加勁土壤基礎受正斷層錯動之有限元素法分析與設計方法建立

Abstract

斷層錯動使得地盤上、下盤形成相對位移，產生之剪裂帶向地表發展會造成嚴重的地表變形，使結構物損壞並倒塌。當線性結構物無可避免地必須通過斷層帶時，若以具有柔性特質之加勁土壤基礎(Geosynthetic-reinforced soil foundation)替代剛性混凝土結構將可減緩斷層錯動造成之地表差異沉陷、降低地表角變量並維持上部結構之穩定性及通行性。 本研究以有限元素法分析加勁土壤基礎減緩斷層錯動引致之地表變形行為，探討加勁材抵抗正斷層錯動之力學機制，包含阻斷效應及張力膜效應。首先透過物理模型試驗結果驗證本研究數值分析方法之合理性，接著透過一系列之參數分析，改變加勁材鋪設長度、勁度、最大張力強度、土壤基礎厚度、回填土模數以及界面強度折減因子，評估不同土壤參數及加勁材參數抵抗正斷層錯動之效果，並用以提出加勁土壤基礎之設計方法。數值分析之基本模型(Baseline case)為土壤厚度3 m、平均鋪設三層地工合成材料之加勁土壤基礎，受傾角60°之正斷層錯動，最大錯動量為S = 1.5 m而最大垂直錯動比S/H達50 %。透過觀察地表最大角變量、剪裂帶發展過程、加勁材應變發展長度以及最大張力應變值評估加勁土壤基礎抵抗斷層錯動之效果。 研究結果顯示，所建立之數值模型可以有效模擬加勁土壤基礎受正斷層作用下之剪裂帶發展及地表變形行為。當加勁材鋪設長度超過正斷層錯動引致之張力區範圍時，加勁材即可有效減緩主要沉陷區之地表角變量。鋪設較長之加勁材則能有效發揮阻斷效應，阻止基礎內剪裂帶繞行至加勁材末端並向上發展至地表，有效防止地表因剪裂帶繞行造成二次沉陷的產生，亦可同時避免加勁材拉出作用。此外，由數值分析結果可得知地表最大角變量將隨著加勁材張力發展增加而降低，兩者大約呈線性關係，證實了加勁材之張力膜效應。最後，本研究透過多變量迴歸分析獲得預測地表最大角變量及加勁材最大張應變之迴歸式，建立加勁土壤基礎在正斷層錯動下抗斷裂與抗拉出之設計方法。

A fault rupture may bring serious structural damages. To prevent potential damages, one possible solution is to use geosynthetic-reinforced soil (GRS) foundation under linear infrastructure to cross the fault. The GRS foundation was adopted to accommodate the differential settlement, reduce the fault-induced angular distortion and maintain the stability and serviceability of the embankment. This study focused on the performance of GRS foundations subjected to fault-induced differential settlement. A series of numerical simulations are conducted in finite element program and validated with physical model tests. The validated numerical model is then used to develop the baseline case properties which is a GRS foundation in 3 m height, with 3 layers of reinforcement, subjected to normal fault displacement in a dip of 60°. Afterwards, a serious parameter study is conducted through changing the parameters which are reinforcement length, foundation height, reinforcement stiffness, reinforcement tensile strength, soil stiffness and soil-reinforcement interface strength reduction factor to investigate the mechanism of reinforcement preventing fault rupture propagation. The results show that the numerical analysis methodology is well-validated and could simulate the surface deformation of GRS foundations subjected to normal fault movement. While the reinforcement length exceeds the range of tension zone caused by normal fault, GRS foundation can effectively reduce surface angular distortion at primary settlement. Build up longer reinforcement can limit the shear ruptures propagate up to the ground surface or winding around at the end of reinforced zone and then avoid secondary settlement take shape. In addition, the numerical results indicate that ground surface maximum angular distortion decrease with reinforcement tensile force increase, which confirmed the tension membrane effect. Finally, multiple regression analysis method is used to obtain the regression formula to predict maximum angular distortion and reinforcement maximum tensile strain. The responses are adopted for the design method of GRS foundation subjected to normal fault.