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Negative skin friction on piles in consolidating ground

Authors Chan, Sze Ho
Issue Date 2006
Summary Pile foundations located within consolidating ground are commonly subjected to negative skin friction (NSF) and failures of pile foundations related to dragload (compressive force) and downdrag (pile settlement) have been reported in literature. In addition, they are normally subjected to axial load and NSF simultaneously. A field study was carried out to investigate load transfer behavior of piles in end-bearing and floating 2-pile groups in consolidating ground. Moreover, load transfer mechanisms of piles subjected to NSF was investigated in detail by centrifuge tests and axis-symmetric and three-dimensional numerical analyses to achieve three objectives: to investigate response of a single pile subjected to NSF with different pile tip locations with respect to end-bearing stratum, to study behavior of floating piles subjected to NSF with and without shielding by sacrificing piles, and, to study effects of axial load on load transfer mechanism along a single floating pile and shielded center pile within a group of sacrificing piles experiencing locked-in NSF. Based on field monitoring, downdrag developed on floating piles is 70% more than that on end-bearing piles and neutral plane (NP) is located deeper with stiffer end-bearing layer. Measured maximum β values ranged from 0.2 to 0.3 for piles located inside the end-bearing and floating pile groups. Based on centrifuge tests results, the maximum dragload developed at a single end-bearing pile is 60% more than that at a single floating pile, whereas downdrag developed on single floating pile is 200% greater than that on a single end-bearing pile. Both centrifuge and field monitoring results show larger downdrag obtained for floating pile than that of end-bearing pile. From centrifuge modeling results, the measured maximum β value at a single end-bearing and floating pile was similar and slightly smaller than 0.3. On the other hand, smaller β values of 0.1 and 0.2 were mobilized at the shielded center piles for pile spacings of 2.5D and 3.0D, respectively, where D is center pile diameter. Consistent results of β values are obtained between centrifuge modeling and fielding monitoring. The use of a constant β value to estimate dragload on piles will not be realistic as a single β value cannot take into account of the reduction of dragload at the lower part of a pile. Based on centrifuge tests results, NP of a single floating pile was located at around 0.75L, where L is embedded pile length into clay stratum. This is consistent with the results as obtained by theoretical solutions, which ranged from 0.63-0.80L. The location of NP of shielded center piles does not appear to be significantly affected by the presence of sacrificing piles. The measured maximum dragload of the center pile in the group at 2.5D and 3.0D spacing was only 53% and 75% of the measured maximum dragload of an isolated single pile, respectively. Correspondingly, the measured downdrag of the center pile was reduced to about 57% and 80% of that of the isolated single pile. Deduced ultimate pile capacity of shielded center pile is smaller than that of a single pile. The closer the pile spacing inside the groups, the larger is the reduction in dragload, downdrag as well as ultimate pile capacity of the center pile. This can be explained by numerical simulations of centrifuge modeling. Sacrificing piles "hang-up" the soil between the piles in the group and thus, the vertical and horizontal effective stress in the soil acting on the center pile are reduced. At the same time, smaller confining pressure at the pile tip of a center pile located inside the group causes the smaller soil stiffness and shear strength of soil, so as the ultimate pile capacity of the shielded center pile.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2006
Language English
Format Thesis
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