Dynamic and Static Load Tests on Large Diameter Bored Piles

C.S. ChenAssociate Director, SSP Geotechnics Sdn Bhd, Level 6, Wisma SSP, No.1, Jalan SR 8/3, Serdang Raya Seksyen 8, 43300 Seri Kembangan, Selangor, Malaysia. E-mail: cschen@sspsb.com.my

C.S. LimAssociate, SSP Geotechnics Sdn Bhd.

___________________________________________________________________________Abstract: High strain dynamic load test (PDA test) on driven piles is a common test procedure for evaluating the pile capacity and integrity. The derived pile capacity generally shows satisfactory agreement with the capacity derived from static load test. However, the dynamic load test on large diameter bored piles is less common. This is mainly because the large diameter bored piles are cast in-situ and the design pile capacity is generally high. An uncommon relatively heavy hammer will be required in order to perform high strain dynamic load test. This paper presents the results of dynamic load test on two numbers of 1.2m diameter bored piles. These two bored piles were also tested using conventional static load test. The results are presented in the paper. The subsoil condition is briefly described. In general, the bored piles were constructed in a tropical residual soil formation with about 10 to 20m thick soft to medium stiff cohesive soil overlying very hard soil stratum. Occasionally weathered rock could be encountered below the very hard soil stratum. One of the bored piles was fully instrumented for the evaluation of the load transfer behaviors of subsoils at various depths. Details of the static and dynamic load tests are described in the paper. A comparison of the results of dynamic and static load tests is presented. Generally the settlement predicted by dynamic load test is lesser as compared with the observed pile top movement from static test.__________________________________________________________________________________________

IntroductionHigh strain dynamic load test (or sometimes it was loosely called as PDA test) has become a common pile test procedure for evaluating pile capacity and pile integrity for the driven piles. The derived pile capacity generally shows satisfactory agreement with the static measured capacity (Rausche et al., 1985). In addition, the test also investigates the hammer efficiency and driving stresses during the installation of pile. The most attractive advantages are the cost of the test is much cheaper as compared with the cost of conventional static load test and the duration of the test is very short. The dynamic test is common for driven piles mainly because similar driver or hammer used for the pile installation can be used for the test. As for the cast-in-situ piles especially the large diameter bored piles, since the piles are formed by boring a hole in the ground and subsequently filled with concrete and steel reinforcements, the dynamic test is relatively less common because extra effort to bring in hammer for the test is required. In addition, the design pile capacity for large diameter bored pile is generally large, a very heavy hammer is required. As a general guideline, in order to verify the pile capacity, the required hammer weight is about 1.5% of the pile static resistance (Hussein et al. 1996). A 300 kN hammer will be required if a pile is designed to have ultimate pile capacity of 20,000 kN. It is not an easy task to lift up the 300 kN hammer and strike it onto the bored pile.

This paper presents the results of dynamic and static load tests on two numbers of 1.2m diameter bored piles installed in a residual soil formation that generally shows complex soil characteristics. Load settlement behaviors and load distributions from both tests are presented.

Subsoil Conditions

The test piles are located in Kenny Hill formation of carboniferous age. This formation is a residual soil formation initially formed by a sequence of interbedded sandstone, siltstone and shale. With the regional metamorphic event, the sandstone and siltstone or shale had been changed to quartzite and phyllite or schist respectively. The rocks turn into residual soil as a result of intensive weathering process. Depends on the mineral composition of the parent rocks, this formation mainly composed of clayey silty sand and sandy silty clay. Details of engineering properties of this residual formation may refer to Wong and Singh (1996) and Komoo (1989). Boreholes carried out at the site revealed that the subsoil can be simplified into two major strata: soft to stiff silty clayey soil or loose to medium dense sandy soil overlying very dense or hard soil layer with Standard Penetration Test (SPT) results of more than 50 blows/300mm penetration. The thickness of the soft to

stiff soil layer is generally in the range of 7m to 15m. The Liquid Limit (LL) and Plasticity Index (PI) for the cohesive soil generally vary from 30% to 60% and 10% to 20% respectively. The cohesive soil is generally of low plasticity to intermediate plasticity. Figure 1 shows the Atterberg Limits of the cohesive subsoil at different depths.

Figure 1. Atterberg Limits of the cohesive soil.

The Dynamic and Static Load Tests

High strain dynamic load test was carried out using a 30 tons hammer with a few separated impacts. The hammer was positioned on top of the test pile by a steel cage with 4 H-beam welded together as shown in Figure 2. A very low drop height will be applied first for the assessment of signal quality and alignment of the drop

hammer with the bored pile. The permanent displacement or “set” after each impact will be measured for the assessment of activated capacity. The forces and motions due to the drop impact were recorded by Pile Driving Analyzer (PDA). The pile capacity was estimated at site using CASE method. CAPWAP, the more rigorous signal matching computer program, was used to compute the pile capacity in later stage based on the recorded data.

The static load test was carried out using kentledge as the reaction system. The applied load on top of the bored pile was measured by a calibrated load cell. Each load increment is about 10% of the designed working load and minimum duration of each load is about 30 minutes. The bored piles were required to be tested to two times the designed working load for normal working piles and three times designed working load or failure for preliminary test piles. For preliminary test pile, vibrating wire strain gauges (VWSG) and tell-tale extensometers were installed internally in the bored pile to measure the strain development and shortening behavior of the bored pile during testing. For normal working pile, only the pile top movement was measured. Figure 3 shows the setting up of the static load test.

The Load Tests Results

Two numbers of bored piles with 1.2m diameter were tested by

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the dynamic and static methods. The Test Pile A was a preliminary pile while the Test Pile B was a normal working pile. The designed working load for the 1.2m diameter bored pile was 6500 kN. Preliminary test pile was conducted prior to the construction of working piles. The main purpose of the preliminary pile test was to

confirm the assumed shaft frictions in the design calculation and also to study the pile performance under the applied load.

Test Pile A

Test Pile A was a preliminary pile and was constructed on 16 December 2001.The total pile length was 21m below the existing ground surface. The bored pile was reinforced with 18 numbers of 25mm high yield steel bars as longitudinal reinforcement and 10mm diameter circular links at 300mm spacing. The concrete strength used for this test pile was 45 MPa. The bored pile was purposely over bored to 21.5m and filled with soil so that the pile could have a soft toe. This was to ensure that most of the test load would be resisted by shaft friction. Two types of instruments namely vibrating wire strain gauges (VWSG) and tell-tale

extensometers (TT) were installed in the Test Pile A. The VWSGs were installed at six levels and each level consists of 4 numbers of VWSG. Two numbers of TT containing a 20mm inner GI pipe housed in 50mm steel pipe each were installed at the bottom of the test pile. Figure 4 shows the levels of the installed instruments and the subsoil condition. Static load test was performed on 11 January 2002 while the dynamic load test was carried out on 8 March 2002. The Test Pile A was statically tested to a maximum load of 2000 tons. The estimated ultimate pile capacity was slightly more than 2000 tons based on Davisson’s method (1972). The maximum pile capacity as determined from the dynamic load test was about 1710 tons. Apparently the dynamic load test had not fully mobilized the shaft friction due to very minimum penetration of pile after each impact of the drop weight. The top displacements of the Test Pile A measured from the static load test and estimated using CAPWAP from the dynamic load test are presented in Figure 5. Apparently the estimated pile top displacement by CAPWAP is quite consistent with the result from the static load test when the test load was low i.e. within design working load. When the test load became higher and closed to the ultimate capacity, the settlement predicted by CAPWAP was less than the pile top displacement measured from static test.

Figure 6 shows the measured load distribution curves from the static load test at different test loads and from the dynamic load test. Apparently the load distribution from the dynamic load test is quite consistent with the load distribution from the static test at similar test load, i.e. at about 17520 kN. The total shaft friction and toe resistance from the dynamic load

Figure 3. Setting up of the static load test.

Figure 4. Subsoil condition and levels of instruments.

test are 15650 kN and 1530 kN respectively. Whereas the shaft friction and toe resistance from static load test at similar test load are 16450 kN and 1070 kN respectively.

The unit skin frictions at different depths analyzed from both static and dynamic load tests are as shown in Figure 7. Larger discrepancy can be observed especially below 12m depth. This may due to the different response of subsoil under static loading condition and when subjected to a dynamic impact.

Test Pile B

Test Pile B was a working pile and was constructed on 4 January 2002. It was reinforced with 24 numbers of 25mm diameter longitudinal steel bars at the top 12m and 12 numbers of 25mm diameter longitudinal steel bars for remaining pile length. The concrete strength was 40 MPa. Figure 8 shows the subsoil condition and the pile length. Due to some construction problems, the pouring of concrete was delayed resulting the bored hole collapsed. Based on the volume of concrete used for this pile, bulging was expected. As this may affect the shaft friction and the pile capacity, dynamic load test was performed on 13 March 2002 for the assessment of the pile capacity as well as the pile integrity. The estimated pile capacity was about 23900 kN. Apparently the shaft friction had not been fully mobilized due to very minimum pile penetration after the dynamic impact. Static load test was carried out only up to 2 times the designed working load, i.e. about 13000 kN, on 12 April 2002. The measured pile top settlement and the estimated load settlement behavior from the dynamic load test are shown in Figure 9. Similar to Figure 5, the pile top displacements from both tests are quite consistent when the test load is low. However, with the increase in test load, the pile settled more based on the measurement from the static load test as compared with the predicted pile settlement from dynamic load test.

Discussions and Conclusions

High strain dynamic load test is often requested as an alternative to conventional static load test due to the time consuming and high cost of the static load test. The dynamic load test also having additional advantage on the assessment of the pile integrity. Although many researches had shown that the derived pile capacity from the dynamic load test agreed well with that from the static load test, exceptions have also been reported (Davisson 1991, Gue and Chen 1998). Care should be taken in comparing the pile capacities derived from both tests. As

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Figure 5. Settlement of pile top for Test Pile A.Figure 6. Load distribution curves.

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Figure 7. Unit shaft frictions from the tests.

the shaft friction could only be fully mobilized if the penetration of pile after each impact is more than 2.5mm (Likins et. al. 2000), the derived pile capacities from the dynamic load tests for Test Piles A and B could be underestimated due to very minimum penetration of piles after each dynamic impact. Comparison of pile capacities could not be carried out in this case.

The load settlement behaviors of piles as predicted from the dynamic load tests show good agreement with the pile top settlements measured from the static load tests when the test load is low, i.e. within design working load. When the test load increased, the dynamic load test may underestimate the settlement of piles.

The load distribution curves along the pile shaft from both tests show small variation. However, in assessment of the unit shaft friction, larger discrepancy was observed. This implies that the dynamic load test may be not suitable for evaluating the shaft friction for different types of soil unless a proper correlation had been performed.

In engineering practice, due to time consuming and high cost, very limited number of static load test can be carried out in a bored pile project. Quick and much cheaper dynamic load test with additional advantage of evaluating the integrity of pile provides an alternative tool for quality control. However, the dynamic load test should be used with care. In determination of unit shaft friction and the load distribution characteristic, static load test to failure is more appropriate. In assessment the sufficient of pile capacity and pile integrity, dynamic load test could play a major role.

References

Davisson, M.T. (1972). “ High capacity piles,” Proceedings, ASCE Lecture Series, Innovations in Foundation Construction, Illinos Section.

Davisson, M.T. (1991). “Reliability of pile prediction methods,” Proceedings of Deep Foundation Institute Conference, Chicago.

Gue, S.S. and Chen, C.S. (1998). “A comparison of dynamic and static load tests on reinforced concrete driven pile,” Proceedings of the 13th Southeast Asian Geotechnical Conference, Taipei, Taiwan, 497-501.

Hussein, M., Likins, G., and Rausche, F. (1996). “Selection of a hammer for high-strain dynamic testing of cast-in-place shafts,” Proceedings of Fifth International Conference on the Application of Stress-Wave Theory to Piles, Orlando, Flo rida, USA.

Komoo, I. (1989). “Engineering geology of Kuala Lumpur, Malaysia,” Proceedings of the International Conference on Engineering Geology in Tropical Terrains, Bangi, Malaysia, 262-273.

Likins, G., Rausche, F. and Goble, G. (2000). “High strain dynamic pile testing, equipment and practice,” Proceedings of the Sixth International Conference on the Application of Stress-wave Theory to Piles, Sao Paulo, Brazil.

Rausche, F., Goble, G. and Likins, G. (1985). “Dynamic determination of pile capacity,” Journal of Geotechnical Engineering, ASCE, Vol. 111, No.3, 367-383.

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Figure 8. Subsoil condition for Test Pile B.Figure 9. Pile top settlement of Test Pile B.

Wong, J. and Singh, M. (1996). “Some engineering properties of weathered Kenny Hill Formation in Kuala Lumpur,” Proceedings of the Twelfth Southeast Asian Geotechnical Conference, Kuala Lumpur, 179-187.