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Assessment of loads
28|The Structural Engineer 19 October 2004
paper: fung et al
SynopsisIn the last decade, Hong Kong has seen many advancements inthe technology of pile installation and driving. Examples arethe introduction of hydraulic hammers, use of plastic cushionsand the introduction of high capacity, Grade 55C steel, H-piles.The use of hydraulic hammers has though created somedifficulty in the assessment of pile capacity. Questions are oftenasked as to whether the Hiley Formula remains appropriate,albeit commonly used in Hong Kong for many years adoptingcoefficients developed in the early fifties. This paper presentsdifferent methods of pile capacity assessment with a view torecommending a practical approach applicable to all drivenpiles. The results show that CAPWAP (CAse Pile Wave AnalysisProgram) analysis is a fairly accurate method for capacityprediction. And the results also show that, providedappropriate parameters are used, the Hiley Formula remainsapplicable with good accuracy when hydraulic hammers areused.
IntroductionThe use of Dynamic Pile Testing (commonly known as the PDAtest in Hong Kong) in the assessment of pile capacity has beenfound successful overseas. But its application in Hong Kong isstill in development. Before this method can be accepted widely,its reliability in local soil conditions has to be assessed andconfirmed. Against this background, studies were conducted tocompare the results of CAPWAP capacity predictions with staticload tests. Additionally, the reliability of the Hiley Formula wasinvestigated by correlating its pile capacity predictions againstcomparable CAPWAP predictions. This paper will discuss find-ings from the above studies on 327, Grade 55C steel H-piles(with a yield strength of 430MPa) of size 305 305 180kg/m,driven by hydraulic hammers across 18 different sites.
Comparison of CAPWAP and static load test resultsThe studys objective was to investigate the reliability ofCAPWAP analysis in pile capacity prediction as compared withresults from static load tests. Selected results have previouslybeen presented in the paper Fung et al. (2004). The study, usedthe failure criterion proposed by Davisson (1972) to define thefailure load in static load tests. Furthermore, for a meaningfulcomparison to be made with such tests,CAPWAP results neededto be from restrike tests rather than end of driving ones.Restrike tests are considered more capable of reflecting real situ-ations since pile capacities usually change with time after instal-lation due to soil setup or relaxation (Likins et al. 1996).
In this study, 13 steel H-piles from seven different sites werestatic load tested to failure. So as not to overstress or perma-nently damage the steel, piles were loaded to Davissons failureload or to 90% of the steel yield strength, whichever was thesmaller.Amongst the 13 piles tested,10 were loaded to Davissonsfailure load whereas the other three reached the 90% yield limitbefore the Davissons failure load could be identified on the loadsettlement curve.The capacities so obtained were compared withCAPWAP predictions and the results are summarised in Fig. 1.
From the static load test results of the 13 piles, it is found thatthe CAPWAP method can predict the Davisson failure load fairlyaccurately. Hence once the reliability of the CAPWAP method isestablished, it can be used not only in the assessment of the pilecapacity but also in evaluating other assessment methods, e.g.Hiley Formula, and this will be discussed in the following para-graphs.
Comparison of pile capacity predicted by CAPWAPanalysis and by the Hiley formulaBecause of its simplicity and applicability in varied soil conditions
and pile lengths, the Hiley Formula is the most commonly useddynamic formula in Hong Kong. It is used in the following form:
Hiley Formula: Rs C C C
EW P
W PeWH
21
c p q
h
2
=+ + +
++
J
L
KK
_
N
P
OO
i...(1)
where Eh = efficiency of drop e = coefficient of restitution of the hammer cushion R = driving resistance W = weight of the ram H = drop height P = weight of pile and helmet s = set per blow Cc = temporary compression of cushion Cp + Cq = temporary compression of pile and soil
In this formula, there are two unknowns (e, Eh) while otherparameters are either known or measurable. The value of Cc isassumed constant, 5mm, in this study.
Studies were conducted on 18 sites where the ultimate capac-ities of 327 piles predicted by CAPWAP analysis were taken tobe the failure loads.For each site, the coefficient of restitution (e)and drop efficiency (Eh)were obtained by back analysis such thatthe average of pile capacities predicted by the Hiley Formulaequaled the average of the capacities predicted by CAPWAPanalysis. As the back calculation is a correlation process, thevalues of e and Eh are termed correlated values (see Table 1).
Once the correlated values of (e) and (Eh) for each site had beenobtained, the pile capacity of each of the 327 piles was recalcu-lated using the Hiley Formula.These values were then comparedwith CAPWAP predictions and the results are summarised in Fig2 below. The results for each individual site are shown in Table2.
The results show good agreement between the capacitiesobtained by the Hiley Formula and those from CAPWAP analy-sis. The ratio of Hiley Capacity/CAPWAP Capacity varies from0.83 to 1.17,with a mean of course equal to 1.0:the standard devi-ation of 0.057 is also very low. This means that 94% of the pileshave capacities predicted by the Hiley Formula deviating fromtheir corresponding CAPWAP capacities by less than 10%.
While it is acknowledged that parameters so obtained may notreflect the exact physical properties of the driving system, it cannevertheless be concluded that when these parameters areapplied to the Hiley Formula, the capacities are close to thosefrom CAPWAP predictions, as noted from the results of the 327piles from the 18 different sites.
In most cases, the correlated values of coefficient of restitution(e) are relatively higher while those of the drop efficiency (Eh) areslightly higher than those recommended in CP4 Foundations
Assessment of load carrying capacity ofdriven piles a practical approach
W. K. Fung
C. T. Wong
M. K. WongAll at the ArchitecturalServices Department, theGovernment of HKSAR
Keywords: Drivenpiles, Load tests, Hileyformula, Hong Kong,Capacity, Forecasting
W. K. Fung, C. T.Wong, & M. K. Wong
This paper is a shortenedversion of the paper thatwas presented at theJoint Structural DivisionAnnual Seminar, NewPerspectives in theDesign & Constructionof FoundationStructures, held in HongKong in June 2004.
CAPWAP is a computerprogram first developed byRausche (1970) at CaseWestern Reserve University.The program uses therecords of force and velocityobtained with the PileDriving Analyser. With pileforce and pile motionquantified, the CAPWAPprogram can determine theboundary conditions includethe pile capacity, the soilresistance distribution, andthe soil quake and dampingcharacteristics.
Fig 1. Comparison ofCAPWAP predictionswith static load test(SLT) resultsThe Davissonfailure load was notreached
19 October 2004 The Structural Engineer|29
paper: fung et al
(1954) (refer to Appendix A), but they are reasonably consistentwith the recent findings by A.R.Biddle (1997) (refer to AppendixB).Please note that Monocast MC 904 cushions were used for all18 sites included in the study.
These higher e (and Eh) values are by no means arbitrary;theyhave physical significance and reflect the performance (of thepiling equipment) and quality of the cushion.High e values indi-cate a good plastic cushion and a high Eh value indicates high effi-ciency of the hydraulic hammer system.The combined effect canbe observed in the measured EMX value. Where the correlatedparameters are low,as is the case at the sites in Kowloon Bay (1)and West Kowloon, the overall efficiency of the driving systemshould also be low when compared with other sites. It may beworthy of note that the Energy Transfer Efficiency (ETR) of thedriving systems, EMX/WH, for these two projects, i.e. KowloonBay (1) and West Kowloon are only 0.74 and 0.66 respectively.
The Energy Transfer Efficiency (ETR) is defined as:
ETR = EMX/WH ...(2)
where EMX is the maximum energy transmitted through thetransducers (at pile head) and can be calculated by the PDAfrom dynamic measurements (acceleration and strain in thesteel); W is the ram weight and H is the drop height (Abe andThendean, 1996).
The coefficient of restitution, e, of the hammer cushionsdecreases after sustained use and the rate of ageing is alsoaffected by the way the cushion is used. A cushion used for 30days at say 10hrs a day would deteriorate faster than one usedfor 100 days at 3hrs a day.The results in Table 1 show that corre-lated e values range from 0.77 to 0.99.The spread of e values can
be explained by noting that cushion conditions used for the 18sites are all different, resulting in different correlated e values.Sometimes, the correlated e and Eh values are high.This may bedue to the fact that actual input energy is higher than thatassumed in the calculation, possibly because during measure-ment of the final set, the ram may have been raised to a positionhigher than the assumed level: it is very difficult to raise it to anexact position. Furthermore, the temporary compression of thepile, soil and hammer cushion (Cp + Cq + Cc), and the permanentset, both allow an extra distance for the ram to travel.
The importance of adopting e and Eh values reflective of actualdriving system performance has been demonstrated. For the 18sites covered in this study, the variation of Energy TransferEfficiency (ETR) among the hammers is large,ranging from 66%to 99%. It is therefore essential that appropriate values of e andEh for each set of driving equipment are determined.
Comparison of Hiley formula predictions and staticload test resultsThe Hiley Formula together with correlated e and Eh from Table1 was used to predict pile capacities of the aforementioned 13piles which had been subjected to static load test up to theDavissons failure load. The predicted capacities were thencompared with the Davissons failure loads and the results areshown in Fig 3.Conclusions drawn from Fig 3 are similar to thosefrom the comparison with CAPWAP discussed in previoussections.
Practical approach to pile capacity predictionIn practice, the Hiley Formula and CAPWAP analysis can beused hand in hand for quality control of piling works. Theircombined use is simple in application and may induce greaterconfidence.At the initial stage of piling works, if static load testshave not been carried out,trial/preliminary piles should be drivento determine the appropriate driving system values of e and Ehfrom a CAPWAP analysis. It is also recommended that only 85%of the CAPWAP ultimate bearing capacity be used instead of thefull value. This factor of 85% accounts for possible deviations ofCAPWAP predictions from static load test results and for thegradual decrease in driving system efficiency due to deteriorationin the condition of the hammer cushion. Normally, the e value ofcushion decreases with sustained use and the efficiency of thedriving system becomes lower when compared with thatobserved during trial pile installation. The last column of Table1 shows values of e and Eh established on this basis. For practi-cal reasons, the e and Eh values used should not be higher than0.65 and 0.98 respectively. Once these parameters are deter-mined, a final set table can then be established for the remain-ing piles as is the case in traditional practice.
Table 1: Correlated values of e & EhProject No. of Average Average Average 100% CAPWAP 85% CAPWAPtitle piles embedded CAPWAP ETR capacity capacity
studied length (m) capacity (=EMX/ Correlated Correlated Correlated Correlated(kN) WH) (e) (Eh) (e) (Eh)
Science Park Building 5 9 25.9 7997 0.82 0.83 0.95 0.63 0.90 Science Park B 7 & 8 16 27.9 7543 0.96 0.96 0.98 0.74 0.95 Schools in Fanling 22 28.3 8630 0.91 0.80 0.93 0.55 0.90 Science Park Building 6 13 28.9 7902 0.76 0.80 0.90 0.60 0.86Project in Shatin 20 30.6 8261 0.99 0.99 0.99 0.80 0.95 Science Park Building 4 15 32.8 7786 0.93 0.92 0.98 0.72 0.95 Schools in Tung Chung 14 34.7 8596 0.91 0.94 0.99 0.76 0.95 School in San Po Kong 15 40.0 6936 0.94 0.79 0.95 0.62 0.90 Schools in Kowloon Bay (1) 14 41.4 6298 0.74 0.83 0.75 0.55 0.75 Schools in Tin Shui Wai 18 43.2 7956 0.87 0.79 0.95 0.60 0.90 Schools in West Kowloon 14 44.7 6529 0.66 0.80 0.72 0.52 0.72Schools in Kowloon Bay (2) 15 44.9 6761 0.90 0.85 0.95 0.67 0.90Schools in Lai Chi Kok 20 45.7 6737 0.99 0.88 0.95 0.71 0.90Project in Tin Shui Wai 44 49.7 7665 0.94 0.92 0.98 0.72 0.95School in Cheung Sha Wan 14 50.0 6181 0.99 0.77 0.95 0.62 0.90Schools in Tokwawan 17 50.2 6810 0.95 0.90 0.95 0.64 0.95Science Park Building 9 14 51.6 7128 0.93 0.91 0.98 0.72 0.95School in Tseung Kwan O 33 53.0 7402 0.97 0.92 0.95 0.70 0.95
Notes : ETR : Energy transfer efficiency. EMX: Maximum energy transmitted.
Fig 2. Comparison ofCAPWAP capacitiesand Hiley capacitiescalculated using thecorrelated e and Ehvalues
30|The Structural Engineer 19 October 2004
paper: fung et al
Under some special or difficult ground conditions, the HileyFormula may be unusable for assessing pile capacity.Situations arise, where the Formula gives odd results or themeasured final set is out of the set table range even whenappropriate e and Eh values have been used, all this suggest-ing that the Hiley Formula has reached its limitation. Underthese circumstances, PDA tests and CAPWAP analyses shouldbe used instead. Moreover, at least one of the piles with thelowest CAPWAP capacity should be load tested for verificationand acceptance.
Before the start of pile driving, visual inspection of the typeand quality of the hammer cushion is essential. The ram andhammer helmet should be weighed. PDA tests should then becarried out for trial piles (3% of the total number of piles or fivepiles, whichever is greater), and these piles should be spreadevenly over the site including the area where the longest piledepth is expected. The Energy Transfer Efficiency of thedriving system, EMX/WH, should be recorded, checked andreviewed to ensure that driving system efficiency during finalset matches that for the trial piles. Thereafter, PDA testsshould be carried out regularly during the pile constructionperiod. Changes in the Energy Transfer Efficiency should beclosely monitored so as to detect any significant changes in thedriving system efficiency.
The energy used to drive piles at PDA restrike tests shouldbe the same as that during final set (i.e. using the samehammer with the same drop height). Normally the set meas-ured at restrike would be expected to be less than that at finalset due to the build-up of soil resistance. If the set values of therestrike tests are larger than those obtained during the finalset, it could mean that the pile had been driven with reducedenergy towards final set and an investigation should beconducted accordingly.
To have accurate pile capacity predictions, PDA tests andCAPWAP analyses should only be performed by properlytrained and experienced personnel.
Conclusions Based on the static load test results, it is noted that
CAPWAP analysis is a fairly accurate method for driven pilecapacity prediction.
It has been shown that for piles driven by hydraulichammers, the Hiley Formula remains applicable for theassessment of pile capacity with good accuracy, providedappropriate e and Eh values are used.
As a practical approach to assessing pile capacity, trial pilesshould be driven at the beginning of any piling contract.Theefficiency of the hammer drop (Eh) and the coefficient ofrestitution of the hammer cushion (e) are determined by
using CAPWAP analysis. The combination of Eh and eshould be chosen such that when these values are substi-tuted into the Hiley Formula, the average predicted bearingcapacity of the trial piles is not higher than 85% of theaverage CAPWAP capacity prediction.
It is worth noting that variation of Energy TransferEfficiency (ETR) amongst hammers is large, ranging from66% to 99% for the 18 sites covered in this study. It is there-fore essential to assess each set of driving equipment todetermine the appropriate values of e and Eh.
Table 2: Comparison of CAPWAP capacities and Hiley capacities calculated using thecorrelated e & Eh values for each individual site
Project Hiley capacity Project Hiley capacityNo. of CAPWAP capacity No. of CAPWAP capacitypiles Mean Standard piles Mean Standard
deviation deviationProject in Schools inShatin 20 1.00 0.042 Kowloon 15 1.00 0.053
Bay (2) Science Park Schools inBuilding 6 13 1.00 0.055 West Kowloon 14 1.00 0.055Schools in School inTung Chung 14 1.00 0.045 Tseung 33 1.00 0.078
Kwan O Science Park Schools inBuilding 5 9 1.00 0.035 Fanling 22 1.00 0.059Science Park Schools in Building 9 14 1.00 0.052 Tokwawan 17 1.00 0.068Project in Schools inTin Shui Wai 44 1.00 0.060 Kowloon 14 1.00 0.032
Bay (1) Science Park Schools inB. 7 & 8 16 1.00 0.059 Lai Chi Kok 20 1.00 0.050Schools in School inTin Shui Wai 18 1.00 0.044 San Po Kong 15 1.00 0.076Science Park School inBuilding 4 15 1.00 0.043 Cheung Sha 14 1.00 0.043
Wan
Abe, S. and Thendean, G.: Hammer performance evaluations,5th Int. Conf. Application of Stress-Wave Theory to Piles, Orlando,Florida, 1996
Biddle, A. R.: Steel Bearing Piles Guide, Steel ConstructionInstitute, 1997
Davisson, M. T.: High capacity piles, Proc. Lecture Series,Innovations in Foundation Const., (1972), SM & FD, ASCE, IllinoisSection, Chicago
Fung, W. K., Wong, C. T. and Wong, M. K.: A study on capac-ity predictions for driven piles, (2004), Trans. HKIE
Likins, G., Rausche, F., Thendean, G. and Svinkin M.: CAPWAPCorrelation Studies, 5th Int. Conf. Application of Stress-WaveTheory to Piles, Orlando, Florida, 1996
Pile driving analyzer manual, (2000), Pile Dynamics, Inc., 4535Emery Industrial Parkway, Cleveland, OH 44128
REFERENCES
Appendix B Parameters for hammers and cushion materials Steel Bearing Piles Guide, by A. R. Biddle (1997)
Efficiency of drop (Eh) Drop hammer winch operated 0.7Drop hammer trigger fall 0.9Hydraulic hammer 1.0
Cushion materials Coefficient of restitution (e) Micarta plastic 0.8 Greenheart oak 0.5 Other timber 0.3
Appendix A Traditional parameters for hammers andcushion materials
Type of hammer Efficiency of drop (Eh)Drop hammer winch operated1 0.8Single acting hammer1 0.9
Cushion materials for steel pile Coefficient of restitution (e)Driving cap and short dolly covered by steel plate1 0.32Driving cap with standard plastic or green heart dolly2 0.5 Driving cap with timber dolly2 0.3
Notes: 1 CP4 Foundations (1954); 2 BSP Handbook
Fig 3: Comparison of HileyFormula predictionswith static load testresultsThe Davissonfailure load was notreached
7000