4.4 TEST RESULTS ON FLOATING PILE AND SOCKETED PILE
4.4.3 Stage 3: NSF Due to Soil-reconsolidation
Test ES on the end-bearing pile has clearly revealed that re-consolidation of the re- moulded clay after pile installation leads to large mobilization of NSF along the pile shaft but negligible pile settlement since the pile was bearing on a rigid base.
The increment of axial forces with time at various elevations along pile shaft after pile installation is presented in the left portion of Fig. 4.29 for test FS. It can be seen that
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similar to what was observed in previous test ES, the axial load kept on increasing after pile installation as the soil re-consolidation proceeded. Although certain fluctuation of the signals was discernable, the general trend of development still clearly revealed a reducing rate of increment with time and tapering of the data towards the end of soil re- consolidation. Different from that observed in test ES, the maximum increment of axial load occurred at gauge level-3 instead of level-1 near the pile tip. The downdrag load mobilized along the pile shaft in test FS at various selected dates during soil re- consolidation stage is shown in Fig. 4.30. A maximum dragload of about 360 kN was observed at the end of soil re-consolidation for the floating pile. This was only about 60%
of the corresponding maximum dragload of 610 kN in the case of the end-bearing pile in test ES (see Fig. 4.12). Also, unlike the case of end-bearing pile with a neutral point located at the pile toe, the neutral point for the floating pile was located at about 12 m below the ground surface, that was at approximately 75% of the pile length. The β curve plotted in Fig. 4.30 is found to fit the final dragload profile up to a depth of about 10 m below the ground surface and start to deviate drastically from the test data below that depth. At the neutral point 12 m below ground surface, the calculated maximum dragload is about 540 kN, which grossly overestimates the measured maximum dragload of 360 kN by about 50%. This is again consistent with the observation in test ES that the NSF around NP is far from full mobilization and utilization of β method which assume full mobilization of NSF down to NP will inevitably lead to an gross overestimation of dragloads. It should be noted that the total stress α method was not used to fit the test data any more as it has been demonstrated in test ES that the β method is relatively more consistent than the α method.
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However, a trade-off for the much smaller mobilization of NSF on the floating pile is the substantial final pile settlement of about 28 mm as shown in Fig. 4.31(a). In the figure, the pile settlement appeared to be more than the soil settlement simply because the latter refers to the soil settlement measured by the potentiometer located at 12 m away from the center of the pile.
The increment of axial forces with time at various elevations along pile shaft after pile installation is presented in the left portion of Fig. 4.32 for the socketed pile in test SS, while the downdrag load profiles along the pile shaft at selected times are shown in Fig.
4.33. The maximum dragload of about 515 kN observed at the end of soil re-consolidation for the socketed pile lies in-between the corresponding maximum dragload of 610 kN for the end-bearing pile in test ES and the maximum dragload of 360 kN for the floating pile in test FS. The neutral point was observed to be located much deeper than that of the floating pile in test FS and was essentially stabilized at about 14.5 m below the ground surface, or 90% of the pile length within the settling soils. The effective stress β method was used to fit the test data as shown in Fig. 4.33. Again, the measured maximum downdrag load at the neutral point was substantially smaller than that calculated by the β method as shown in Fig. 4.33. The calculated maximum dragload is about 665 kN, which overestimates the measured maximum dragload of 535 kN by about 24%, which lies in- between the overestimation of 16% for the end-bearing pile in test ES and 50% for the floating pile in test FS in the corresponding stage. It appears that the softer a pile toe condition, the larger is the over-estimation by the β method against the measured data.
The ground heave due to pile driving and subsequent soil subsidence with soil consolidation for test SS as shown in Fig. 4.32(b) is consistent with those measured in test
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ES and test FS. The pile downdrag settlement during soil re-consolidation was about 6 mm, which again lied in-between that of the end-bearing pile of less than 2 mm and that of the floating pile of 28 mm.