樁基礎為軟弱地盤中常採用之基礎型式，其主要功能為將上部荷載以點承或摩擦的方式傳遞至底部或周圍土層。然而，上部結構除了垂直荷載外也會有水平荷載。為了符合基樁的使用性，必須將容許側向位移限制在某一限度，本研究旨在探討環狀地盤改良法對基樁側向位移的改良成效，並兼顧經濟性及實用性，歸納出最佳改良範圍。
　　本研究以不同水泥配比之攪拌處理方式，對樁周土壤進行環狀改良，以增加土壤強度，探討對基樁水平側向位移之改良成效。其中改良區域配合網格為輻射狀，假設成圓柱體。改良區域以三個參數作為控制變數，改良壁體中心與樁中心點之距離、改良壁體深度及改良壁體厚度，皆以樁徑作為基本單位，運用有限元素軟體ABAQUS，進行108種不同改良範圍及4種改良配比，共432組之分析。
　　在基樁承受最大荷重（Q=1000kN）時，由數值分析結果得知，改良配比為16%，改良寬度為1m，改良深度為5倍樁徑，改良距離為4倍樁徑之改良範圍，對樁頭側向位移之改良成效達13.89%，顯示對樁周土壤進行環狀改良，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 foundation is often adopted in soft ground, it can transfer the weigh of structure by the way of skin friction and end bearing resistance to the bottom or the soil around. However, the structure will load in either vertical or horizontal direction. To conform to the practicality, the lateral displacement of pile must be restrained in a certain limit. This study focused on the treated effects in lateral displacement by annular ground improvement, and figured out the best range of treatment by considering its economy and practicability with FEM analysis.
In this study, different cement contents were used to treat soil ring around pile to increase the lateral resistance of ground. The FEM mesh of treated zone is annular. The range is governed by treated distance, treated depth and treated thickness. To perform the numerical analysis, FEM based software named ABAQUS was used in this research. This research carried out a series of numerical analysis including 108 kinds of treated range and 4 kinds of cement content.
The numerical analysis results of pile in cohesive soil indicated that with the cement content of 16% and the treated range with the dimension of 1m in thickness, 5R in depth, and 4R in distance (where R is the diameter of pile), it can achieve an economical improvement effects in reducing the lateral displacement of pile. The local soil improvement can increase the lateral resistance of ground and reduce the lateral displacement of pile under laterally loading condition.

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