本研究利用FLAC2D有限差分軟體，觀察土壤受壓密沉陷時，對基樁所產生的負摩擦力影響。模型土層之上層為軟弱黏土層，下層為承載卵礫石層。基樁承載採軸對稱分析模式，分別探討摩擦與點承兩種型式基樁之負摩擦力行為，並釐清不同的加載模式和承載條件彼此相互影響的關係。 研究結果發現，負摩擦力大小與中立點位置和樁、土間之相對位移有很大的關係，增加載重大小或加載面積造成的樁身沉陷量與下拉力(dragload)皆會變大，使中立點往下移動。若在相同條件下改變摩擦樁長，則只有樁身沉陷量會改變，土壤沉陷大小幾乎相同。點承樁與摩擦樁相比，中立點位置較靠近樁底，樁身沉陷小而造成之下拉力大，須考慮樁材強度是否足夠。 改變承載與壓縮層土層勁度比，發現在10倍勁度比內，沉陷量隨著勁度比提高而減少，下拉力增加；而超過10倍之勁度比後即無明顯變化發生。若改變地表荷重(surcharge)與樁頭荷載之加載先後順序，以先加地表荷重再加樁頭荷載的下拉力值較小，而此兩種方式之最終沉陷量差異不大，故可作為降低下拉力大小之方法之一。 This research uses finite difference program FLAC2D to observe negative skin friction behavior of a single pile. The numerical soil model consists of two layer soils, the upper compressible soft clay and the lower gravel bearing layer. The aim of this research is to find the differences of negative friction behavior between friction and end-bearing piles, and to clarify the influences of different loading patterns and bearing conditions. The result shows that the downdrag force of a pile and neutral point location depend on the relative displacement in the interface of pile and soil. Increasing loading magnitude and area will cause more dragload and settlement, while the neutral point moves down in the meantime. Compared with friction pile, end-bearing pile has more dragload and smaller settlement, it is necessary to check the buckling potential of pile material. By varying the stiffness ratio between compressible and bearing layers, the larger the stiffness of bearing layer, the less the pile settlement and the more the pile dragload within 10 times stiffness ratio, but there is no obvious variation when the stiffness ratio is greater than 10. Moreover, applying axial loading then surcharge will have larger dragload than the reverse loading sequence but with similar pile settlement. Therefore, changing loading sequence might be chosen to be one of the ways to reduce dragload.
