軟弱地盤為了增加土壤承載力常採用土壤改良方法，而砂柱具備施工快且經濟的地盤改善優勢。近幾十年來，已證實除了可提高土壤承載力、加速黏土層壓密沉陷外，亦可以有效地降低地盤液化的潛能。傳統砂柱雖然可承受上方構造物給予的軸力及剪力，但砂柱應用於軟弱黏土層時，砂柱樁體上部分易因周圍軟弱土壤提供的束制力不足而產生腫脹破壞。利用具有抗張特性的地工合成材包裹砂柱外圍，以增大砂柱的束縛應力，提升砂柱承載能力，即為可行且有效的解決方法。 本研究針對軟弱黏土層中外包加勁砂柱的應用進行模型試驗，探討砂柱經外包加勁方式所提升的承載能力。為使模型試驗能夠符合現地真實行為，透過模型相似律之因次分析，選擇適當的模擬材料及尺寸。試驗內容分別以5種不同勁度的加勁材進行模型試驗，以探討不同加勁材張力特性對軟弱黏土層中加勁砂柱的載重-變形行為。除外包加勁外，也進行純黏土、純砂土及無加勁砂柱等承載力試驗，以作為加勁後砂柱力學行為的比較。 模型試驗結果顯示，外包加勁的方式確實能有效地提高軟弱黏土中砂柱的承載性能，其中加勁材勁度越高，柱體側向擴張越小，承載力越大。而低強度且延展性較差的加勁材料，加載過程中可能因加勁材張力破壞，柱體大幅側向擴張，使得承載力因而衰減。 此外，本研究基於孔穴擴張理論，以解析方式詮釋外包加勁砂柱於軟弱黏土層中是如何獲得圍束應力，使得砂柱試體能夠抵抗上方載重的施加。砂柱側向應力的提供除來自於周圍土壤外，主要利用砂柱本身的側向擴張，引致外包加勁材發揮環箍張力，因而提供砂柱圍束應力，增加砂柱的承載性能。分析結果與試驗觀測相近，提供業界應用於外包加勁砂柱工程設計的參考。 In order to increase the bearing capacity is often used the soil improvement methods in soft ground. The sand column is a rapid and cost-effective construction method for improvement of weak soils. In recent decades, the sand column has been verified can effectively providing higher load-carrying capacity, accelerating soil consolidation, and reducing the liquefaction potential. However, insufficient lateral support at a shallow column depth often caused bulging failure in the top portion of the column. The bearing capacity of a sand column is increased by encapsulating the column with geosynthetics that is feasible and effective solution. In this study, the model tests on single geosynthetic-encased sand columns installed in very soft clay was used. In order to the model test results correspond with the field columns behavior. Selecting the appropriate modeling material through dimensional analysis method of model similar law is used. The model tests were conducted on five different stiffness reinforcement for encased sand columns. The clay layer, sand layer, and un-reinforcement column for load - deformation behavior was also evaluated. The model test results show that encased sand columns can be significantly to enhance the bearing capacity on soft clay. The higher stiffness of the reinforcement caused the smaller the lateral expansion of the column and the larger carrying capacity. The relative assessment of the load-carrying capacity for different reinforcement and a contrasting evaluation of column failure mechanism in the different cases, are salient features of this study. In addition, exactly how to get the variation of confining stress in soft clay layer is interpret based on the theory of cavity expansion. The increase in axial deformation causes lateral column expansion and compression of the surrounding soft clay, subsequently causing circumferential stress from the reinforcement; meanwhile, compression of the surrounding clay induces additional earth pressure to act on the column. The analytical results close to the experimental observations. This study provides the estimation of bearing capacity for encased sand columns in soft clay layer.