本研究利用離心模型試驗，探討位於飽和砂土層中之既存基樁，當鄰近施工基樁發生坍孔、以及抽降地下水對既存基樁的影響。分別探討(1)施工基樁在不同的坍孔深度以及跟既存基樁在不同的距離情況下，對既存基樁的影響。(2)不同降水深度對既存基樁軸向力的變化情況與樁身沉陷和地表沉陷。 研究結果顯示，在自重壓密階段，地盤的沉陷量都比樁體的沉陷量還大，各組試驗中立點也都位於樁尖附近，故其樁身軸向力都會隨著深度的增加而增大，且與理論分析差異不大。在既存基樁施加荷載階段，因為既存基樁沉陷大於地表沉陷，導致樁身摩擦力都往正摩擦力發展，且樁身量測的軸向力也與理論分析有良好的一致性。在施工基樁發生坍孔階段，可以發現當施工基樁中心與既存基樁中心水平距離1.5m時，既存基樁的樁身軸力，會因鄰近施工基樁發生坍孔而受到影響，也因為發生坍孔而有負摩擦力產生；但當施工基樁中心與既存基樁中心水平距離3m時，其既存基樁樁身軸力受到鄰近施工基樁則影響較小；另外，當鄰近施工基樁發生坍孔，既存基樁樁身摩擦力的發展會比只有單一既存基樁還小，這是因為土壤側向力變小的緣故。在地下水下降階段，樁身軸力會隨著降水深度的增加而增加，而且當既存基樁鄰近有施工基樁時，樁身摩擦力發展也比只有單一既存基樁還小，而且與理論分析也有良好吻合。另外，試驗量測土壤沉陷量與理論計算土壤沉陷量也相當吻合；且在既存基樁樁頭施加荷載階段、地下水下降階段，所估算的土壤之楊氏模數，與前人提出的土壤之楊氏模數相當吻合。 A series of centrifuge modeling tests were performed to study the effect of collapse of the adjacent constructing pile and the groundwater lowering on the existed pile in saturated sandy ground. Two topics are investigated. The first is the effect of the collapse of constructing pile on the existed pile. The second is the changes of axial force and settlement on the existed pile and the variation of the ground surface settlement as a result of different groundwater withdraw level. The test results show that, at the self-weight consolidation stage, the settlement of the soil is greater than that of the existed pile and the neutral point of pile in all the tests is located at near the pile tip, leading to the axial force of existed pile increases with the increasing depth. In the next stage while the work load was applied on the existed pile head, the settlement of the existed pile is greater than that of the soil resulting in the positive skin friction developed along the pile. When the distance between the existed and the constructing piles is 1.5 m, the collapse of constructing pile leads to the significant changes of the axial force within the existed pile and the occurrence of the negative skin friction along the existed pile. But it would disappear when the distance of piles is 3 m. Besides, the collapse of constructing pile leads to the decrease in the skin friction of the existed pile because of the decrease in the lateral earth pressure, and so does the lowering groundwater level resulting from the increase in the axial force. In addition, the measured ground surface settlement, skin friction and axial force of pile are in good agreement with that obtained from the theoretical analysis. The estimated Young’s modulus Es at each stage of testing procedure also corresponds with that proposed in the past studies.