- 學位論文
參數變異性對邊坡穩定分析之影響 以國道3號師公格山順向坡崩塌為例
Slope Stability Analysis Considering Variability of Geotechnical Property: A Case study of the Mt.Shih-Gong-Ge Landslide on Fwy. 3
摘要
邊坡工程中主要控制邊坡穩定性的參數為地質構造、地層材料參數、地下水位與設置的穩定設施等。然而，地層材料參數受組成材料、形成環境影響具有變異性，試驗值較人工材料分散。在進行邊坡穩定分析時，亦易因對地形、地層與地下水位的幾何分布、地層參數的取樣密度限制，未能以正確的資訊進行邊坡穩定分析，以致所得到的邊坡穩定分析結果與事實有偏差。本研究以2010年4月25日發生於國道3號3.1K師公格山的順向坡崩塌事件為例，探討參數變異性對邊坡穩定分析結果之影響。 本研究首先蒐集案例基本資料，瞭解案例邊坡的歷年地形變化，蒐集多時期案例邊坡之地形高程模型與案例邊坡內部的地層分布狀況。判定案例邊坡為平面破壞，並確認地層之真傾角與案例邊坡破壞時滑動面位置、所處地層與滑動方向，作為反算分析之依據。由監測資料瞭解案例邊坡地下水位之波動範圍，與案例降雨指標建立關係公式，並由滲流公式假定該情境下，案例邊坡地下水位面高程。自鄰近氣象站之歷史降雨資料模擬案例邊坡發生破壞前一週，案例邊坡地下水位變動狀況。接著統計本研究所蒐集之案例邊坡與鄰近區域之地層材料試驗參數資料，確認試驗值的位置與分散情形，並探討地層材料參數之變異性，決定進行邊坡穩定分析時引用之地層材料參數特徵值。 根據前述資訊進行邊坡反算分析，首先建立邊坡穩定分析反算模型，將參數之最可能值代入邊坡穩定分析模型，計算出案例邊坡崩塌前，最可能之邊坡穩定安全係數，並逐項調整地錨強度衰減程度與地下水位高度，推斷案例邊坡可能因為地層材料強度偏低，地錨鏽蝕強度衰減與事故發生前兩日之降雨造成地下水位上升，導致邊坡發生破壞。 確認案例邊坡崩塌機制後，首先討論滑動面所處地層剪力強度參數變異性造成之影響，計算各情境下，案例邊坡之破壞機率。接著進行敏感度分析，依序討論岩石材料單位重、地下水位高程變動、剪力強度參數對邊坡穩定分析結果造成之影響。並與前人選擇的分析剖面進行比較，確認所取的剖面為最具代表性的分析剖面。 最後，比較各項參數之變異性，並以Duncan法進行簡易之破壞機率分析，得到案例邊坡之破壞機率。
並列摘要
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.