The deformation of overburden soil induced by thrust faulting and its impact on structure
|Keywords:||逆斷層;砂箱實驗;數值分析;三角形剪切帶;土壤與結構互制;危險因子;thrust fault;sandebox experiment;numerical analysis;triangular shear zone;soil and structure interaction;dangerous factor.||Issue Date:||2007||Abstract:||
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.
|Appears in Collections:||土木工程學系|
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