PROBABILITY ANALYSIS OF SLOPE STABILITY FOR ROCK CUTS ON INTERSTATE 40, WESTERN NORTH CAROLINA

The deterministic analysis based on the factor of safety concept is a common practice in rock slope engineering. It uses a single value for each input parameter without regard to the extent of uncertainty involved in each parameter. However, parameters governing rock slope stability, such as discontinuity orientation, discontinuity geometry, and rock strength parameters, do not have single fixed values but may occur as various number within a certain range. Probability analysis provides a powerful means to quantify and model uncertainties prevalent in variables of rock slope parameters. In this study a probabilistic analysis approach was developed by extending the Monte Carlo simulation, and was used to analyze rock slope stability for road cuts along Interstate Highway 40 (I-40), Western North Carolina. This probabilistic procedure consists of two parts: kinematic stability analysis and kinetic stability analysis. The two probabilities are then combined to provide an overall stability for the rock slope. For each analysis, probabilistic stability analysis is accomplished using a probability density function for each discontinuity parameter. Random strength and geometric parameters for discontinuities were obtained from engineering judgment and derived from field measurements and analysis using the statistical inference method. Six geometries for the cut slopes on I-40 were evaluated, involving the original 75º slope, along with 70º, 65º, 60º, 55º and the present 50º slope developed over the past 40 years of weathering. The probabilities of rock failure were compared with different geometries, and the influence of weathering processes on rock slope stability for each discontinuity was evaluated. Since intensified rainfall triggered many rock slides in North Carolina, the effect of pore water pressure on rock slope stability was also considered using the probability analysis.