4.3 TEST RESULTS ON END-BEARING SINGLE PILE
4.3.3 Stage 3: NSF Due to Soil Re-consolidation
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The pile installation generated excess pore pressure with a maximum at the pile shaft surface and reduced exponentially with distance away from the pile. Thus, after pile installation, the excess pore pressure would dissipate by both vertical and outward radial consolidation (Randolph et al., 1979). Fig. 4.9 shows that the excess pore pressure dissipated quickly after pile installation and essentially stabilized after 156 days. At the location of the potentiometer at 12 m from the center of the pile, the soil settled by about 18 mm due to the soil consolidation in contrast to the initial upheaval of 6 mm during pile installation, resulting in a net soil settlement of about 12 mm, as shown in the left portion of Fig. 4.10. As expected, the corresponding downdrag settlement of the end-bearing pile was negligible as shown in the figure.
The development of NSF on piles due to re-consolidation of the remolded clay after pile installation has long been recognized. However, very few test data is available so far in the literature to quantify such effect. Fellenius (1972) observed a dragload of about 300 kN developed in his field test pile due to soil re-consolidation after pile driving. It is noted that in that case history, a regional soil subsidence also contributed to the development of dragload on the test pile. In the present study, sufficient time has been allowed for the excess pore pressure to essentially fully dissipate before pile installation. Thus, any development of NSF on the pile should be caused solely by the effect of soil re- consolidation after pile driving. The increment of axial forces with time at various elevations along the pile shaft after the pile installation is presented in the left portion of Fig. 4.11. It can be seen that the axial forces kept on increasing after pile installation as soil re-consolidation proceeded. The rate of increase reduced with time and tended to taper off towards the end of soil re-consolidation. It is clear that the maximum increment of
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axial load occurred at gauge level-1 near the pile toe, and the increment reduced towards the pile head. The development of downdrag loads along the pile shaft can be best viewed by plotting the load transfer curves along the pile shaft as shown in Fig. 4.12 at some selected time after pile installation. It is evident from Fig. 4.12 that the downdrag load along the pile shaft increased with time as soil re-consolidation proceeded. A maximum dragload of 610 kN was observed to develop near the pile toe at the end of soil re- consolidation. The neutral point (NP) persisted at the pile tip, which was in line with findings by other researchers like Johannessen and Bjerrum (1965), Lee et al. (2002). The α and β curves are used to fit the downdrag loads along the pile shaft at the end of the soil re-consolidation. It should be noted that in fitting the α and β curves to the test data, no difference was made to the 2 m top sand layer as the downdrag load contributed by the top sand layer was observed to be not substantial probably due to the low overburden stress.
The same phenomenon was observed by Johannessen and Bjerrum (1965). For the α method, the undrained shear strength measured after self-weight soil consolidation (see Fig. 4.2(a)) was used and the derived α value was 0.95, which is very close to that derived by Leung et al. (2004) for the same clay. The derived β value was 0.24, which is very consistent with β values reported for clays by various researchers such as Endo et al.
(1969), Burland (1973), Garlanger (1974), and Leung et al. (2004). It is noted that around the NP, namely around the pile toe in this case, the α and β curves deviates from the measured final dragloads. The NSF appears not fully mobilized around the NP due to the small relatively pile-soil relative settlement around the neutral point. As such, either the α or β method, which essentially works on the assumption of full mobilization of NSF up to the neutral point, tends to over-predict the maximum dragload. For the present case, the
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calculated maximum dragload was about 705 kN which is about 16% higher than the measured value of 610 kN. This may be the reason which motivates some pile design codes like Singapore Code of Practice for Foundations (CP4: 2003) to adopt a mobilization factor to discount dragloads calculated by α or β method. More in-depth exploration of the mobilization factor will be conducted with the aid of FEM numerical analysis in Chapter 6.
Re-call that at the end of pile installation, the pile head was not totally unloaded.
Owing to the self-weight of some accessories like the coupling connector, the pile head was subjected to a load of about 1800 kN in prototype scale. Leung et al. (2004) established that the load transfer along the pile shaft due to the applied load at the pile head and the subsequent development of downdrag loads along the pile shaft can be treated independently and the overall axial load distribution along the pile shaft is the combination of the above two developments. The present observation apparently further supports such postulation, as illustrated in Fig. 4.13. The load transfer curve immediately after pile installation was shown in Fig. 4.13(a) with a load of P0 =1800 kN acting on the pile head and a load transfer along the pile shaft of about 410 kN as denoted by P+. The induced downdrag load due to soil re-consolidation after pile driving was shown in Fig.
4.12(b) with a maximum dragload of 610 kN developed near the pile toe (denoted by P- in the figure). The superposition of the above 2 curves results in Fig. 4.13(c), which is the overall axial load distribution along the pile shaft. It can be seen that load transfer along the pile before the development of NSF turns out to be beneficial in reducing the maximum load at the neutral point. In the present case, the maximum axial load at the NP is equal to P0 + P- - P+ = 2070 kN, instead of P0 + P- = 2480 kN, as shown in Fig. 4.13(c).