樁基礎為各項工程中最常使用之基礎型式，當樁尖深植於較堅硬之土層或岩盤中，可承受上部結構物傳遞的荷重。但對於軟弱地盤而言，樁基礎抵抗側向力的能力會受到限制。為解決此一問題，本研究對樁周土壤進行改良，以增加其側向支承力及減少樁身側向位移。 本研究係以ABAQUS有限元素軟體模擬基樁埋置於軟弱黏性土層中，承受側向荷重後之力學行為，並對樁周土壤以水泥混合攪拌之方式進行局部改良，以探討改良前後基樁側向支承力之變化及側向位移之改良成效。 在基樁承受最大荷重（Q=1000kN）之條件下，由數值分析結果得知，當改良配比為12%，且地盤改良範圍控制改良直徑為8倍樁徑，改良深度為4倍樁徑以內之圓柱體，對於減少基樁側向位移可達到78%的改良效果。顯示對樁周土壤進行局部改良，可增加淺層土壤的剪力強度，提供水平阻抗，進而有效減少樁身之側向位移。 Pile Foundations are often used in large constructions. It can transfer the weight of structions directly to stiff layer or rock ground. However, the lateral resistances of piles in soft ground are limited. This research tended to improve the shear strength of the surrounding soil of single pile to increase the lateral resistance and reduce the lateral displacement of pile. The lateral bearing behaviors of single long pile in cohesive soil layer were analyzed by finite element method. To perform the numerical analysis, FEM based software named ABAQUS was used in this research. The local soil improvement was proposed by using cement, and the behaviors of lateral bearing capacity and lateral displacement were compared with those in untreated cases. The numerical results of the pile in cohesive soil indicated that with the cement content of 4% and the treated range with the dimension of 8D in diameter and 4D in depth (where D is the diameter of pile), it can be achieved an economical improvement effect in reducing the lateral displacement of pile. The local soil improvement can increase the shear strength of soil surrounding the pile near the ground surface and reduce the lateral displacement of pile in the laterally loading conditions.