不同降雨形態對異向性水力傳導之淺層邊坡信賴度評估

Reliability Analysis of Anisotropic Hydraulic Conductivity of Shallow Landslides under Different Rainfall Pattern

葉信富(Hsin-Fu Yeh) ； 沈岡陵(Kang-Ling Shen) ； 李振誥(Cheng-Haw Lee)

《農業工程學報》 60卷1期 (2014/03) Pp. 15-28

https://doi.org/10.29974/JTAE.201403_60(1).0002

摘要

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參數不確定性增加。本研究利用信賴度分析，以機率分佈輸入土壤力學參數，其不同於傳統方法僅考量單一定值，進而評估淺層邊坡信賴指標（reliability index）。本研究主要考量三種不同的降雨型態，分別為前峰雨型、中峰雨型和後峰雨型，分別以砂土、粉土及黏土三種類型土壤邊坡進行降雨入滲模擬，本研究設定土壤水平水力傳導係數（kx）和垂直水力傳導係數（ky）比值（kx / ky）分別為2、10及20三種異向性比值。本研究應用數值分析軟體Geo-Studio 模擬降雨入滲至未飽和土壤邊坡內部，首先進行邊坡暫態滲流分析，將所評估之孔隙水壓空間分布進行邊坡穩定分析，計算出各時間點的邊坡信賴指標，進而探討降雨型態對於不同土壤水力傳導異向性邊坡穩定性的影響。本研究結果顯示，砂土邊坡在三種降雨型態中，異向性高的土壤邊坡（kx / ky比值越大）其信賴指標發生明顯下降的時間皆較異向性低的土壤邊坡延後，且於模擬時間內，異向性最高的土壤邊坡（kx / ky = 20）其信賴指標值達到最小。另外，對粉土邊坡而言，異向性對於信賴指標發生明顯下降的延遲時間與砂土相似，異向性高的土壤邊坡信賴指標發生明顯下降的時間較異向性低的土壤邊坡晚，於模擬時間內，邊坡的信賴指標變化的趨勢皆為持續下降，以異向性低的土壤邊坡下降幅度最大。最後，黏土邊坡模擬結果顯示，於案例中的信賴指標皆沒有明顯的變化，可推知黏土水力傳導的異向性對於邊坡穩定性的影響並不大，本研究之分析結果可以提供坡地防災之參考。

關鍵字

水力傳導異向性 ； 未飽和邊坡 ； 信賴指標 ； 降雨型態

並列摘要

Stability of the slope decreases due to the suction decreases with the rainfall infiltration. As the result, many parameters used in analysis their uncertainty will increase. In this study, reliability analysis is used for the unsaturated soil slopes. The numerical software Geo-Studio is used to simulate rainfall infiltrated inside the unsaturated soil. The analysis was performed on three typical rainfall patterns, including the advanced, normal, and delayed, with considering three different soil types (sand, silt, and clay). Anisotropic studies are defined the vertical anisotropic ratio as the ratio of vertical (ky) to horizontal (kx) hydraulic conductivity (kx / ky = 2, 10, 20). The finite element method and limit equilibrium method are used to calculate the reliability index with time. The process of infiltration into a slope due to rainfall patterns and its effect on soil slope stability is examined using numerical simulation.The reliability index of the sand unsaturated slope is 7.7 before the start of the rainfall patterns. The time for reliability index reduction becomes noticeable is early for low anisotropic ratio (kx / ky = 2) but during the simulate time the high anisotropic ratio (kx / ky = 20) has the largest reduction in the reliability index. For the silt unsaturated slope, the reliability index exist obvious reduction in the reliability index when the anisotropic ratio is low (kx / ky = 2). During the simulate time all the silt slope cases are sustained decreased in reliability index. Finally, effect of anisotropic conductivity for the clay slope and the reliability index, the result indicated that there is not evident different between these three anisotropic ratio.

並列關鍵字

anisotropic conductivity ； unsaturated slope ； reliability index ； rainfall pattern