All the soil preparation, pile group configuration as well as instrumentation for the socketed pile group tests are essentially the same as those for end-bearing pile group tests, except that the rigid acrylic plate has been replaced by a uniform underlying dense sand layer, and the pile group was jacked in half a pile diameter (0.64 m in
n,single n,ave n,single
P - P Average GRF =
P
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prototype scale) into the bearing sand layer, as schematically shown in Fig. 5.1(b).
Four socketed pile group tests comprising 3, 5, 9 and 16 piles were conducted in the present study with corresponding test ID of SG-3, SG-5, SG-9 and SG-16, respectively.
Details of the test results are given in the Appendix. As an illustration, the downdrag load profiles along the pile shaft of the corner pile, side pile and inner pile of the socketed 16-pile group are plotted in Fig. 5.11, together with the downdrag load profiles for the socketed single pile in the previous test SS presented in Chapter 4. It can be seen that regardless of pile position in the group and test stage, the NP persists at about 90% of the pile length within the settling soils, the same as that for the socketed single pile. Comparing the dragload of socketed piles presented in Fig. 5.11 with those of end-bearing piles in Fig. 5.8, substantially smaller dragloads are induced on socketed piles for all pile positions. Using Eq. (5.10), the NSF group reduction factor for the corner piles, side pile and inner piles is calculated to be 30%, 36% and 43%, respectively, at the end of the re-consolidation stage. These values are about 5~6% lower than those of the corresponding end-bearing piles. At the end of water drawdown stage, the respective NSF group reduction factor reduces to 27%, 32% and 38%, about 8~12% smaller than those of end-bearing piles. At the end of surcharge, the NSF group reduction factor drops substantially to 12%, 17% and 24% for the corner pile, side pile and inner pile, which is about 5% lower than those of the corresponding end-bearing piles.
The dragloads for all test stages for the corner piles, side piles and inner piles from all the socketed pile group tests (test SG-3 to SG-16) are summarized in Table 5.2. The corresponding NSF group reduction factor for individual piles in a group determined using Eq. (5.10) and average dragload and average group reduction factor calculated using Eq. (5.11) and Eq. (5.12) are summarized in Table 5.2 as well. The results are
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also plotted in Fig. 5.12 which shows a clear trend of decrease in dragload with increase in pile size. Again, it is noted that the reduction in dragload appears to taper off when the pile number exceeds nine. By and large, there is a difference of about 1~8%, with an average of 4% of NSF group reduction factor going from corner piles to side piles and from side piles to inner pile. Comparing Fig. 5.12 for socketed pile groups with Fig. 5.9 for end-bearing pile groups reveals smaller dragloads and smaller NSF group reduction factors prevail for the socketed pile groups. By and large, the average NSF group reduction factors for the socketed pile groups are about 2~10%
smaller than those of corresponding end-bearing pile groups with an average value of about 5%.
The reduction in NSF for pile groups would lead to a reduction in induced pile group downdrag settlement. However, the pile group settlement measured by the local miniature potentiometers mounted on top of the pile cap shows some inconsistent readings with some measured pile group settlement due to NSF even larger than that of the single socketed pile test-SS presented in Chapter 4. Due to the space constraint at the model pile head assembly (see Fig. 3.12), a lever arm has to be extended from the pile center for the 2 local miniature potentiometers to mount on so as to measure the pile head settlement. It is believed that the susceptibility of the extended lever arm to some random twist and tilt movements during a test attributes to the measured inconsistent readings of pile head settlement in the present study.
Again, it should be noted that the measured dragloads and NSF group reduction factors presented in Fig. 5.12 and Fig. 10(b) derived from the present socketed pile group tests are only applicable to the present specific pile and ground conditions and should not be applied indiscriminately to other conditions.
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Table 5.2 Measured dragloads and group reduction factors from socketed pile group tests