以個別元素法界定 凝聚性覆土材料於正斷層 之地表及土中變形帶

Abstract

基盤上覆土材料之性質及厚度為控制斷層錯動後覆土中剪切帶發展之主要因素。土壤主要可分為凝聚性及非凝聚性材料，前者，如黏土，受力後會形成張力裂縫，使張力裂縫之尖端與基盤初始小位移錯動時形成之破裂面相連。 應用個別元素法於斷層模擬可直接觀測土體內之破壞機制及變形帶之發展。且數值模擬較物理試驗易於取得模型內之受力、變形資料。本研究以個別元素法搭配前人之離心機模擬成果，以離心試驗為真，校正數值模擬參數，使兩者之產狀相符，進而探討不同覆土深度、覆土材料性質之正斷層土中變形帶演育歷程，以推估未來可能錯動斷層之土體變形機制。 數值及離心模型之尺寸相同，為斷層面傾角60度之正斷層，於1g狀態下，其覆土厚為0.2米，施加離心力後，對應之現地覆土厚最多可達16米。模型於錯動率(上盤之垂直位移量除以總覆土厚)達到25%時錯動完成。覆土材料包含純砂層、純黏土層、砂黏土互層。 本研究證明個別元素法之數值模擬工具可應用於離心機斷層砂箱模擬。同樣的基盤錯動率及覆土厚下，砂性覆土層具有較寬廣的剪動帶，地表之變形剖面為平緩且具連續性，斷層崖由安息角主控；黏土性覆土會在極小基盤錯動率時於地表發展張力裂縫，並與斷層基盤尖端之初始剪動帶相連，並於地表發育坡度陡峭之斷層崖，使地表變形剖面不連續。另也利用數值模擬建立具離心環境的生長斷層，以凝聚性及非凝聚性覆土材料觀察其地表及土中變形差異。

The depth and character of the overlying earth deposit contribute to fault rupture path. For cohesive soil, for instance, clay, tension cracks on the ground happen during faulting, limiting the propagation of fracture in soil mass. The cracks propagate downwards while the fracture induced by initial displacement of faulting propagates upwards. The connection of cracks and fracture will form a plane that is related to tri-shear zone. With application of distinct element method the mechanism of fault propagation in soil mass and the development of ground deformation zone can be observed directly in numerical analysis of faulting. The information of force and deformation in the numerical model are also easier to be obtained than centrifuge modeling. Therefore, we take the results of centrifuge modeling as the field outcrop then modify the micro-parameter of numerical analysis to make sure both of them have the same attitude. A series of centrifuge tests and numerical modeling are conducted at this study with maximum acceleration conditions of 80g (equals to 16m thick of overburden soil) and dip angle of 60° on normal faulting. The model is with total overburden soil thick, H, 0.2m, vertical displacement of moving wall, ∆H. At the beginning, hanging wall and the left-boundary wall moves along the plane of fault. When ∆H/H equals to 25%, both of the walls stop moving. This study proved that the numerical analysis can be applied on simulation of centrifugal modeling and overburden soil deformation by normal faulting.