| Summary: | 博士 === 國立中央大學 === 土木工程研究所 === 88 === Pile foundations have been frequently used to support bridges constructed on the soft ground or ground with saturated sand. During earthquakes the ground deforms and generates excessive pore water pressure, which leads to the reduction in effective stress and sometimes liquefaction of ground, affecting the bearing capacity of the pile foundations. Over the past years many damages of bridges due to this effect have been reported. The purpose of this study is to develop an effective-stress based nonlinear 3D finite element model to investigate the seismic behavior of pile foundations of bridges in liquefiable ground.
Considering the soil as a two-phase porous media with solid grain and water, the effective-stress based nonlinear 3D finite element model is developed using the Biot theory. The nonlinear soil behavior is assumed to follow the cap model and the pore pressure model proposed by Pacheco is adopted. The viscous boundary derived considering the soil as porous media is developed to simulate the infinite lateral extent of ground. This model can be used to analyze the soil-pile-bridge system. The developed program was validated by comparing with the one-dimensional analytical result by Zienkiewicz and recorded data of two different sites.
The seismic responses of pile foundations are then studied for regular ground and river valley. It is found that the irregularity of ground affects the seismic responses of pile foundations significantly. The lateral displacement of piles in the direction of valley and the vertical displacements of piles are amplified about 35% and 100%, respectively, as compared with those of regular ground. Similar observations can also be made for the bending moment, the shear force and the axial load of piles. The increase in bending moment and shear force is about 15% and that in axial load can be as high as 100%, as compared with those of regular ground. Therefore, in the seismic design of pile foundations for bridges in the river valley, special care must be taken.
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