Nonlinear Soil Resistance of Pile Group Subjected To Load in Different Directions Based on Nonlinear 3D Fem

Abstract eng:
The stress on any given pile of a pile group foundation varies during an earthquake because the mechanisms of horizontal soil resistance differ from each other depending on the pile location. In order to calculate the seismic response of the structure supported on a pile group, a beam-spring model with nonlinearity on the subgrade reaction along the pile shafts is needed. Because the structure is subjected to a multi-directional earthquake motion, it is also necessary to obtain the relationship between the subgrade reaction and the loading direction. In this paper, static analyses using the nonlinear three-dimensional finite element method (3D FEM) were performed to obtain the nonlinearity in the horizontal soil resistance of a pile group foundation subjected to arbitrary directional loading. Analyses were conducted on a single pile, and 2 x 2, 3 x 3, 5 x 5, 7 x 7 and 9 x 9 pile groups placed in sand and clay soil, for spacing to diameter ratios of 2.5 and 6.0. To calculate nonlinear soil springs, the piles were subjected to forced displacement in arbitrary directions including diagonal directions. In these analyses, shear force at the pile head and subgrade reaction along each pile shaft were calculated. The major findings obtained from these analyses can be summarized as follow: 1) The pile group effect on the subgrade reactions was larger than that of shear forces at the pile head. The number of piles and the soil type affected the pile group coefficient, while the pile displacement and its loading direction had small impact on it. 2) The shear force at the pile head and the subgrade reaction along the pile shaft differed depending on the pile location and became larger in the case of the front piles during the loading. These differences could be clearly observed in a pile group with narrower pile spacing in sand. 3) For a pile located on the edge of a pile group, the shear force at the pile head and the subgrade reaction along the pile shafts differed depending on the direction of loading. When the pile became a front pile, the shear force at the pile head and the subgrade reaction became larger. These differences could be clearly observed in the pile group with the narrower pile spacing in sand. 4) The hysteresis curve of the pile subgrade reaction-displacement relationship located in sand and clay had different characteristics. When the piles were placed in sand, the hysteresis of the pile located on the edge of the pile group showed an asymmetric loop, while those in the middle of the pile group were symmetrically-shaped. When placed in clay, the hysteresis of all piles showed an almost symmetric loop.

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