杭位置の違いによる群杭の非線形水平地盤抵抗のモデル化

Abstract

1. Introduction Pile foundation response during earthquakes is strongly affected by nonlinear soil-pile foundation interaction. The damages to pile foundations during the 1995 Hyogo-ken Nanbu Earthquake and the 2011 Tohoku-Chiho Taiheiyo-Oki Earthquake were obviously attributed to nonlinear interaction of soil-pile foundation-superstructure. The dynamic nonlinear behavior of soil around each pile affects lateral load distribution and displacement of each pile. Therefore, shaking table tests were conducted to clarify the dynamic nonlinear behavior of soil around piles, further earthquake response analyses were conducted to simulate the effect of the nonlinear soil-pile interaction system on performance of the superstructure supported by pile group. 2. Outline of shaking table tests 25-pile group foundation model was set up in Toyoura sand deposits within a laminar container. The sand deposits were prepared by air pluviation method in the laminar container. The model piles were acryl cylinder with 12mm in diameter and 400mm long. 25 piles were arranged in squares and pile spacing was 2.0 times pile diameter. The superstructure was modeled as a rigid body. The input waves were seismic waves in notification with random phase of Japan and the Hyogo-ken Nambu earthquake. 3 input acceleration levels were used to investigate the influence of input motion level. Tests with and without the mass of superstructure were conducted to investigate lateral load distribution of each pile. 3. Outline of shaking table tests simulation Two analysis methods were conducted for simulation of tests, one is nonlinear 3D-FEM, and the other is lumped mass model. Two case of Lumped mass model are conducted, one is with soil springs around pile group which were different according to pile arrangements and direction of loading, and is with soil springs without pile group effect. Soil springs were calculated by using nonlinear 3-D FEM, which were modeled appropriately according to the location of pile. 4. Conclusions The concluding remarks of these shaking tests and analytical studies are as follows. (1) In shaking tests, lateral subgrade reaction around piles depends on the location of each pile in pile group and the direction of loading. That of the corner pile is remarkably larger than that of the middle pile. (2) In shaking tests, hysteresis curve of soil spring at the corner pile is found to be asymmetric loop, while that at the middle pile be symmetric loop. (3) In shaking tests, bending moment at each pile head depends on the location of each pile in pile group. That of corner pile head is remarkably larger than that of middle pile head. As the input level of acceleration increase, bending moment at each pile head is equalized. (4) The proposed lumped mass model of the soil-pile foundation system, which consists of beam elements and nonlinear interaction springs considering the location of pile, is verified validation. It is confirmed that this proposed model well represents response of the superstructure supported by pile group.