving,days

Pile

Table 4.2 Effects of pile age as assessed in 1st loading tests at Dunkirk

The multiple re-tests made in Phases II and III at Dunkirk show that thetime-dependent growth of shaft capacity is disrupted and retarded by pre-loading tofailure. Although the load-displacement curves seen in first-time loading tests were

45

1 The data points are from the 22m long CLAROM piles tested in tension five yearsafter installation, without any prior test having being performed (see Chow et al1998).

generally ductile, or only slightly brittle, none of the piles could re-mobilise its peakcapacity when reloaded shortly after completing a test to failure.

Figure 4.1 Summary of tension capacity-time data: first-time tests at Dunkirk

Figures 4.2 and 4.3 illustrate the brittle response, and the relatively slow and partialrecovery of shaft capacity after pre-failure. Normalised capacity-time trends arepresented for pile R1 (which was subjected to a single static tension test during eachof the three phases of field work) and piles R3 and R4 which were loaded to failure(both cyclically and statically) for the first time during Phase II, and then re-testedstatically in Phase III.

46

Notes:

R1(1) - virgin path for pile R1 (end point proven by first test)R1(2) - decrease in capacity following first test (projected from other field tests)R1(3) - increase in capacity (end point proven by second test)R1(4) - decrease in capacity following second test (projected from R1(5)R1(5) - increase in capacity (end point proven by third test)

line drawn parallel to that proven for R3(3)R1(6) - presumed decrease in capacity following third testR3(1) - virgin path for pile R3 (end point estimated to be similar to that for R6)R3(2) - decrease in capacity from two phases of cyclic testing (end point proven by rapid test)R3(3) - increase in capacity

(end point proven by tension test)R3(4) - presumed decrease in capacity following tension test

Figure 4.2 Summary of tension capacity-time data: re-tests on piles R1 and R3

47

Notes

R4(1) - virgin path for pile R4 (end point estimated to be similar to that for R6)R4(2) - decrease in capacity for two phases of high-level cycling

(end point proven by rapid test)R4(3) - increase in capacity (end point estimated by drawing line drawn parallel to that proven for

R3(3)R4(4) - increase in capacity from low-level cycling

(end point proven by rapid test)R4(5) - presumed decrease in capacity following rapid test to failure

Figure 4.3 Summary of tension capacity-time data: re-tests on pile R4, including1000 low-level loading cycles

Tables 4.3 to 4.6 give further details of the assessment made of the variations ofstatic pile capacity with time and the effects of pre testing to failure (under cyclic orstatic conditions). The tabulated estimates made use of all the available informationincluding, when necessary, projections based on the local Jardine and Chowpredictions and the Dunkirk time-capacity trends shown in Figure 4.1.

48

Final tension capacity 68% less thanestimated undisturbed tensioncapacity.

515 kN, after2nd two waycyclic loading

Further 27% losses caused by two(two-way) cyclic tests.

620 kN, after1st two waycyclic loading

Tension'68 day' 2nd test capacity 16% lessthan expected after compression test.

Tension840 kN '68day'

Tension999 kN - '68day'

Extensometer data do not allowconclusive split between base and shaftcapacity.

68 day shaft capacity projected fromFigure 4.1 to be 83% higher than J &C estimates, ie 2335 kN; Implied base= 514 kN which seems improbablylow.

Compressions2849 kN

Compression2592 kN(1276 kN shaft1316 kN base)

C1

Preliminary extensometerinterpretation suggests base capacitybetween 2300kN and 4500kN,depending on grout column stiffnessand load-transfer characteristics.

65 day shaft capacity projected fromFigure 4.1 to be 82% higher than J &C estimates, ie 1881 kN; Implied base= 3369 kN

Compression5250 kN

Compression4894kN(1034 kN shaft,3760 kN base)

JP1

CommentsMeasurementsof capacity

Jardine & Chow1996 estimates

Pile

Table 4.3 Preliminary interpretation of tests on JP1 and C1

49

Similar to Jardine and Chowpredictions

Final staticcapacity1644 kN

Cyclic failureat peak of2000 kN in 9cycles

Far stronger than predicted. Staticcapacity reduced to 2500 kN by brittlefirst failure?

3147 kN '235day' (Brittle)

1644 kN - '235day' First test

R2

As above.1632 kN're-test after239 days'(Brittle)

1775 kN -'239-day'

Recovery of capacity, but nowmarginally below prediction due todisrupted ageing

1500 KN're-test after57 days'(Brittle)

1523 kN -'57-day'

'9 day' 1st test close to '50' dayprediction.

1450 kN '9day' (Brittle)

1198 kN -'9-day' First test

R1



Pile

Table 4.4 Commentary on variations in pile capacity due to age, testing and cycling:piles R1 and R2

50

Full recover and substantial gain withtime, aided by 1000 low level cycletest at 242 day age.

1000 stablelow-levelcycles then2491 kN on'retest after242 days'

2055 kN - '242day'

Losing further 23% (wrt 2100 kN)1625 kN '85day after 2ndcycling'

Falling 31% to 2100 kN during 1st

cycling>2000 kNafter firstcycling

'85 day' 1st test capacity far greaterthan expected, estimated as 2960 kN

>2000 kN '85day' (Brittle)

1842 kN - '85day' First test

R4

Full recovery and substantial gain withtime

1986 kN're-test after241 days'(Brittle)

1570 kN -'241-day' Retest

Losing further 19% (wrt 2050 kN)1650 kN '85day after 2ndcycling'

Falling 12% to 2050 kN during 1st

cycling>1900 kN '85day after 1stcycling'

'85 day' capacity greater thanexpected, estimated as 2320 kN 85 daycapacity

>2000 kN '85day'

1431 kN -'85-day' Firsttest

R3



Pile

Table 4.5 Commentary on variations in pile capacity due to age, testing and cycling:piles R3 and R4

51

Static capacity greater than expected;recovery developed between 81 and244 days lost through cyclic loading tofailure.

Staticcapacity>1400 kN(cyclic failurein 206 cycles),1426 kN onstatic re-test

1506 kN -'244-days'

Overall loss = 47%.1300 kN '84day, after 3tests andcycling'

35% loss of capacity due to 1st and 2nd

tests.2000 kN '81day, 2nd test'

'81 day' 1st test far greater thanexpected.

2450 kN '81day'

1351 kN - '81day'

R6

Full recovery and substantial gain withtime.

1794 kN're-test after234 days'

1672 kN -'234-days'Re-test

Further 32% loss through 2nd cycling.1350 kN '85day after 2ndcycling'

19% losses through 1st cyclic loading;2000 kN '90day after 1stcycling'

'90 day' 1st test capacity far greaterthan expected, estimated as 2464 kN.

>2000 kN '90day'

1514 kN -'90-day' Firsttest

R5



Pile

Table 4.6 Commentary on variations in pile capacity due to age, testing and cycling: piles R5 and R6

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4.5 Comments on combined effects of age and pre-testing to failure

Significant observations on the effects of ageing and pre-testing to failure include:

1. All of the static tension tests showed a tendency to creep as they approachedfailure. A detailed analysis of the test records is required to investigate someunexpected time delay and creep phenomena.

2. Most of first-time tests to failure (involving pause periods between 9 and 250days) followed significantly 'brittle' load-displacement curves. However, thepeak capacities developed in first-time tests could not be recovered in re-testsperformed shortly afterwards: the load-unload sequence undoubtedlydamaged the piles' capacities.

3. The gains of tension (shaft) capacity with time noted in tests on previouslyun-failed (ungrouted) piles were far larger than had been anticipated.

4. Some recovery of tension capacity took place with time after both testing tofailure and high level cyclic load testing. But the recovery curves laggedbehind the capacity-time trend exhibited by 'virgin' piles.

5. The load-displacement curves followed during tension re-tests performedafter an extended pause periods showed a pronounced 'peak' in capacity,which developed at a relatively small pile head displacement, after which theload carrying capacity dropped to an apparently stable ultimate value.However, it is thought that a lower capacity would be available afterunloading (see 4 above).

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4.6 Comments on effects of cyclic loading

The cycle loading tests proved the following:

1. High level loading can lead to large, and long lasting, reductions in shaftcapacity.

2. High level two-way loading produces the most extreme effects. In theextreme case of pile C1, the combination of two-way static loading to failureand two-way cycling applied during Phase II led to an estimated 68% loss oftension capacity.

3. Low level cycle loading can have a beneficial effect on pile capacity, perhapsaccelerating the process of ageing. This was proven in the Phase II test onpile R4.

4. As cyclic loading levels increase, fewer cycles are required to lose a givenamount of capacity.

5. Rapid tension tests performed after cyclic loading failures did not show clearpeaks, but tended to develop a stick-slip pattern of failure after reachingrelatively small displacements, often with the average resistance climbinggradually as displacements increased. This behaviour may have been due toeither the faster loading rate, or the pre-cycling.

Conclusion (4) is explored further in Figure 4.4 which was interpreted from thePhase II cyclic testing. Contours are shown for the number of cycles required tocause failure (N), plotted in cyclic loading co-ordinates where Qcyclic is the (single)amplitude of pile head cyclic load, Qaverage is the average of the peak and trough loadextremes and Qmax static is the best estimate of the static capacity available (in tension)just before the cycling starts.

The co-ordinate values are given Table 4.7; note that the estimated static capacitiesare taken from Tables 4.3 to and 4.6.

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Figure 4.4 Cyclic failure interaction chart developed from the Phase II field tests

Points to note in connection with Figure 4.4 are:

1. The diagram offers an interpretation based on a relatively sparse data set.Three of ten co-ordinate sets rely on projected static capacity estimates.

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2. The plot combines tests on virgin piles, and tests on pre-failed piles.Generally, even after normalising by the best estimate for the current shaftcapacity, the 'virgin piles' show a stronger resistance to cyclic loading.

3. The plot combines data from two tests on the 10m long C1 pile with eight on19m long reaction piles and data are given from first-time tests alongsidere-tests.

4. The contour lines represent a preliminary interpretation. The contours areglobal 'best estimates' established by eye. A more reliable interpretationcould be made if a greater number of piles had been driven, and a moreextensive testing programme performed.

Figure 4.5 presents the three data points from the Phase III cyclic tests. Generallythese tests reinforce the pattern seen in Phase II. Additional features to note include:

1. Piles R4 and R6, which were allowed to rest after pre-testing in Phase IIshowed a relatively stiff response.

2. Piles that had recently been failed showed a distinctly softer response. Thecyclic loading applied to R2 (immediately after its first time loading tofailure in Phase III) led to failure in a relatively small number of cycles.

3. Low level cycling applied to a pile that had been allowed to rest did notdegrade its capacity. Indeed, the net effect of the 1000 (relatively low level)cycles applied to R4 was a 17% gain in capacity. It is suggested that lowlevel cycling may improve capacity in the medium term by accelerating thebeneficial ageing process.

The last point is of considerable practical significance. While a typical North Seastorm includes at least 10,000 significant waves, a calibration exercise (performed byWS-Atkins with advice from Imperial College for the 'Lomond' idealised structure)showed that even in the 100 year or 1000 year return period event, only a minorfraction of the total number of waves cause cyclic pile loads that exceed the cyclicdegradation threshold values projected from the Dunkirk field tests. Taking

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account of the threshold cyclic loading levels leads to substantially reducedpredictions for each storm's impact on the piles' capacities.

2060.420.4216503.R6.CY6No failure0.190.1921103.R4.CY690.400.4025003.R2.CY2120.680.06202.C1.CY5410.71-0.058402.C1.CY3240.440.4415852.R6.CY410.620.3820002.R6.CY2270.350.3520002.R5.CY33450.510.3024642.R5.CY230.590.3621002.R4.CY32210.340.3429632.R4.CY2130.460.4620502.R3.CY32000.300.3023172.R3.CY2

No of cyclesto failure, N

Qaverage/QmaxQ cyclic/QmaxMeasured or estimatedpre-cycling static tensioncapacity Qmax'KN

Cyclic piletest

Table 4.4 Summary of cyclic co-ordinates for cyclic tests; Phases II and III

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Figure 4.5 Cyclic failure interaction data for Phase III tests, contours interpretedfrom the Phase II are also shown

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5. SUMMARY

Three phases of field pile tests have been carried out, or supervised, by ImperialCollege at the Dunkirk experimental site between September 1998 and April 1999.The comprehensive programme of research was fully successful. Seven importantconclusions may now be drawn:

1. Forming a jet grouted bulb at the base of a 10m long steel tubular pile drivenin dense sand led to an approximate doubling of its compressive capacity. Italso doubled the 'creep yield' load of the pile.

2. The medium-term compression tests on piles JP1 and C1 gave marginallylarger capacities (7 to 10%) than expected by Imperial College.

3. The static and cyclic shaft capacities of plain driven piles increase steeplywith age. 'Virgin' (ie not previously failed) piles showed steeper increasesthan anticipated by Jardine and Chow (1996).

4. The plain driven piles suffered brittle failures when tested statically orcyclically. This was most marked when testing aged 'virgin' piles.

5. Pre-testing a pile to failure disturbs and retards the growth of shaft capacitywith time.

6. High level cyclic loading leads to pronounced and progressive degradation ofshaft capacity.

7. Low level cyclic loading appears to enhance shaft capacity, perhaps byaccelerating the beneficial ageing processes.

The beneficial effects of pile again and low level cyclic loading may be able to offsetthe degradation caused by severe cyclic loading. But the balance between thesecompeting actions will depend on the structure's type and age, on the sitegeotechnical conditions, the pile layout and pile slenderness ratios, the expectedstorm climate and the adopted pile design methodology. Further parametric studieswould help to identify those circumstances (if any) under which cyclic action has asignificant impact on system reliability.

Static full-scale field tests on aged piles that had supported platforms through manycyclic loading events (carried perhaps during a platform removal exercise) wouldprovide a valuable final check on the applicability, under field conditions, of thetrends identified in this report.

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6. REFERENCES

Chow, F C (1995), "Field measurements of stress interactions between displacementpiles in sand", Ground Engng, 28(6), pp 36-40.

Chow, F C, Jardine R J, Nauroy J F & Brucy F (1997), "Time-related increases inthe shaft capacities of driven piles in sand", Géotechnique, Technical Note,Geotechnique, 47, No 2, pp 353-361.

Chow, F C (1997), "Investigations into displacement pile behaviour for offshorefoundations". PhD Thesis Univ London (Imperial College).

Parker, E J, Jardine R J, Standing J R and Julian X (1999), "Jet Grouting to improveoffshore pile capacity: GOPAL project". Offshore Technology Conference, Houston,OTC 10828.

Jardine, R J (1991)The cyclic behaviour of offshore piles. Chapter in 'The Cyclic Loading of Soils', EdsBrown & O'Reilly, Blackie & Son, Glasgow.

Jardine R J (1994)Review of offshore pile design for cyclic loading: North Sea Clays. HSE OffshoreTechnology Report, OTN 94 157.

Jardine, R J and Chow F C (1996)New design methods for offshore piles. MTD Publication 96/103, MTD (nowCMPT) London.

Lehane, B M (1992)'Experimental investigations of pile behaviour using instrumented field piles' PhDThesis, University of London (Imperial College).

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