當高速公路穿過山區時，有些橋樑的基礎將會無可避免地興建在坡地上，其中井樁是較常用的基礎。一般而言，除了承載橋樑結構之垂直荷重外，井樁也會因為風力及地震力而受到側向水平力的作用，因此有必要就井樁承受水平側向荷重的行為進行探討。
本研究乃在運用大地工程離心機進行實驗，以探討在砂土層中井樁在承受反覆側向載重後，對樁身彎矩及樁頭變位的影響，並藉著與單一方向施加水平載重的行為比較，探討在反覆載重與單向載重作用下，井樁行為的差異。實驗結束後，將於樁身各測點所量得的彎矩，利用改良式多項式法求得表示彎矩分布的數學式，然後再將此數學式分別微分兩次和積分兩次以求得剪力、土壤反力和樁身的側向變位。
由研究結果可看出，本實驗具有可重複性，而隨著反覆水平荷重作用次數的增加，樁體的彎矩增量會逐漸下降，且水平荷重的大小會漸漸趨一定值，至於樁頭變位則會隨次數的增加而變大。在相對密度較低的地盤中，於卸載階段，樁體彎矩值衰減的較慢，且殘留彎矩值較相對密度大的地盤來的高。

When free way is constructed in the mountain area, the foundation of bridge will be inevitably erected in the slope and one of most frequently used foundation is the caisson-type pile foundation. In addition to supporting the gravity load transmitted from the bridge structure, the caisson-type pile foundation will be subjected to lateral load induced by wind, earthquake and so on. Therefore, it is important that the behavior of caisson-type foundation under the action of lateral load be investigated.
In this study, the behavior of caisson-type pile foundation subjected cyclic lateral load is investigated using the geotechnical centrifuge. The distribution of bending moment along the shaft of caisson-type pile is obtain by using the modified polynomial approach and the measured values, which can then be used to find the soil reaction by differentiating twice and the deflection by integrating twice.
The results show that the purposed test procedure has a very good repeatability. As the number of cycles increases, the increment of bending moment decreases and the lateral load will approach a constant value, while the pile-head displacement increases. For caisson-type pile embedded in a low-density sandy soil stratum, the decrease in moment will be smaller and the residual moment will be larger, as compared with that embedded in high-density sandy soil stratum.

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