the capacity of driven steel h-piles_l.j. endcott (2014)

8/10/2019 The Capacity of Driven Steel H-Piles_L.J. Endcott (2014)

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THE CAPACITY OF DRIVEN STEEL H-PILES

L. J. Endicott and X. Zhang

AECOM Asia Company Ltd

The Hiley formula, which is fundamentally based on transfer of energy from the

hammer to the pile, has been used for estimating the capacity of driven piles for

decades. In the early years the formula gave reasonable results, notably for short

piles. However as piles have become longer and of higher capacity and as new

techniques have developed the formula has become less reasonable and some

users have modified the formula. Pile Dynamic Analysis (PDA) was devised as a

mean of numerically modeling the dynamic effects of driving piles. Instrumentation

for PDA provides a direct measurement of the forces transferred to the top of the

piles during driving. Comparison between results of many static load tests withPDA has validated the PDA method of determining the capacity of the piles. As a

consequence PDA is now widely adopted as the method for acceptance of capacity

of driven piles in Hong Kong.

As a consequence, in addition to dozens of results of static load tests, results of

many hundreds of PDA tests are available. These results provide the opportunity

to examine some long held concepts such as the smaller the final set, the higher

is the capacity of the pile. By contrast there is substantial evidence that in many

cases the driving force in the piles can be independent of their final set over quite

a wide range of values of final set.

Based on a study of the test results for many piles it is proposed that it is timeto change methods of determining capacity of piles by using modified versions of

the Hiley Formula and to use the PDA method. Moreover the PDA method can

be used to determine maximum stresses in piles and thereby establish criteria to

prevent damage to piles during driving.

Keywords:Hiley Formula, Pile Dynamic Analysis, Capacity of Piles.

1. INTRODUCTION

Pile Dynamic Analysis (PDA) testing has been in use for several decades to check the

integrity of piles, Reference Goble et al (1996). As the result of comparison between hun-dreds of static load tests with PDA tests analyses during the last decade it has been observed

that analysis is a fairly accurate method for pile capacity prediction, Reference Fung et al

(2004). It has become a common practice in Hong Kong to adopt PDA testing for check-

ing the ultimate capacity of driven piles and it is quite common nowadays for contracts

whereby every driven pile is subjected to PDA testing. As a consequence there are many

hundreds of PDA testing results.

PDA testing involves instrumentation with strain gauges and accelerometers attached

to the top of each pile such that during the fractions of a second whilst the hammer strikes

Advances in Foundation Engineering

Edited byK. K. Phoon, T. S. Chua, H. B. Yang and W. M. Cham

Copyright 2014 Research Publishing Services.

ISBN: 978-981-07-4623-0 :: doi:10.3850/978-981-07-4623-0 049 345


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the pile the driving force in the pile can be deduced form the strain gauges and the acceler-

ation can be measured and the velocity and displacement can be computed, Pile Dynamics

Inc (2000). From this data, the maximum driving force, FMx, at the top of the pile can be

determined. The analysis is configured in order to estimate the frictional forces down theshaft of the pile and the reaction at the toe, for which the maximum force at the toe is

RMx. The analysis requires input estimates of dynamic properties of the ground and trials

are carried out to match the recorded FMx vs. time recorded at the top of the pile. In the

past, calibration of the PDA analysis was carried out by comparison with static load tests.

Comparison of PDA results with static load tests has identified dynamic properties of the

ground which give good agreement with the static load test results and now less static load

tests are carried out.

Notwithstanding the use of PDA testing to prove the capacity of driven piles, many con-

tracts require the use of the Hiley Formula, or similar, to estimate the final set for drivingaccording to the size of pile, length of the pile, size of hammer and the strike of the hammer

to be used.

This paper makes use of PDA test data from three sites in Hong Kong where steel

H-piles of 350 mm 350 mm 180 kg/m size were driven to depths generally in the

range of 20 m to 60 m using hydraulic hammers ranging from 16tonnes to 25tonnes and

with strikes of 0.4 m to 4 m. Test results are available for driven piles with a set of the order

of 400 mm per 10 blows down to less than 10 mm per 10 blows.

2. MODIFIED HILEY FORMULA

The Hiley Formula has been used for decades to estimate the capacity of driven piles.

It considers the energy of WH from the hammer, of weight Wand strike or drop height of

H, transmitted to a pile of weight P, with a coefficient of restitution ofe and an efficiency

ofEh. It also considers the absorption of the energy by the set s, and by half of the tempo-

rary compression of the cushionCc, and the temporary compression of the pile Cpand of

the soilCs. The energy transmitted divided by the absorption gives the driving reaction R

which is taken to be the capacity of the pile. The Hiley Formula can be stated as follows:

R=Eh

W+Pe

2

W+P

W H

S+ 12 (Cc+Cp+Cs)(1)

By inspection of this formula, the set, s, is in the denominator and thereforeR, the driving

resistance or capacity, is inversely proportional to the magnitude of the set.

It is common to estimate the temporary compression of the pile by computing the elastic

compression under the imposition of the driving force as follows:

Cp= R L

E

A

(2)

Where L is the length of the pile, E is the elastic modulus of the pile and A is the cross

sectional area of the pile.

Cpis proportional toL and appears in the denominator of the Hiley Formula and there-

foreR, the computed driving force, is related inversely to the length of the pile.


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The Capacity of Driven Steel H-Piles 347

It is a common practice to determine the set to which a pile shall be driven to achieve

a specified capacity and to produce a tables with values of set decreasing for increased

lengths of pile for different weights and drop of the hammer.

3. PDA TEST RESULTS

Instrumentation data from the PDA tests include values for FMx. This is the measured driv-

ing force at the top of the pile and, according to the Hiley Formula, it should be inversely

related to the set. This conception has been examined by combining the data from all three

sites and includes the results from 496 PDA tests. Figure 1 shows a plot of values of FMx

vs. set (mm/10 blows) for a 16 tonnes hammer.

It is evident that, for sets in the range of about 50 mm/10 blows to nearly 400 mm/10

blows, the driving capacity is very close to 6000 kN. This result is contrary to the conceptionthat as the driving force is inversely related to the set. A similar plot for the use of a 25

tonnes hammer is shown in Figure 2.

This data also does not reflect an inverse relationship between the set and the driving

force.

Both sets of data show significantly increased forces as the set is reduced below about

10 mm/10 blows. This set is defined as refusal (CP, 2004). Such hard driving normally is

associated with end bearing on very hard material such as rock whereby the compression

wave in the pile is reflected at the toe and travels up the pile and thereby increases the

maximum force at the top of the pile.Based on this interpretation, the basic driving force in a pile with little or no reflection

from the base might be considered to be of the order of about 6000 kN for the 16 tonnes

hammer and about 7000 kN to 7300 kN when using a 25 tonnes hammer.

It may also be considered that when driving to refusal, reflection from the base increases

the maximum force at the top of the pile to more than 8000 kN for these two cases.

Another consideration is that the driving force is proportional to the mass of the hammer

and the strike. Figures 3 and 4 show plots of FMx vs. Strike for the 16 tonnes hammer and

25 tonnes hammer respectively from the data with final set over 25 mm. These plots show

a trend of increasing maximum driving force with increasing strike. However it is evident

that FMx is not proportional to strike as adopted in the Hiley Formula.The maximum force at the base of the pile is not measured directly, it is computed by

a process of estimating by trial the shaft friction and matching the measured force/time/

deflection relationships measured at the top of the pile. However the comparison of PDA

Figure 1. FMx vs. Set utilizing a 16 T hammer. Figure 2. FMx vs. Set utilizing a 25 T hammer.


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Figure 3. FMx vs. Strike of 16 T hammer. Figure 4. FMx vs. Strike of 25 T hammer.

Figure 5. RMx vs. set for 16 T hammer. Figure 6. RMx vs. set for 25 T hammer.

Figure 7. RMx and FMx vs. Pile length.

test results with static load tests indicate that the computations are generally reasonably

good, Fung et al. (2004). Figure 5 shows a plot of RMx vs. set for the 16 tonnes hammer.

The results are similar to the values of FMx with some differences. For sets (shown in Fig. 1)

greater than refusal (10 mm/10 blows) some values of FMx are less than 6000 kN, the value

for FMx at the top of the pile, and this may be attributed to a reduction of force in the piles

due to shaft friction. Likewise, when driving to refusal (less than 10 mm/10 blows) the

RMx is increased with values generally similar to those recorded at the top of the pile but

marginally greater.Likewise Figure 6 shows a plot of RMx vs. set for the 25 tonnes hammer. In this plot there

is a tend with a lot of results lying above the value of RMx = 7200 kN and when drivingto refusal the maximum values rise to about 8500 kN which is marginally more than that

recorded for FMx as shown in Figure 2.


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The Capacity of Driven Steel H-Piles 349

Pile length varies in the studied area. Figure 7 shows the differences between RMx and

FMx value for different pile length .The plot shows that the influence of pile length on RMx

and FMx are not significant.

4. CONCLUSIONS

Based on this study, it can be concluded that 305 mm 305 mm 180 kg/m H-piles

driven by 16 tonnes and 25 tonnes hammers can achieve a capacity of 6000 kN to about

7200 kN without driving to refusal for a range of set values from 10 mm/10blows to as

much a 120 mm or even 350 mm (the range of data available). Importantly the driving

force is almost constant, unrelated to the set, until refusal is achieved and reflection from

the toe increases both the driving force and the bearing capacity. The actual driving force

and capacity can be increased by increasing the strike, but the force and capacity are notdirectly proportional to the strike, and are not related to the length of the pile, as adopted

in the Hiley Formula.

By driving to refusal, i.e. to sets of 10 mm/10 blows or less the forces in the piles are

computed to increase by as much as 2000 kN. Such hard driving runs the risk of damage

to the tips of the piles unless they are reinforced.

It is considered that, with the continued use of PDA testing for driven piles, the selection

of weight of hammer and strike could be determined empirically from previous PDA data

and that the Hiley Formula, even with modifications is no longer a useful predictor.

REFERENCES

1. CP2004, Code of Practice for Foundations, Buildings Department, the Government of theH.K.S.A.R., October 2004.

2. Fung, W. K., Wong, M. K. and Wong, C. T., A study on Capacity Predictions for Driven Piles,H.K.I.E. Transactions Volume 11 Number 3(2004), 1016.

3. LI, W. W., Wong, M. K. and Chan, Y. K., The Application of PDA/CAPWAP to Ensure Qualityand Capacity in Driving Long Steel H-piles. H.K.I.E. Transactions Volume 18 Number 2(2010),pp. 1016.

4. Goble, G. G. and Likins, G. E., On the application of PDA Dynamic Pile Testing,STRESSWAVEConference 1996. Orlando, Fl (1996).

5. Pile Dynamics Inc., Pile Driving Analyser Manual 4535 Emery Industrial Parkway, Cleveland,OH 44128 (2000).

the capacity of driven steel h-piles_l.j. endcott (2014)

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