樁基礎為橋梁工程常用的基礎型式之一，其所在的地盤，則常為軟弱地盤或飽和砂層，地震時，地盤受力變形，孔隙水無法及時排出而致土壤之孔隙水壓升高，降低了土壤之有效應力，甚至發生地盤液化而影響樁基礎的之支承能力，在多次大地震中，皆有因地震時地盤強度降低或液化的關係，而造成重大橋梁災害。本研究的目的為發展有非線性三維有效應力分析模式，以分析地震時地盤及樁基礎的反應，或液化後之橋梁樁基礎的反應。 本研究考慮土壤土壤係由固相與液相所構成之多孔介質，並以Biot多孔介質理論推導土粒構架之變形與孔隙水流動之耦合關係式。土壤之非線性行為遵循帽蓋模式，並以Pacheco 孔隙水壓模式模擬地震時孔隙水壓的變化，至於側向邊界則以多孔介質理論推導之黏滯邊界模擬。本分析模式可以進行橋墩─樁基礎─地盤之地震反應互制分析。以本研究所發展之程式，計算Zienkiewicz一維地盤例、羅東大比例尺核能電廠圍阻體模型及台北捷運基隆河橋等之受震反應，並與相應的分析結果或實測資料比對後，驗證了本計算程式的正確性。 藉由水平地形及河谷地形之橋梁樁基礎的地震反應分析，探討了橋梁樁基礎在不同工址地形之受震行為。由分析的結果可知，水平地形及河谷地形之橋梁樁基礎的受震反應，依其所在工址位置不同而有差異。河谷地形橋梁樁基礎在沿河谷方向的水平變位會比水平地形者大35%，而在上下方向則會大到一倍之多；至於樁身彎矩和剪力值，也有相同的現象，惟其值約略大15%，而軸力方面則最大值有大到一倍之多。因此在河谷地形之橋梁樁基礎的耐震設計上，有其特別考慮的必要。 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.
