樁基礎常見使用於各項公共工程，如風力發電、海域鑽油平台、橋梁、核能電廠等設施之基礎，其中，基樁須能承受可能發生之垂直力、水平力，並考慮振動載重及地震作用所造成之動態反應。地震發生時，樁體受上部結構物慣性力、以及地盤反覆土壓力相互作用下，會使樁身產生側向變形；群樁基礎若埋置於液化土層時，因液化土壤剪力強度的喪失，使樁體被動土壓力減少。尤其樁基礎位在緩和坡地或岸邊土層時，則會產生側向滑移(lateral spreading)，造成樁基礎產生嚴重破壞。因此樁基礎於地震力作用時，應考慮土壤與結構物互制效應。本研究採用乾式儲存基座作為模擬與試驗對象，透過模型因次分析，製作縮尺樁-土-結構模型。並利用中大地工離心機進行群樁離心模型振動台試驗。 試驗結果顯示，(1)在受到相同的基盤輸入加速度時，位於乾砂地盤中的樁其最大樁身彎矩，會大於埋置在液化土層中樁基礎的最大樁身彎矩；(2)當乾砂地盤中受到相同的基盤輸入加速度，樁基礎在樁帽埋置於土壤中時的最大樁身彎矩，會小於樁帽裸露於地表之樁基礎的最大樁身彎矩；(3)當飽和砂地盤中受較小的基盤輸入加速度，樁帽裸露或樁帽埋置於地表，其樁身彎矩分布皆與埋置在乾砂地盤的試驗結果相似；(4)當飽和砂地盤受到較大基盤輸入加速度而發生液化時，由於砂層液化喪失絕大部分的剪力強度，樁帽裸露或樁帽埋置於地表，其樁身最大彎矩亦無明顯差別。(5)超額孔隙水壓激發時，地盤內隨著深度增大，樁身軸力亦逐漸下降，直至超額孔隙水壓開始消散，樁身軸力方回復並接近至振動前的軸力。 ;Group piles foundation is widely used in public projects, such as wind power generation, oil drilling rig, bridges and nuclear power plants. It is important to consider its dynamic response under seismic loadings. During earthquake, piles deform laterally subjecting to the inertia force of superstructure and cyclic earth pressure of ground. Furthermore, the passive earth pressure of group piles foundation would loss as well as the shear strength when the soil liquefies. Sometimes, the lateral spreading occurs leading to the serious damage of piles. A series of centrifuge shaking table tests in this study was performed to investigate the seismic behavior of group piles with pile cap and superstructure.
According to the test results, the following conclusions can be drown. (1) The maximum bending moment of group piles in dry sand is larger than that in liquefied sand. (2) The maximum bending moment of group piles with pile cap embedded in the ground is smaller than that with pile cap exposed above the ground surface. (3) Subjecting to the small input base acceleration, the bending moment distributions of group piles in dry and saturated sand deposits are almost the same. (4) Under the larger input base acceleration, the soil liquefies and leads to that the maximum bending moment of group piles with embedded pile cap is almost the same as that with exposed pile cap. (5) When the soil liquefies, the excess pore water pressure increases with the increasing depth, and the axial force decreases with increasing excess pore water pressure along depths. After the dissipation of pore water pressure, the axial force increases to as large as that before shaking.
