一般的邊坡地下水位皆處於深處，位於地下水位以上的土壤為不飽和狀態，其力學特性會受到土壤顆粒、孔隙水及孔隙空氣三種材料之互制行為所影響，非傳統飽和土壤力學所能詮釋。Fredlund等人(1978)提出不飽和土壤之廣義莫爾-庫侖破壞準則，定義 為基質吸力，為不飽和土壤力學重要參數之一。在大雨過後邊坡可能發生淺層滑動破壞，其原因為雨水入滲在邊坡表面形成之浸潤帶，使得基質吸力下降，而導致邊坡於浸潤帶內發生淺層近似平面的破壞面。本研究針對一個現地邊坡進行位移、地下水位及基質吸力監測，以了解其相互關係，並藉此探討不飽和土壤邊坡之工程行為。
另外本研究利用ABAQUS模擬降雨入滲不飽和土壤邊坡之假設案例分析，由分析結果得知接近地表處基質吸力下降的速度會在瞬間發生，主要原因為接近地表處吸力受水影響最快也最直接，而在地表中其基質吸力下降的幅度也隨深度增加而趨於緩和，與本案例現地量測比較後其趨勢一致。

Usually the groundwater level in the slope is deep under ground surface. The soil above the groundwater level is unsaturated and the characteristic of mechanics will be affected by the interaction behavior of soil particles, pore water and pore air. The situation can not be explained by the traditional saturated soil mechanics. The unsaturated soils Mohr-Coulomb failure criterion was presented by Fredlund et al.(1978).In this thesis, one of the important parameters is the matric suction that is defined as . After rainfall, shallow slip failure might happen on the slope because of the rainfall infiltration through the surface of the slope. Rainfall infiltration forms a wetting band lowering the matric suction and causes a nearly flat shallow failure plane. This study focuses on the field monitoring of slope displacement, groundwater level and matric suction of a well instrumented slope. The measured results are then used to understand their relationship and investigate the engineering behavior of the unsaturated soil slope.
Besides, computer program ABAQUS is used to analyze the suction variation due to rainfall infiltration of a hypothetical case. Results shows that the matric suction decreases in a very short time near the surface. The main reason is that the surface suction is affected by rainfall infiltration most directly and quickly. When the depth increases, the decrease amount of the matric suction tends to slow down. This trend agrees with the behavior observed from field monitoring.

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