THESIS
2004
xx, 237 leaves : ill. ; 30 cm
Abstract
The development of Negative skin friction (NSF) is regarded as a common problem in the design and construction of pile foundations in consolidating ground. Several authors have reported failure of pile foundations related to pile settlement (downdrag). To date most of the current design approaches consider mainly on ultimate limit state (dragload) and overestimate the effect of NSF on pile behaviour. With an aim to clarify aspects of pile behaviour subjected to NSF, four centrifuge tests have been conducted. The research includes investigations of NSF developed on single piles with different pile tip location with respect to the bearing layer as well as changes of dragload and downdrag of the centre pile inside the 3x3 pile group under the application of axial load. Moreover, shielding...[
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The development of Negative skin friction (NSF) is regarded as a common problem in the design and construction of pile foundations in consolidating ground. Several authors have reported failure of pile foundations related to pile settlement (downdrag). To date most of the current design approaches consider mainly on ultimate limit state (dragload) and overestimate the effect of NSF on pile behaviour. With an aim to clarify aspects of pile behaviour subjected to NSF, four centrifuge tests have been conducted. The research includes investigations of NSF developed on single piles with different pile tip location with respect to the bearing layer as well as changes of dragload and downdrag of the centre pile inside the 3x3 pile group under the application of axial load. Moreover, shielding effects on dragload and downdrag in 3x3 pile groups at 2.5D and 3.0D spacing are investigated. The observed pile behaviour is to be compared with relevant previous studies and numerical back analysis.
Combined with the findings from this study and other reported test results in literature, it suggests that the smaller of the distance of pile tip from the bearing stratum, the more significant the effect from the bearing stratum on the development of dragload and the larger the magnitude of dragload and vice versa. In contrast to the variation of dragload with pile tip location, the magnitude of downdrag with pile tip located at 0.5D away from the bearing stratum is about twelve times as large as the one with pile tip rest on bearing stratum. Regarding the combined effect of applied axial load and dragload on a pile where its pile tip is located at 0.25D from the bearing stratum, dragload developed on the centre pile is completely eliminated for the cases where pile spacing equal to 2.5D and 3D when axial loads of 150% and 175% of the maximum dragload developed prior to the application of axial load (Pmax) is applied. The values of required axial load are comparatively close with published test result of floating pile but are about halve the value of end-bearing pile. (i.e., 3Pmax). For the two 3x3 pile groups at 2.5D and 3.0D, the maximum dragload of the centre pile is 53% and 36% of the maximum dragload of the single isolated pile respectively. At the same time, the shielding effects on downdrag of the centre pile is about 40% and 19% of a single isolated pile in the case where the pile spacing equal to 2.5D and 3.0D respectively. Shielding effect on downdrag is smaller than shielding effect on dragload regardless of pile spacing and shielding effect on downdrag is largest for the centre pile as compared with other piles inside the group and the isolated pile.
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