Application of Stress-Wave Theory to Piles, Niyama & Beim (eds) Cl 2000 Balkema, Rotterdam, ISBN 90 5809 150 3

A comparative study of static, dynamic and statnamic load tests of steel pipe piles driven in sand

A. Shibata - Kubotu Corporation, Ichikawu, Japan N. Kawabata -Nippon Steel Corporation, Tokyo, Japan YWakiya - Kawasaki Steel Corporation, Tokyo, Japan YYoshizawa - Sumitorno Metal Industries Limited, Tokyo, Japan M. Hayashi - NKK Corporation, Kawasaki, Japan T. Matsumoto - Department of Civil Engineering, Kanazawa University, Japan

ABSTRACT Comparative static, dynamic and the Statnamic load tests on three steel pipe piles driven in sandy ground were conducted. In this test program, dynamic load tests were performed at various time intervals after the initial driving to investigate the variation of the pile capacity with different hammer driving energies, and the increase in the pile capacity with elapsed time after the initial driving Two different computer programs were employed for the wave matching analyses to estimate the static load-displacement curves for the piles, which were compared with the static load test results The Statnamic test was also performed on one of the test piles that had underwent the static load test. This paper will discuss the uses of the dynamic and the Statnamic load tests as alternatives to the static load test

1 INTRODUCTION

More than 20 comparative cases of the dynamic and the static load tests on driven steel pipe piles were collected by the Japanese Associatioa for Steel Pipe Piles (JASPP) to examine the use of the dynamic load testing to estimate the static load-displacement curve for steel pipe piles. The data were collected from the field tests with various pile configurations, various soil conditions, various driving hammers used, and various rest periods after the initial driving for the dynamic load test (Wakiya et al. 2000).

The collected data suggested that the reliability of the static load-displacement curve estimated through the wave matching analysis of the dynamic load test signals depends on hammer energy, rest period before the re-driving test, the computer program used for the wave matching analysis and soil test data available for the site.

Therefore, JASPP conducted their own test program of various load tests on three open-ended steel pipe piles in a relatively uniform sandy ground at Hasaki (the test ground of Sumitorno Metal Industries), Japan, in 1993, to evaluate the use of the dynamic load test and the Statnamic load test for piles in sandy soils as an alternative to the static load test. The emphasis was placed on the following goals in this particular test program:

1. To evaluate the influence of different hammer driving energies on the estimated pile capacity.

2. To measure the increase in the bearing capacity of the pile with elapsed time after initial pile driving, (the so-called "set-up" phenomena).

3. To investigate the differences between the results of various computer programs used for the wave matching.

2 TEST DESCRIPTION 2 1 The fes f sife md f es f pr1e.r Figure 1 shows the soil profile and the results of site investigations at the test site The Standard Penetration Test (SPT) and the Cone Penetration Test (CPT) were conducted immediately before and 7 weeks after the pile installation The test ground consisted of fine to gravel sands from the ground surface to a depth of 20 m The SPT N-value measured prior to the pile installation was relatively high and uniform at depths greater than 7 m The ground water level existed at a depth of 5 m from the ground surface The variation with depth of the tip resistance, qc, from the CPT before the pile installation seems to be similar to the variation of the SPT N-values The sleeve friction, A, tends to increase linearly with depth to a depth of 9m and level off for larger depths

The layout of the test piles, the location of site investigations and a piezometer are shown in Figure 2 PThree test piles, designated piles D for dynamic load testing, S for static load testing and M for soil measurements, were prepared The test pile specifications are listed in Table 1 Pile S and pile D were individually prepared, so that the dynamic and static load tests could be carried out at the same elapsed time after initial pile driving to minimize the

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Figwe 1. Soil profile and results of site investigations at the test site.

Figure 2 Lavout of the test piles, the location of site inwtigations and a piezometer

Table 1 Specifications of test piles Propert) Value Length L ni 13 0 Outer diaineter U inin 100 0 Wall tluckness t,, Illill 12 0 Cross-sectional area ,A ni2 0 017 Youngs modules E MNlin 206x10 Mass densih p t/in 7 85 WaL e 1 elocity c m/s 5120 Mass .U ton 1.78

Cross-sectional area- ;1, includes the cross-sectional area of steel channels for protection of strain gages.

influence of the loading history for each pile. Pile S was instrumented with strain gages at a total of 6 levels. Steel channels were welded to the outside of the pile shaft for the protection of the strain gages. Note that steel channels were attached to piles I) and

M also, so that piles D and M had the same configuration as pile S, although piles D and M were not instrumented with strain gages. The steel channels increased the net cross-sectional area of the test piles to 0.017m2.

Pile M was prepared for the purpose of investigating the change in the soil conditions around pile M before and after the pile driving. The CPTs and the SPTs were conducted at a distance of 0.4m from the center of pile M immediately before and 7 weeks after the pile driving. There was little increase in N-values and qc-values, while there was a clear increase in&-values after pile driving to depths deeper than 8m. The excess pore pressures measured by the pre and post CPTs were very small indicating the relatively high permeability of the sand.

An electric pore pressure transducer (piezometer) was placed in the ground at a depth of 11.5 m from the ground surface prior to the initial pile driving. The horizontal distance between the center of pile D and the piezometer was 0.4m, which was equal to the outer diameter of the test pile.

2 2 Test sequences and procedures A series of dynamic load tests was performed on pile D after 8 different time intervals, 0 min, 5 min, 15 min, 30 min, 1 hour, 3 hours, 20 hours, 6 days and 30 days, after the initial driving. In the dynamic load tests, the hydraulic hammer (HK65), which allows the control of the hammer driving energy, was used. Pore pressures were measured during the

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pile driving. The dynamic signals (time variations of accelerations and strains at the pile head) were monitored by a PDA (Pile Driving Analyzer) and an FPDS (Foundation Pile Diagnostic System), which are widely used in the world on a commercial basis. Two different computer programs, CAPWAPC and TNOWAVE, were employed for the wave matching analyses of the dynamic load test signals to estimate the static load-displacement curve for the pile.

A series of static load tests were conducted on pile S, separately. The first test was carried out 6 days after the initial piIe driving and the last test was conducted 30 days after the initial pile driving. The measured load-disp~acement curves were compared with those estimated from the dynamic load tests on pile D to examine the applicability of the dynamic load test to the estimation of the static load- displacement curve for a pile.

The Statnamic test was also carried out on pile S after the completion of the series of static load tests, 52 days after the initial pile driving (Nishimura et al. 1995).

3 STATIC LOAD TEST RESULTS

A total of 5 static load tests on pile S were carried out 6, 7 and 30 days after the initial driving as shown in Figure 3. The maintained step load test method was employed for the first and the last (5th) test Each load step was maintained for 15 min. The number of load steps was 5. The quick maintained load test method was employed for the Znd, 3rd and 4th tests, in which the pile head load was increased to the ~aximum load in about 20 minutes. It can be seen fiom Figure 3 that the curvatures of the load- displacement curves after the yield loads obtained from the first and the last tests are similar and they are more moderate than the other tests in which the quick ~aintained loading method was used. It is also seen from the comparison of the 4th test and the last (5th) test that the yield load in the last test is smaller than that obtained from the 4th test, althoug~ the rest period for the last test is longer than the 4th test. This fact indicates that the influences of the loading rate andior the Ioading duration are not negligible even for sandy ground when it is saturated.

Figure 4 shows the distributions of the shaft resistance, 2, obtained from the first static load test and the last static load test. The shaft resistance, 2, was estimated from the measured axial strains of pile S when it penetrated a distance equal to 10 percent of the pile diameter in each test. There is a slight change in the distributions of the shaft resistance after the rest periods of 6 days and 30 days, except for depths shallower than 3.5m. Accordingly, it is thought that set-up has completed within 6 days after the initial pile driving. The loss

of the shaft resistance along the top 3 5m of the pile at 6 days after the pile driving may be attributed to the fact that a gap between the pile shaft and the surrounding soil was generated by lateral movements of the pile near the ground surface during the pile driving. The shaft resistance along the top 3.5m of the pile recovered after a rest period of 30 days, although it is difficult to comment on this mechanism.

4 DYNAMIC LOAD TEST R E S a T S

4.1 The resirifs dirring the irzitiai driwig Figure 5 shows the change in the total resistance, Rt, with penetration depth of the test piles during the initial driving. The total resistance, Rt, was estimated by the Case method (Goble et al. 1975). The total resistance of each test pile increases almost

Load on pile head (MN)

Figure 3 Load 4splacenient c m e s obtained from static load tests (pile S).

Shaft resistance. z-(kN/m3) 0 50 100 150 200 250 300

6 daj s after EOID

"I ' - 4 E W

I I

Figure 4. Distributions of the shaft resistance obtaincd from the static load tests conducted 6 and 30 days afrer initial driving (for pile S).

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linearly with the increasing penetration depth and attains about 2 MN at the end of initial driving. Therefore, the bearing characteristics of all the test piles are comparable with each other.

4.2 The ii$'iierrce of hammer driving energy The static pile capacity, R,, estimated from the dynamic load test may vary with the change in the driving energy actually transmitted to the pile To investigate this aspect in detail, the dynamic load tests were carried out on pile D changing the hammer driving energy after different time intervals from the initial pile driving

Figure 6 shows the change in the static resistance, R,, estimated from the wave matching analysis of the dynamic load test signals using CAPWAPC, as a function of the nominal hammer driving energy, EH. The measured values of set per blow, S, are also shown in Figure 6 The static resistance and the set per blow increase gradually with increasing hammer driving energy to 75 kNm. The static resistance attains it's peak value of 2 MN when the hammer energy is greater than 80 kNm On the contrary, the set per blow increases steeply for hammer driving energies greater than 75 kNm, indicating that the pile capacity is fully mobilized by hammer driving energies greater than approximately 75 kNm.

It is recommended that the relations (EH vs R, and E I ~ vs 5') as shown in Figure 6 be obtained prior to the actual pile driving, in order to assure that the pile capacity is fully mobilized by the selected driving hammer. If EH versus R, alone is measured, it is difficult to judge that the peak value of R, is the actual pile capacity or it is a limitation due to the insufficient hammer driving energy. The relations shown in Figure 6 are useful for the selection of an appropriate hammer for each site.

4.3 Set-irp phenomerza Dynamic load tests were performed after 8 different time intervals measured from the end of initial driving, in order to investigate the set-up phenomena of a steel pipe pile (pile D) driven in sandy ground. Figure 7 shows the change in the total resistance, Rt, which was estimated using the Case method, with elapsed time after the pile driving. Two dynamic monitoring systems, PDA and FPDS, were used to record the dynamic signals. The total resistance, Rt, reaches it's peak 60 min after the initial pile driving, and remains almost constant after that time. The peak total resistance is 1.2 times R, measured at the end of initial driving, that is to say, the "set-up ratio" for the total resistance is 1.2 in this case.

Figure 7 Increase 111 total resistance. R,. nit11 elapsed tiine after inihal driving (pile D)

Figure 8 shows the change in excess pore pressures with elapsed time after each blow for the different depths of the pile tip, during the initial driving of pile D Note again that the piezorneter was placed at a depth of 11.5m from the ground level and at a horizontal distance of 0.2rn from the outside of the pile shaft When the pile tip level is above the level of the piezometer, the magnitude of the first peak of the positive excess pore pressure, Au, increases, and the time instant of the peak of Air becomes earlier, as the pile tip approaches the piezometer level. When the pile tip level is below the piezometer level, the value of the peak positive excess pore pressure, Au, decreases as the pile

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E -0.1 .-. z Depth of pile tip = 10.0 in

mn Depth of pile tip = 10.5 in

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2 E - & 4 - ' 5 3 - : 2: 5 0 . 8 - 1 :

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> 2 0.1 2 0.0 $ -0.1

-(b) 6 days after EOID 30 days after EOID

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Time (s) Figure 8 Changes in elcess pore pressures uith elapsed time dunng each blou at different depths of the pile tip (pile D)

penetration proceeds It is interesting that negative excess pore pressure is generated after the peak positive pressure, although the magnitude of the negative excess pore pressure decreases as the pile penetration proceeds

Although the measured excess pore pressures have not been fully interpreted, the influence of the excess pore pressures on the pile penetration resistance may not be negligible even for sandy ground One of the interesting features of the measured pore pressure is the fact that the pore pressure dissipated almost completely within 30 ms As indicated later, the duration of each dynamic load test was about 30ms also These facts show that the excess pore pressure is not accumulated during successive pile driving, resulting in the small set-up ratio of 1 2 for this site

The set-up ratio for steel pipe piles driven in clay grounds is in the range of 3 to 5 (Wakiya et a1 1992, Matsumoto et a1 1995) In these cases, the existence of the accumulated excess pore pressures was confirmed It may be thought that the set-up is completed just after the end of each driving for sandy ground

4.4 Wave matching ctimlyses to estmiate the static

Wave matching analyses of the re-driving test signals recorded at 6 days and 30 days after the

load-d s p l n c e ~ ~ l t cim'es

initial driving of pile D were conducted to estimate the static load-displacement curve for the pile for each time interval after the initial pile driving.

Figure 9 shows the measured wave signals of the re-driving tests. Figure 10 shows the results of the wave matching analyses. The wave matching analyses were conducted using CAPWAPC and TNOWAVE programs. The calculated and measured upward traveling forces are compared in Figure 10.

6 da\ss after EOID.

2 0 1 5 -

T 0 5 l 0 a) g 0 0

LL -0 5 -1 0 (a)TNOWAVE \\as use -1 5

0 10 20 30 4 0 Timc. t (ins)

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0 10 20 30 40

Time, t (ms) Figure 10 Coinpanson betu eeii ineasured and calculated upv ard tral eling forces in pile D (6 daj s after EOID)

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Shaft resistance, z (kN/niL) o O 100 200 300

Shaft resistance, z (kN/m) 0 100 200 300

(a) 6 days after EOID (b) 30 days after EOID

Figure 11. Distributions of shaft resistance estimated from re- driving tests using TNOWAVE. together with static load test results (piles D and Sj.

Figure 12 Load -displacement cunes obtained from static load tests. and denved froin d!naniic load test using CAPWAPC and TNOWAVE (pile Dj

Figure 11 shows the distributions of the shaft resistance, z, estimated from the wave matching analyses using TNOWAVE and measured in the static load tests. Although the total shaft resistance estimated from the wave matching analysis is comparable with the static load test results, the

calculated distribution of the shaft resistance along the pile shaft does not coincide with the static load test results.

The distributions of the shaft resistance, z, estimated from the wave matching analyses using CAPWAPC were similar to Figure 11. In these wave matching analyses, the traditional empirical soil model proposed by Smith (1960) was used. The maximum soil resistance, the spring value and the damping constant for the Smith model were estimated only from the agreement between the calculated and measured signals. These factors may well be the cause of the difference between the estimated and the measured soil resistance as shown in Figure 11.

The static load-displacement curves estimated using the wave matching analyses are compared with the load-displacement curves measured in the static load tests in Figure 12. The estimated load- displacement curves are comparable with the measured curves. Especially, the estimated initial pile head stiffness is fairly coincident with the measured values. This fact is usefbl in the limit states design and the performance based design of pile foundations, in which the estimation of the load- deformation relation will be a vital issue (Kusakabe 1998).

5 STATNAMIC TEST RESULTS

A loading device having a loading capacity of 8 MN was used in the Statnamic test conducted on pile S 52 days after the initial pile driving

The measured variations with time of force, F,[,, displacement, w, velocity, 11, and acceleration, a, at the pile head during the Statnamic test, are shown in Figure 13 The loading duration is about lOOms The peak ofF,,, is 3 68 MN, the maximum pile head displacement is 37 mm, and the residual displacement is 16 mm which corresponds to 4% of the pile diameter The maximum downward velocity is 1 m/s The maximum downward acceleration is 60 m/s2 while the maximum upward acceleration attains 120 m/s2 The FAln - w curve is shown in Figure 14

The pore pressures were measured during the Statnamic test also The magnitude of the pore pressure was very low, o 0 1 MN/m2 in maximum

The Unloading Point method analysis (Kusakabe & Matsumoto, 1995) and the wave matching analysis of these Statnamic test signals were conducted to derive a static load-displacement curve for the pile The KWAVE program developed by Matsumoto & Takei (1991) was used for the wave matching analysis Figure 15 compares the derived load-displacement curves and the load-displacement curve obtained from the static load test The curves compare well for practical purposes

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Pile head load (MN)

U

0 50 100 150 200 250

i

0 50 100 150 200 250

.5i' 1.0 . E 0.5 v "- 0.0 0' -0.5 0 a, -1.0 > _.

0 50 100 150 200 250

0 50 100 150 200 250 Time, t(ms)

Figure 13. Statnarnic test signals of pile S.

Pile head force, Fll,, (MN) 0 1 2 3 4

4 0 ' " " ' " Figure 11. Frl,, versus II '

6 RELIABILITY OF DYNAMIC LOAD TEST

The ultimate bearing capacity of the test pile obtained from the static load test is compared with the bearing capacity derived from the dynamic load test and various pile design codes in Figure 16. OIn Figure 16, Q is the ultimate capacity which is the sum of the ultimate end bearing capacity, Qp, and the ultimate shaft capacity, 0,.

Four Japanese design codes, which are based on the SPT N-values, predict well the total pile

m - ro 73 (d 30 I: a,

ei

Figure 15 Load 4isplaceineiit cun es obtaiiied froiii the static load test, derir ed from Statnanic load test using Uilloadiiig Point method (ULPM) and derived from the wave matching anal! sis

- - 40

capacity, Q, from the static load test, but overestimate the end bearing capacity, Qp, and underestimate the total shaft capacity, Qy. It is seen that the dynamic load test predicts well the total pile capacity as well as the proportions of the total shaft capacity and the end capacity.

7 CONCLUSIONS

The results of a comparative study of the static load test, the dynamic load test and the Statnamic test on three open-ended steel pipe piles driven in relatively uniform sandy ground have been presented, and the uses of the dynamic load test and the Statnamic load test in sandy ground as an alternative to the static load test has been discussed in this paper. The following conclusions were derived from this study:

The hammer driving energy should be enough, so that the pile capacity is fully mobilized. A method to select an appropriate hammer driving energy was proposed, in which the mobilized static resistance as well as the measured set per blow are utilized to determine the minimum required driving energy (see Figure 6). Excess pore pressures are generated during pile driving even in sandy ground. However, the generated excess pore pressures dissipate within the duration of each driving without accumulation of residual excess pore pressures. Therefore, the 'set-up ratio' is relatively low in this case study. The set-up phenomena was completed within 60 min after the end of the initial driving process.

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