國內水庫集水區發生崩塌之邊坡大多被不飽和崩積土壤所覆蓋，乾季時存在著基質吸力維持邊坡穩定。降雨之雨水入滲使得崩積土壤因飽和而失去基質吸力，進而產生淺層崩塌。本研究選取石門水庫砂崙仔崩塌地附近的崩積土壤作為試驗之土樣，進行了一系列的室內、現地及土壤水分特性曲線(SWCC)試驗，以瞭解同一集水區內不同位置崩積土壤的巨、微觀特性。接著應用SWCC來推求不飽和剪力強度參數，再以STEDwin 2.64程式進行降雨入滲對邊坡穩定影響之分析。另外，本研究也初步研製一套應力相依土壤水分特性試驗儀，以便後續研究應力狀態對於SWCC之影響時使用。
試驗結果顯示，同一集水區內崩積土壤的土壤礦物皆以伊利石為主，且成份含量百分比趨勢略同；重模崩積土及原狀殘積紅土之保水能力比原狀崩積土來得佳。邊坡穩定分析結果顯示，安全係數隨降雨浸潤帶的加深而下降，而潛在滑動面則隨浸潤帶加深而上移。因此，降雨浸潤帶是造成不飽和崩積土壤邊坡產生淺層滑動破壞的最重要因子。

Former studies have indicated that unstable slope locating near the watershed of a reservoir is often covered by unsaturated colluvium soils. During the dry season, high matric suction exists in the ground and maintains its stability. However, after rainfall the collapse of soil slope can occur due to infiltration and saturation of the surface soils. In order to understand the colluvium soil characteristics including both macroscopic and microscopic properties, this study adopts the colluvium soil samples nearby the Sha Lun Tsai landslide area in Shihmen reservoir. The variation of soil characteristics is investigated by samples taken from different location. A series of experiments including both field and lab tests were conducted. The soil water characteristic curve (SWCC) was used to estimate the unsaturated shear strength parameters. The slope stability analyses were, then, performed using STEDwin 2.64 program at various rainfall infiltration levels. In addition, this study also preliminary develops a set of stress-dependent SWCC equipment for future study to evaluate the effect of stress state on the SWCC.
The test results exhibit that the colluvium soil samples of the same watershed mainly contain illite mineral with similar percentage of content. Moreover, the water retention capacity of the remold colluvium soil and undisturbed residual soil is better than that of the undisturbed colluvium soil. Slope stability analyses indicated that the factor of safety decreases with the increase in rainfall infiltration depth, and the potential slip surface becomes shallower when the infiltration depth increases. Therefore, it can be concluded that the “wetting-front” of rainfall infiltration is the most essential factor that can induce shallow slip failure of the colluvium soil slope.

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