Probabilistic Slope Stability Analysis of Anisotropic Rock Slopes Using Directional Shear Strength Model

The highly dissected Northeastern Himalayan ranges witness landslides almost every monsoon. One such notable landslide is the Chumoukedima-Ghaspani area (NH-29) Dimapur rockfall that occurred on September 8, 2021. This study aims to address critical issues of the Dimapur landslide by considering it as a case study and understanding the anisotropic variation of engineering properties using directional strength models and their influence on the stability of the slopes. These strength models are dependent on the orientation and strength of the anisotropy plane, intact rock strength, etc. In addition to directional anisotropy, the study considers three primary uncertainties, namely, cohesion, friction angle, and hydrological conditions—as random variables. To avoid unsafe or overconservative estimates resulting from extreme values of the data, the undertaken work employs a probability-based slope stability analysis using the limit equilibrium method. Monte Carlo simulations were found to be an efficient tool to incorporate the entire range of uncertain data available from the chosen site. It could be inferred from the study that the spacing and frequency of joints (the major discontinuity) had a significant impact on the probability of failure but only till a certain spacing interval (of 30 to 35 m). Joint intervals below 20 m would result in highly unstable slopes. Also, material spatial variability expressed in terms of correlation length is a major factor of concern. Equal correlation length in both axes, meaning isotropic conditions, proved to be the most underestimated configuration conditions, giving a low risk of failure, while any other proportions increased the risk value for more realistic anisotropic conditions. Hence, discontinuities, both micro and macro, have a significant impact on slope stability.