不飽和邊坡入滲之數值模擬

Abstract

本研究利用數值分析結合室內實驗成果，對不飽和邊坡因入滲破壞之機制作更定量的了解。研究方法係以有限元素軟體ABAQUS模擬不飽和邊坡入滲情形，並以兩組室內砂箱試驗作為模擬對象；其中之一為均質邊坡，另一邊坡為具有傾角10°的岩層面。模擬方式係假設模型邊坡於入滲前已有初始飽和度，接著以2mm水壓於坡頂平台進行入滲。數值模型於左右邊界設置輥接，下邊界設置鉸接；在上方平台處給予水壓值，並於坡面與坡趾堆積區設置排水邊界。 由程式模擬結果，可觀察到靠近上方平台的水流方向為垂直向下，而上邊坡附近水流方向約與坡面平行，且於下邊坡位置呈現水流由邊坡內部往外流出的趨勢；坡趾處的水流甚至有向上湧的情形。由觀察孔隙壓力隨入滲時間變化的情形，發現於均質邊坡中，於上下坡交界處與靠近坡趾位置首先出現初始降伏的情形，此時孔隙壓力突然增加，所對應之主應力值亦大幅降低，顯示土壤有效應力下降，使得邊坡穩定性降低；在有岩層情況中，亦出現孔隙壓力值突然增加之情形，惟先出現降伏的區域為上下坡交界處。另由塑性區之分佈與砂箱試驗結果比較，模型邊坡出現局部破壞的位置與數值模型開始發展成降伏情況之位置相符。

Numerical analysis was employed and combined with laboratory results to understand the mechanism of the infiltration process in partially saturated slopes. The finite element analysis software, ABAQUS, was used in this study to simulate the process of infiltration into unsaturated slopes. Two infiltration tests performed in sandbox were simulated ; one slope was homogeneous and the other had bedding planes dipping at 10° with the horizontal. Rollers were set up at the left and right boundaries of the numerical model and the bottom boundary was hinged. Furthermore, water pressure form overland flow was applied on the crest of the slope. The drainage boundaries were the slope surface and the deposition area. The results of numerical analysis show that the water infiltrates vertically downward at the crest and flows parallel to the slope face in the upper part of the slope, but it flows outward from the slope in the lower part. Moreover, in the homogeneous slope, the initial yielding point occurred at the intersection of upper and lower slope faces and at the toe of the slope. Consequently, the principal stresses and the effective stresses decreased at the same time. In the intersection slope with bedding planes, the initial yielding point occurred only at the upper and lower slope faces. In addition, the development of plastic zones is accordance with the experimental results.