NEGATIVE SKIN FRICTION ON PILE FOUNDATIONS EMBEDDED IN A LAYERED HALF-SPACE
LIM CHING HENG
LIM CHING HENG
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Abstract
It has been long recognized that negative skin friction is induced in a pile when the soil surrounding the pile settles more than the pile. This can happen when the pile is installed in soft soil which undergoes consolidation settlement. In this thesis, a simple discrete element approach utilizing the subgrade reaction method to model the soil response has been developed to analyze negative skin friction on single piles embedded in a layered half-space. The upper soil layer undergoes consolidation due to surcharge placement and induces downdrag force on the pile which is bearing on or socketed in a stiffer bearing stratum of infinite depth. The effects of the compressibility of the bearing stratum and the embedded length of the pile in this stratum are investigated. This simple approach is verified by comparison with rigorous numerical methods which model the soil as an elastic continuum. The applicability of this approach to analyze actual cases has been studied by comparing the results obtained using this approach with reported field measurements. This simplified approach has been extended to the case of pile groups. The main objective of the group analysis is to examine the approaches in which pile-soil-pile interaction between the group piles can be modelled. For this purpose, a layered-soil model and a "hybrid" model have been investigated. In both models, the individual piles are represented by the discrete elements and the soil response is modelled using the subgrade reaction method as in the single piles. The difference between these two models lies in the manner in which the soil medium is dealt with. The layered-soil model treats the soil as independent horizontal layers which permit pile-soil-pile interaction to take place within the same layer only. The continuity of the soil in the vertical direction is thus discounted. On the other hand, the continuity of the soil medium is preserved in the "hybrid" model by using the following methods to determine the interaction effects between the group piles: (i) the solutions of Chan et al ( 1974) for a point load within a layered half-space and (ii) the approach of using Mindlin's equation with average E-value of the influenced and influencing nodes. Comparisons of the present approaches with the more rigorous elastic continuum solutions for a two-layer system ( e.g. Chin, 1988} indicated that the layered-soil model generally gives good agreement for the final maximum downdrag force. The group pile-head settlement however gives good agreement for pile groups with end bearing on top of a bearing stratum. It is found that smaller group pile-head settlement is predicted by the layered-soil model as compared to Chin (1988) for socketed using the solutions of Chan et al (1974) are always in excellent agreement with the rigorous solutions. As in the layered-soil model, the "hybrid" model using Mindlin's equation only gives good agreement for the final maximum downdrag force.
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Date
1994
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