當降雨入滲至邊坡土體時，淺層土壤內的基質吸力隨降雨入滲而改變，導致許多推估邊坡穩定性的土壤參數不確定性增加。傳統的邊坡穩定分析在處理這些參數的不確定性大多採用平均值或是過去經驗值輸入極限平衡法來求取安全係數，再以容許安全係數(一般均大於1.0)來概括參數的不確定性，即當安全係數大於容許值時才認定其為安全。本研究在邊坡穩定分析中，考慮各種變數或參數本身之變異性，利用邊坡穩定可靠度分析模式，評估邊坡破壞機率及信賴度指數(reliability index)來判定邊坡穩定。未飽和殘積土經過許多地質作用而形成，因此在各方向存在著不同的水力傳導係數，多數的研究對於土壤水力傳導係數的大多考慮為均向性(isotropy)，對於異向性(anisotropy)的滲流機制的瞭解較不足。因此本研究將針對水力傳導係數的均向性以及異向性進行探討。

本研究設計不同研究案例，應用數值分析軟體Geo-Studio模擬降雨入滲至未飽和均質土壤邊坡內部，以SEEP/W套件進行暫態分析，再將所得的孔隙水壓資料代入SLOPE/W套件分析邊坡穩定，進而探討不同的降雨特性對於異向性水力傳導邊坡的影響機制。首先，本研究考量均向性水力傳導之砂土、粉土及黏土等三種土壤邊坡，並設計三種不同的降雨型態，分別為降雨強度尖峰最先到達的前峰雨型、降雨強度尖峰值位於中間的中峰雨型和降雨強度尖峰位於後段的後峰雨型。研究案例結果指出，不同土壤邊坡於降雨事件發生時，砂土邊坡安全係數下降幅度最大，次為粉土，最後為黏土邊坡。而分別在前峰、中峰及後峰不同雨型的狀態下，對於邊坡穩定發生下降的時間影響最為顯著，穩定性下降時間先後順序為前峰雨型、中峰雨型，最後為後峰雨型。

在探討異向性水力傳導的案例中，結果顯示砂土邊坡對於水力傳導係數的異向性最為敏感。水力傳導係數的異向性對於黏土邊坡的影響不甚顯著。最後，本研究考量當降雨強度改變下，土壤水力傳導係數大於降雨強度時，於降雨事件開始信賴指數就發生明顯下降，於降雨事件開始6小時後，下降幅度趨於平緩，下降為初始信賴指數的23%。另外，當土壤水力傳導係數小於降雨強度時，降雨入滲發生時，信賴指數有一段7小時的遲滯，其後發生一大幅度的下降，下降為初始信賴指數的65%。本研究模擬結果指出當氣候變異造成一強降雨發生時，如果土壤水力傳導係數小於降雨強度時，邊坡發生不穩定的時間會相對降雨強度小較早發生。本研究之分析結果可以提供坡地防災之參考。

The stability of slopes decreases due to the suction decreases occurring with rainfall infiltration. Traditional studies of slope stability have used a general limit equilibrium method to calculate the safety factors and to determine whether a slope is safe. However, sometimes the failure of slope may occur even though the safety factor is more than unity (FS > 1) because the input soil parameters are considered to be the mean value for slope stability analysis. As a result, when many parameters are used in analysis, the level of uncertainty increases. The probability approach used to study geotechnical issues offers a systematic way to treat uncertainties, especially in the case of slope stability problems. In this study, probability analysis is used to evaluate the stability of unsaturated soil slopes. The geological formation of residual soils is mostly in distinctive layers that may have different hydraulic conductivity (ks) in different directions. Furthermore much of the research on this topic has assumed the ks to be isotropic. Therefore, in this thesis, the effect of anisotropic of ks on the slope seepage under the condition of rainfall infiltration is examined.

In this study, the finite element computer program Geo-Studio is used to simulate the process of rainwater infiltrate to the slope. The pore-water pressure results evaluated from seepage analysis (SEEP/W) are imported into the slope stability program (SLOPE/W). In order to quantify the slope stability results probabilistically, the soil strength parameters are provided with a range.

The results of the designed case study indicated that in the case of sand, the rainfall pattern controlled the time for the occurrence of instability of the slope under consideration. The rate of decrease in safety factor versus time was found to be faster in the case of the advanced pattern, followed by the normal and delayed patterns. The results for the anisotropic ratio of hydraulic conductivity indicated that when the anisotropic ratios become higher, the reduction in the reliability index is insignificant. Cases for the sand slope under different rainfall intensities (I) were designed. It was found that while the ks was greater than I, the reliability index decreased immediately, and there was also a decrease in the reliability index by nearly a quarter because the event after 6 hours remained stable. When, the ks was less than I, the reliability index stayed at the beginning level. About 7 hours later, there was found to be a marked downward trend. The reliability index fell by 65%. In the other case, the simulation results indicated that when the ks was less than I, the percentage probability of the occurrence of a landslide was larger than when the ks was greater than I. Finally, in the cases of anisotropic ks, the stability of the high ratio soil slopes was not found to be sensitive to the reliability index variation during the simulation period. Moreover, when the ks was greater than I, the stability of the slope decreased earlier than was the case in the opposite situation.

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