| Summary: | 碩士 === 國立臺灣大學 === 土木工程學研究所 === 103 === Uncertainty is the main problem in slope stability analysis, parameters such as the shear strength and other data might vary due to the inherent variabilities. In addition, the geometrical assumptions including use in the analysis model, including the location of the strata and groundwater level, might also vary. These variation would lead to the uncertainties in the slope stability analysis. Typically, the slopes were designed with high factor of safety to avoid the slope failure from the bias of the estimated ground properties and the assumptions. However, a high factor of safety might not ensure safety of designed slope. A dip slope along Freeway No. 3 slid on April 25, 2010, 12 years after the construction. A case of Mt Shih-gong-ge landslide event was conducted to investigate how the slope stability analysis was affected by the variabilities of the geotechnical properties.
In this study, the geotechnical parameters of the ground strata and the geometrical data such as the ground profile and the groundwater level were collected from the ground investigation reports. The slope was consisted of sedimentary strata. The plane slip surface occurred in the layer of alternations of thin sandstone and shalestone, 19 meters below the ground. The average dip direction and dip angle of slip surface was N113°E／15°SE, different from the dip direction of the ground strata’s bedding plane, N124°E／16°SE. The study confirmed that the direction of most critical profile was parallel to the dip direction of slip surface. The surface of ground-water in the slope was estimated from the water pressure monitor records and Casagrande’s solution for flow through an earth dam. A regression analysis was conducted for the relationship with the highest groundwater level and 24-hours-rainfall-intensity in rainfall events. The characteristic values of the parameters considered in slope stability analysis were estimated. The variabilities of ground properties were also estimated.
The back analysis slope was performed with the limit equilibrium method (LEM) analysis using and the characteristic values of ground properties the factor of safety was calculated as 1.20 without considering rising groundwater level. By raising the groundwater surface due to the rainfall event on 2 days before the landslide, the factor of safety was 1.10. During that scenario, when efficiency of corrosive anchors reduced to about 70%, the factor of safety would be 1.0.
The effects of the variabilities of ground properties and assumptions was analyzed. The results showed the variability of the shear strength of shalestone and rising of groundwater surface would affect the stability of slope significantly. The failure probability of the slope was 18.2% by considering the variability of the shear strength of shalestone without the efficiency reduction of anchors and the rising groundwater surface. The failure probability of the slope would come to 70.7% as groundwater surface raised to the surface of slope.
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