Sensitivity and Uncertainty Analyses of the Translational Slide at the Cidu Section, 3.1k of the Taiwan Formosan Freeway

Long-term water immersion weakens slope rocks and degrades rock-layer parameters such as cohesion (c) and friction angle (ϕ), causing slope failure. The traditional analyses of slope failure generally set fixed values to the required parameters by limit equilibrium, but omit uncertainties of the parameters. These methods fail to fully reflect the slope safety factor, causing unreliable calculated safety factors. We examined the collapse of the Cidu section (3K+100) dip slope on Taiwan Formosa Freeway for the sensitivity analysis of c and ϕ and uncertainties related to the variation of the safety factor before and after water immersion and slope rock weakening. The sensitivity analysis results showed that c has greater effects on the slope safety factor compared with ϕ. In the uncertainty analysis, we used three different point estimate methods and the Monte Carlo simulation to calculate the corresponding slope safety factors. This method established the probabilistic analysis of slope stability. We also explored the effect of ρ (correlation coefficient of c and tan ϕ) on safety factors. The traditional analysis results found that the factor of safety is approximately 1, but the probability of failure of slopes reaches 50%, indicating urgency in using stabilizing measures.