逆斷層錯動引致上覆土層變形行為及對 結構物影響之研究

Abstract

根據近幾年來世界上幾個著名災害性地震之調查結果，顯示近斷層結構物受到地震破壞之原因。除了強地動之慣性力外，另一主要因素為斷盤錯動所導致之上覆土層變形。以921集集地震為例，在斷層作用時之斷層帶附近，大多數結構物均遭受到嚴重損毀。然而少數結構物即使位於斷層帶，結構體本身卻僅有剛體位移或轉動而未受損。因此斷層作用時，覆蓋在斷層帶上面的土層在斷層作用所反應之出來的行為與變形對結構物之影響，實有進一步研究之需要。 由於台灣之活動斷層主要為逆斷層，因此本研究針對逆斷層，使用無凝聚性砂土模擬上覆土層之材料，進行小尺度砂箱實驗，探討斷盤錯動時，上覆土層變形行為及對結構物之影響。並以砂箱實驗配置與實驗所得之材料參數進行數值分析，以測定邊界條件及輸入參數之合理性。再將已驗證之數值分析工具，以全尺度數值分析探討斷層作用時上覆土層變形行為，及其對地下潛盾隧道及淺基礎之影響。 由砂箱實驗結果顯示，逆斷層隨上盤抬昇時，斷層面向上擴展。斷層面路徑為一曲線，隨接近地表而與水平方向夾角由陡變緩。當基盤抬昇比(基盤鉛直抬昇量與土層厚度之比值)為0.3時，產生兩條斷層面，斷層面由上盤向下盤發展。其主要變形區域，集中於一個三角形剪切帶中。其剪切帶之大小(為三角形之頂角角度)與斷層傾角有關，斷層傾角較緩頂角角度越大。當地下隧道位置接近三角形剪切帶時，斷層面擴展路徑受到隧道之影響而產生分支之現象，隧道本身亦產生明顯之變形。同樣地，淺基礎位置接近三角形剪切帶時，亦會造成斷層面路徑改變。其斷層面路徑隨淺基礎位置不同而有向下盤偏移或受基礎角隅牽引等現象。 當以砂箱實驗配置所進行的數值分析結果顯示，由塑性應變集中帶之位置、發展先後順序與砂箱實驗結果相當，可以用來判斷預測砂箱實驗斷層面之位置及發展過程，驗証數值分析工具之可行性。 由全尺度數值分析結果顯示，斷層作用引致上覆土層變形，其塑性應變集中帶大小，受到斷層傾角、覆土材料參數(包括：楊氏模數、柏松比、凝聚力、摩擦角及膨脹角)等參數影響。其中以斷層傾角、楊氏模數及膨脹角等參數，對塑性應變集中帶影響最大。當斷層傾角越小，塑性應變集中帶越大呈負相關，而楊氏模數及膨脹角呈正相關。 當地下隧道位置接近塑性應變集中帶時，會使塑性應變集中帶產生分支之現象，而有明顯之互制行為。當以隧道襯砌所受之軸力與彎矩分析，顯示隧道越接近塑性應變集中帶時，其所受之彎矩量越大，隧道環片破壞程度越高，且位於下盤之隧道破壞程度會較位於上盤者大。 同樣地，淺基礎位置越接近塑性應變集中帶時，亦會使塑性應變集中帶產生分支之現象而有明顯之互制行為。造成剪切帶轉向與基礎位於剪切帶之位置、基礎載重等因素有關。當基礎位於塑性應變集中帶之下盤側，造成塑性應變集中帶轉向所須之載重較上盤側低。當基礎越接近斷盤錯動面時，其所須之載重越大。此外，基礎之剛性，造成剪切帶轉向之現象較為明顯，因此對於接近塑性應變集中帶之淺基礎，增加基礎之剛性，將較可能確保基礎之完整。 在工程實際應用上，常以應力觀念為主，當由危險因子(安全因素之倒數)分析，顯示危險因子可以反映土壤之應力狀態，與即將到達破壞狀態之區域。當以隧道襯砌所受之軸力-彎矩與危險因子作為比對，也得到良好之結果，因此未來工程設計應用方面，可利用危險因子作為土壤與結構互制之初步評估計之參考。 本研究經由砂箱實驗與數值分析所得有一致結果，因此未來在進行特定工程之地中或地表結構物之受震危害度時，可利用現地斷層特性、土壤分佈及土壤力學參數與結構物之特性，配合數值分析工具，進行定量模擬分析。

When we observate the major earthquakes in the last century, the results, in addition to the disasters of intertial force, indicates that some destruction to structures is mainly threatened by the near-fault effect especially near the ground surface movements. For example, the Landers Earthquake in California (1992; Mw=7.3), the Chichi Earthquake in Taiwan (1999; Mw=7.6), and the Duzce Earthquake in Turkey (1999; Mw=7.1). All of them had activated co-seismic fault movement on ground surface, and they accounted for severe damages of structures (such as houses, bridges, and roadway). Even we can find few structures on the top of fault trace were not damaged, and the causes can be subjected to the rigid stiffness of the buildings. But most of the structure were destroyed. As the results, the near-fault effect is a significant topic to be studied. During the period of a large earthquake, where the overburden soil beds situat above a fault are often deformed by the propagation of the bedrock thrusting from the fault. Then the deformed beds form a triangular shear zone. On the basis of previous work by Cole and Lade (1984), this research further explores the processes of thrust faulting within a overburden soil, and it examines the influences of corresponding factors or parameters under a range of boundary conditions. This study proposes a physical model and applies numerical analysis for both small-scale and full-scale configurations. Here, factors explores include uplifting rate, fault dip angle, dilation angle of plastic flow, Young’s modulus, Poisson ratio, cohesive strength, and frictional angle. All of the factors are as well as the location of ground loadings applied on ground surface. Beside the numerical analysis, the experimental results indicate that although one major fault slip surface can be developed, and then the subsidiary faults may also form. The subsidiary faults must be taken special attention when we define the stain patterns. For small-scale, physical models are used to simulate the fault development to simulate the full-scale problem, and the stiffness of model soil are properly scaled down. Moreover, the scaling-down procession still requires further study. This coseismic faulting often causes damage to underground tunnels near the shear zone. The present research studies the deformation behavior of the overburden soil beds and the tunnel, the associated mechanism, and the impact on the safety of tunnel linings induced by a large blind thrust slip. Based on sandbox experimental and numerical studies, it is found that results from numerical analysis are in agreement with the sandbox model tests, the growth of the shear zones within the soil beds, the location of the tunnel in this shear zone, and the deformations of the tunnel. The potential major shear zone may be bent or be bifurcated into two sub-shear zones owing to existence of a tunnel inside the shear zone. Furthermore, the occurrence of back-thrust faulting will threaten the safety of nearby structures. It was also identified that the stiffness of soil and the fault dip angles are among the major factors to control the configuration of shear zones, the stresses within the soil, and the loads on tunnel linings. Based on the identified mechanisms, the strategies for hazard prevention are accordingly suggested and discussed in our studies. Similarily, when the loacation of a shallow fundatin is near the shear zone, the potential major shear zone may be bent or be bifracturted into two sub-shear zones as well. The trend of the shear zone is influencd by the location of the shear zone and the surcharged acted on the fundation. When the fundation is loacted wihin the foot wall, the shear zone will trend to the hanging wall side. Otherwise, it was found that the fundation rigidity is the key factor to influence the trend of the shear zone. Accordingly, it is helpful to ensure the safety of a fundation by increasing the fundation rigidity. In engineering appication, the stress state of soil can be reflected by the dangerous factor, and it is helpful to determine whether the stress state of the soil reachs failure or not. Comparing the dangerous factor with the axial force and the bending moment that acted on the lining, it shows that the dangerous factor is an available indes to evaluate preliminary the reaction between soil and structure.