台灣地區之山坡地多屬不飽和土壤邊坡，而這些處於地下水位上的不飽和邊坡，在遇到降雨後，水分滲流易導致水文特性(如基質吸力等)的改變，進而引發後續邊坡穩定的問題。本研究以現地案例作為分析對象，利用數值分析程式SEEP/W與SLOPE/W模組以不同降雨條件下對不飽和土壤邊坡進行滲流模擬及穩定分析。研究內容除原有警戒方式(降雨量、地下水水位)之模擬探討外，本研究也深入探討降雨入滲造成不飽和土壤邊坡之水文特性以及穩定性之變化，進而找出較為準確、且亦有力學基礎之崩塌災害發生指標，以供相關單位進行防災預警之參考。
研究發現，不飽和土壤邊坡在相同累積降雨量但延時不同，其邊坡穩定分析結果有所差異，故僅以累積降雨量作為崩塌發生與否的指標參數，以本研究案例而言，其準確性仍未臻完善。且在存有厚崩積層或深層地下水之邊坡，在不同降雨情境下，水位的變化都不甚顯著。不飽和邊坡土壤的破壞類型多屬淺層破壞，破壞位置多沿著浸潤帶發生，而浸潤帶的產生時即代表基質吸力的消散，土壤之抗剪強度也會隨之下降，此為造成邊坡不穩定之重要因素之一。分析結果顯示降雨入滲引致之邊坡破壞前體積含水比存在臨界值特徵，而此臨界體積含水比不受降雨條件及不同位置的影響。因此，利用臨界體積含水比當作崩塌災害發生指標之一，應屬可行之方向，值得進一步探討。

ABSTRACT
The majority of Taiwan’s hillsides belong to unsaturated soil slope. Seepage in these unsaturated slopes situated above groundwater level would easily lead to changes of hydrological characteristics (such as matric suction, etc.) when encountered rainfall, and then trigger subsequent slope stability problems. This study adopted a field case and used numerical analysis program SEEP/W and SLOPE/W module to conduct seepage simulation and stability analysis on unsaturated soil slopes under different rainfall scenarios. In addition to simulation and exploration of original alert modes (rainfall, groundwater level), this study also investigates changes of hydrological characteristics and stability in unsaturated soil slopes caused by rainfall infiltration, and then identify more accurate landslide disaster occurrence indicators also having a mechanical basis for potential application by related agencies in implementing disaster prevention and early warning measures.
This study found that the slope stability analysis results of unsaturated soil slope would vary under same cumulative rainfall but different durations. Therefore, only cumulative rainfall was used as indicator parameter of whether landslide will occur or not, in terms of this case study, its accuracy still has room for improvement. And under different rainfall scenarios the changes of water level in slopes with thick colluvium or deep groundwater are not significant. The majority of damage types of unsaturated soil slope belong to shallow landslide, and slope failures mostly occur along the wetting front, while the generation of wetting front represents the dissipation of matric suction, as a result the shear strength of soil will also drop, and this is one of the important factors causing slope instability. The analytical results exhibit critical value characteristic of volumetric water content prior to slope failure induced by rainfall infiltration. In addition, this critical volumetric water content would not be influenced by rainfall conditions and positions in the slope. Therefore, using critical volumetric water content as a landslide disaster occurrence indicator should be a feasible direction and warrants further study.

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