摘 要 樁基礎常用於各項工程中以支承龐大結構物荷重，它能傳遞這些荷重穿越軟弱地盤達到堅硬土層或岩盤中。相對於樁的垂直向支承力，其側向支承力顯得不足。為解決此一問題，本研究針對樁周圍土壤進行改良，以增加樁基礎側向支承力及減少樁頭側向位移。 本研究利用室內土槽進行一系列模型試驗，嘗試以添加水泥之方式增加樁周圍土壤剪力強度，比較改良前後側向荷重和樁頭位移與樁身彎矩之差異，並探討模型樁側向位移、彎矩、改良範圍和改良配比之間的關係。 由試驗結果得知，以寬度6倍樁徑的正方形改良面積，與深度4倍樁徑之改良區域，回填添加水泥配比3%之改良拌合土，可減少樁頭側向位移達58%，並能增加樁基礎側向支承力達1.5倍。顯示對樁周圍土壤進行局部改良，可增加淺層土壤的剪力強度，提高側向阻抗，進而有效減少樁頭側向位移。 ABSTRACT Pile Foundations are often used to bearing construction loading in many kinds of engineering, and it can transfer the loading directly to stiff layer or rock ground. But the lateral bearing capacity of pile is always smaller than its vertical bearing capacity. This research tended to improve the strength of soil surrounding the single pile to increase the lateral bearing capacity of pile. The research carried out a series of model pile tests, and tried to improve the shear strength of soil surrounding the pile by using cement, and compared the lateral displacement of model pile in treated and untreated soil. The experimental results of model pile in treated sand indicated that with the cement content of 3 percent and the treated area with 6D in width of square and 4D in depth (where D is the diameter of pile), it can achieve a well improvement effect in reducing the lateral displacement of pile for 58 percent, and increasing 0.5 times the lateral bearing capacity of pile of untreated sand. The local improvement of soil surrounding the pile can increase the lateral bearing capacity of pile, and reduce the lateral displacement of pile effectively in the lateral loading conditions.