W K FUNG MSc PhD CEng FlStructE FHKIE Email:[email protected]C T WONG MSc CEng MICE MlStructE MHKIE Architectural Services Department, The Government of HKSAR Email: [email protected]ultimate pile capacity by varying degrees. Introduction Generally speaking, there are two approaches to the assessment of pile capacity, viz the pile driving or dynamic formulae and the stress-wave (wave equation) analysis. Different pile driving formulae have been developed in the past of which the Hiley Formula is most widely used in Hong Kong. Fung et al (2004) discussed the application of CAPWAP analysis (stress-wave method) for driven piles in Hong Kong, and the use of the Hiley Formula (with a proposed method for the assessment of parameters) as a handy means to predict the bearing capacity of piles. There are some other researchers who have proposed a Modified Hiley Formula by adopting the energy measured by PDA method (stress-wave method). Against this background, this paper wil discuss the predictions of this Modified Hiley Formula in comparison with static load test results for 15 Grade 55C steel H-piles. It will also discuss the results of the correlation studies of pile capacities predicted by CAPWAP and the Modified Hiley Formula for 566 Grade 55C steel H-piles driven by hydraulic hammers at 20 different sites. The Energy Approach and the Modified Hiley Formula The traditional Hiley Formula assumes that the force/displacement relationship is elasto-plastic (see Fig l). It uses an energy approach and assumes that the ‘energy left after impact by a hammer’ equals to the ‘work done by the pile resistance and losses of the pile-soil system’. In order to estimate the energy left after impact, the Newton's law of impact and some other assumptions are used. While acknowledging the drawbacks in the over-simplification of the driving process, it is generally recognised that the formula is practical and easy to use. Instead of using the energy calculated by the traditional Hiley Formula, Broms 8 Lim (1988) suggested a Modified Hiley Formula (Eq 1) by using the energy measured at the pile head by PDA method. EMX R _ Eql 1 s+—C+C 2<,, Q) Observations on Using the Energy Obtained from Stress-wave Measurements in the Hiley Formula M K WONG MSc CEng M|StructE MHKIE Architectural Services Department, The Government of HKSAR Architectural Sen/ices Department, The Government of HKSAR Email: [email protected]The use of energy obtained from stress-wave measurements in the Hiley Formula to assess load carrying capacity of steel H-piles is discussed. It has been tried out on 566 piles and the predicted results are compared with those obtained by CAPWAP analysis and actual static load tests. The results show that the Modified Hiley Formula tends to overestimate the Keywords: Davisson’s Failure Criterion, Stress-wave Analysis, Pile Driving Analyser (PDA), CAPWAP Analysis, Modified Hiley Formula, Hydraulic Hammer, EMX I-'m'i:e 4 i-1., A Work _ F _ I I___ __ i I l)1's.plac+:n1i:nt Set Tr:n1poi'ary (‘oii1|1re:as|i'iii Figure 1 — Assumed Force vs Displacement at the Top of the Pile where R driving resistance EMX the maximum energy transmitted past the transducers attached at the pile head, calculated by PDA using the measured strain and acceleration. s set per blow Cp + Cq temporary compression of pile and soil in general, the energy E imparted to the pile during the driving process can be obtained by using the measured strain and acceleration at the pile head position. The energy is: E = ftF(t)V(t)dt Eq 2 where F(t) and V(t) are the measured force and velocity at the pile head pod¿on. TRANSACTIONS ~ Volume 12 Number 4
C T WONG MSc CEng MICE MlStructE MHKIEArchitectural Services Department, The Government ofHKSAREmail: [email protected]
ultimate pile capacity by varying degrees.
IntroductionGenerally speaking, there are two approaches to the assessment of pilecapacity, viz the pile driving or dynamic formulae and the stress-wave(wave equation) analysis. Different pile driving formulae have beendeveloped in the past of which the Hiley Formula is most widely usedin Hong Kong. Fung et al (2004) discussed the application of CAPWAPanalysis (stress-wave method) for driven piles in Hong Kong, and theuse of the Hiley Formula (with a proposed method for the assessmentof parameters) as a handy means to predict the bearing capacity ofpiles.There are some other researchers who have proposed a Modified HileyFormula by adopting the energy measured by PDA method (stress-wavemethod). Against this background, this paper wil discuss the predictionsof this Modified Hiley Formula in comparison with static load testresults for 15 Grade 55C steel H-piles. It will also discuss the resultsof the correlation studies of pile capacities predicted by CAPWAP andthe Modified Hiley Formula for 566 Grade 55C steel H-piles driven byhydraulic hammers at 20 different sites.
The Energy Approach and the Modified HileyFormula
The traditional Hiley Formula assumes that the force/displacementrelationship is elasto-plastic (see Fig l). It uses an energy approach andassumes that the ‘energy left after impact by a hammer’ equals to the‘work done by the pile resistance and losses of the pile-soil system’.In order to estimate the energy left after impact, the Newton's law ofimpact and some other assumptions are used. While acknowledging thedrawbacks in the over-simplification of the driving process, it is generallyrecognised that the formula is practical and easy to use.Instead of using the energy calculated by the traditional Hiley Formula,Broms 8 Lim (1988) suggested a Modified Hiley Formula (Eq 1) by usingthe energy measured at the pile head by PDA method.
EMXR _ Eql
1s+—C+C2<,, Q)
Observations on Using the Energy Obtained fromStress-wave Measurements in the Hiley Formula
M KWONG MSc CEng M|StructE MHKIEArchitectural Services Department, The Government ofHKSAR Architectural Sen/ices Department, The Government ofHKSAR
The use of energy obtainedfrom stress-wave measurements in the Hiley Formula to assess load carrying capacity of steelH-piles is discussed. It has been tried out on 566 piles and the predicted results are compared with those obtained byCAPWAP analysis and actual static load tests. The results show that the Modified Hiley Formula tends to overestimate the
Figure 1 — Assumed Force vs Displacement at the Top of the Pile
where R driving resistanceEMX the maximum energy transmitted past the transducers
attached at the pile head, calculated by PDA usingthe measured strain and acceleration.
s set per blowCp + Cq temporary compression of pile and soil
in general, the energy E imparted to the pile during the driving processcan be obtained by using the measured strain and acceleration at thepile head position. The energy is:
E = ftF(t)V(t)dt Eq 2
where F(t) and V(t) are the measured force and velocity at the pile headpod on.
TRANSACTIONS ~ Volume 12 Number 4
EMX is, however, the maximum energy obtained with ‘t’ at the instantwhen ‘V’ changes from positive to negative, ie at zero velocity (the pilehead displacement is also at a maximum value at this instant) (see Fig2). This is the maximum energy imparted onto the pile during the wholedriving process. Thereafter, the stress waves with the imparted energycontinue to pass along the body of the pile, hammer and soil system.Broms Et Lim (1988) considered that after V = 0, energy is returned tothe hammer until the hammer separates from the pile.Paikowsky 8 Chernauskas (1992) discussed an approach similar toEquation 1, and appropriately pointed out that the equation considersonly the elasto-plastic energy losses of the pile-soil system, ie it onlyaccounts for energy losses due to elastic deformation in the pile and thesoil as well as plastic deformation in the soil. As no account is made forthe energy losses due to the dynamic action (eg radiation, soil inertia,true damping etc), the Energy Approach analysis prediction values maybe regarded as the maximum possible resistance. In order to accountfor all dynamic related energy losses, Pa'kowsky 8 Chernauskas (1992)proposed that a correction factor of 0.8 be used to reduce the capacityobtained by Eq 1.
'F79000l—— f A12 F12|\/(£L€p‘)""' 1
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, Fm
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~ T* - ‘ (time)1 \ /1
. \ /
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l
Energy (t)
'E'Z3oo.o)—o (120)-
-------- _--Q--Q--op-Q->—--_-s --_-
' : Displacement (t) EMX/-i/ \\
,/-I
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_ . 1Figure 2 — The Variations of Force, Velocity, Energy and Displacementwith Time at Pile Head Level
Comparison of Pile Capacities Predicted by theModified Hiley Formula with Static Load TestThe objective of this study is to compare the predictions of the ModifiedHiley Formula with those obtained from static load tests. The stress-waveanalysis (CAPWAP analysis on restrike data) was also used to predict thepile capacities for comparison. Some of the results have been discussedin Fung et al (2004).Fifteen steel H-piles from seven sites were static load tested to ‘failure’.The failure criterion proposed by Davisson (1972) was used to define the‘failure load’ of a static load test. However, in order not to overstress orpermanently damage the steel material of the piles, they were loadedto Davisson’s failure load or about 85% of the yield strength of thesteel, whichever was less. Among the 15 piles tested, 11 were loaded toDavisson’s failure load whereas the other 4 reached about 85% of theyield strength of the steel before the Davisson’s failure load could beidentified on the load settlement curve. The capacities predicted by the
two methods were compared with the failure loads of static load testsand the results are shown in Fig 3 and Fig 4 respectively (see AppendixA for test pile data).From the static load test results of the 15 piles, it is found that theCAPWAP analysis can predict the Davisson failure load fairly accuratelyand with reasonable confidence (most of the CAPWAP/SLT ratios liebetween 0.9 and 1.1). By comparison, the Modified Hiley Formula (ieusing measured energy by PDA method) tends to overestimate theultimate pile capacity.
Comparison of Pile Capacities Predicted by theModified Hiley Formula with those by CAPWAPAnalysisIn addition to the studies of Modified Hiley versus static load test, furtherstudies were carried out by comparing the pile capacities predicted bythe Modified Hiley Formula with those obtained by CAPWAP Analysis.Here, the sample consists of a batch of 566 steel H—piles of threedifferent sizes ie 305 x 305 x 223 kg/m, 305 x 305 x 180 kg/m and305 x 305 x 149 kg/m. They were driven by hydraulic hammers at 20
The Hong Kong Institution of Engineers
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1 ‘.-_
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| l
I ‘III 3 I :i 1 I I :I' ilii l 1""l! l= it -. I I
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lJEi 055 [I5 0'55 1 ‘-.05 '-l 11511 TIi-!’.'-1'_'|l351-l1451515515155
Figure 5 — Comparison of Capacity Calculated by Modified Hiley Formulaand CAPWAP Analysis
different sites. The ultimate capacities were calculated using the aboveModified Hiley Formula with the EMX determined for every pile from PDArestrike test. The capacities so obtained were compared with CAPWAPpredictions on restrike data (Linklns et al, 1996) and the results aresummarised in Fig 5. For these 566 piles, the ratio of ‘Modified HileyCapacity/CAPWAP Capacity’ varies from 0.81 to 1.65, with a mean of1.167. The standard deviation of 0.141 with the coefficient of variationof 0.121, is considered quite large.In addition to the global view presented in Fig 5, the study results,grouped by the 20 sites in which they were tested, are presented in Table1 below. The results show that, the coefficient of variation of the ratio,varying from 0.049 to 0.119, is generally large. Furthermore, the variationof the mean among different sites, from 0.99 to 1.44, is also large.The results suggest that the Modified Hiley Formula tends to overestimatethe ultimate pile capacity, and that the degree of overestimation wouldvary from one site to another.
Observations
(i) The Modified Hiley Formula (using measured energy by PDA method)tends to overestimate the ultimate pile capacity and the degree ofoverestimation would vary from one site to another.
(ii) The results show that the coefficient of variation of the ‘ModifiedHiley Capacity/CAPWAP Capacity’ ratios is generally large.
References1. Broms, B.B. and Lim, P.C. A simple pile driving formula based on stress wave
measurements, Proc. 3rd International Conference on Application of Stress-waveTheory to Piles, pp 591-600, (1988).
2. Davisson, M. T., High Capacity Piles, Proceedings, Lecture Series, Innovationsin Foundation Construction, SM 8 FD, ASCE, Illinois Section, Chicago (1972).
3. Fung, W.K., Wong, C.T., and Wong, M.K. A study on capacity predictions fordriven piles. Transactions, HKIE, Volume 11, Number 3, September 2004.
4. Likins, G., Rausche, F., Thendean, G., and Svinkin M., CAPWAP CorrelationStudies, Fifth International Conference on the Application of Stress—Wave Theoryto Piles, Orlando, FL, (1996).
5. Paikowsky, S.G. 8 C1ernauskas, L.R. Energy approach for capacity evaluation ofdriven piles, Proc. Conf. On Application of Stress-wave Theory to Piles, Balkema,pp 595-601 (1992).
SciencePark 21Building 92
49.1 272 1.09 0.087 0.080
Schools inArea 111, 28TSW2
43.6 310 1.09 0.105 0.096
SciencePark 30Building 7& 82
27.9 327 1.12 0.107 0.095
Wetland 44Park2
49.7 295 1.13 0.067 0.059
Schools atFat Cheung 28St.2
45.1 267 1.14 0.084 0.074
Schoohin 39FanHng2
29.1 356 1.14 0.105 0.092
SciencePark 15Building 42
32.8 338 1.15 0.072 0.063
Schools inKowloon 18Bay2
44.6 283 1.16 0.095 0.082
Schools in 29Tokwawan2
50.8 293 1.16 0.112 0.097
School in 33TKO (1)2
52.8 314 1.17 0.101 0.086
Schoolat 29Sze Mei St.2
39.5 299 1.20 0.130 0.108
Schools inTKO (2)2 30 51.2 275 1.22 0.083 0.068
Schools atKai Yan St.2 24 39.9 276 1.23 0.087 0.071
Schools atLai Hong 30St.2
44.3 287 1.25 0.097 0.078
Schools inArea 104, 31TSWl
40.2 300 1.30 0.154 0.119
School in 40C S W2
51.6 264 1.44 0.092 0.064
Total no.of Piles
566 Average 1.167
1 305 x 305 x 149 kg/m stee2 305 x 305 x 180 kg/m stee3 305 x 305 x 223 kg/m stee
Table 1 — Ratio of ‘M
H-pHesH-p esH-pHes(dnven by drop hannnen
odified Hiley Capacity / CAPWAP Capacity’
. 21TRANSACTIONS Volume 12 Number 4
Appendix A — Test Pile Data
Project/Pile No Embedded 6,, + Cq Final Set/Blow EMXLength (m) (mm) (mm)
School in Cheung3Sha Wan/ P59 47.5 53 2.1 234 6216 8171
Science Park B9/P2973 49.0 55 1.0 217 7100 7621
1 The Davisson Failure Load Was Not Reached2 305 x 305 x149 kg/m steel H-pile3 305 x 305 X180 kg/m steel H-pile
$2’n I4-11". '
1
W K FUNG
Ir W K Fung graduated from the Hong Kong TechnicalCollege in 1966. After working for Palmer and Turnerfor 4 years, he joined the Public Works Departmentand worked as Assistant Structural Engineer in theBuildings Ordinance Office. Since his transfer to theArchitectural Office in 1972, he has worked his wayup to his present rank of Assistant Director (StructuralEngineering) of the Architectural Services Department.Between 1997 and 2002 he served as Chairman of theStanding Committee on Concrete Technology. His main
interests are foundations and material science. He completed his PhD programmeon the use of recycled concrete in construction in 2005.
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C T WONGlr Wong Chi-tong is a Senior Structural Engineer ofthe Architectural Sen/ices Department. He graduatedfrom the Hong Kong Polytechnic in 1976. Sincethen, he undertook civil and structural engineeringtraining in Palmer and Turner and became a CharteredEngineer in 1980. lr Wong joined the Government in1981 working in ASD responsible for the design andconstruction of various types of building structures. lrWong has completed many school buildings, markets,offices, in-door game halls, swimming pool complexes
and sport grounds. His main interests are foundations, structural steel and use ofdifferent construction materials.
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M K WONGlr M K Wong is a Structural Engineer of the ArchitecturalServices Department. After graduating from the HongKong Polytechnic in 1985, he joined the ArchitecturalServices Department and worked as StructuralEngineering Graduate. He became a Chartered Engineerin 1989 and obtained a MSc degree in Civil Engineeringfrom the University of Hong Kong in 1994. His maininterests are dynamic pile testing and related researchwork.
