| Summary: | 碩士 === 國立中興大學 === 水土保持學系所 === 97 === The geological structures and hydrological conditions are extremely complicated at landslide area and it is difficult to evaluate the groundwater seepage behavior and the associated stability problems of an exsiting slip surface during rainfall by employing simplified conventional analysis method. This study performed two dimensional finite element seepage and deformation analyses on the existing slip surface along the A-A profile of Lu-Shan Landslide during torrential rainfall of Matsa Typhoon in 2005. At the mean time, incorporating with the limit equilibrium analysis the corresponding stability of the existing slip surface at each time step can be evaluated. Thruough the aforementioned analyses, the variations of groundwater level, pore water pressure, factor of safety and displacement rate of the existing slip surface at each time step of a real time rainfall can be obtained.
The reliability and validity of the proposed numerical model was verified by comparing the numerical results of groundwater variation and displacement rate of the existing slip surface along the A-A profile of Lu-Shan Landslide with those from measurements. In addition, a set of optimum numerical parmeters of alluvium which encompassed saturated hydraulic conductivity, saturated volumetric water content for seepage analyses and Youngs modulus, friction angle, cohesion, Poissions ratio and total unit weight for displacement analyses were determined by the optimization back analysis procedure of objective function.
Based on the objective function of optimization back analysis, it was found that the variation of groundwater level in seepage analyses is most sensitive to the saturated hydraulic conductivity value of alluvium and secondly followed by the saturated volumetric water content value. On the other hand, the displacement variation of soil mass is most sensitive to the Youngs modulus value in displacement analysis of rainwater infiltration, then followed by the Poissions ratio and friction angle values, whereas it is less sensitive to the total unit weight and cohesion values.
Through the time-dependent slope stability analyses during rainfall, it was found that the factor safety of the existing slip surface F(t) along the A-A profile of Lu-Shan Landslide will descend as the groundwater level uplifts and which alternately implies an increase of pore water pressure on the existing slip surface. On the contrary, the numerical results show a tendency of the recovery of the descended F(t) value when the groundwater level is drawdown and the pore water pressure on slip surface is decreasing.
Eventually, a predictive equation (or v(t)~F(t) equation) for displacement rate v(t) from factor of safety F(t) can be derived by a regression analysis. The predictive equation is capable of predicting the immediate displacement rate v(t) of the slope according to the factor of safety F(t) at each timestep t during rainfall. In such manner, the displacement rate of the exsiting slip surface of Lu-Shan Landslide can be immediately evaluated and used as a quantitative reference for landslide warning system.
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