Detailed Stability Analysis on A Major Slide Occurred Near Uyilatti Waterfall, Kookalthorai To Kotagiri Road, The Nilgiris District, Tamil Nadu

Abstract: Landslides threaten hill stations like Ooty in the Nilgiris district of Southern India due to their steep terrain, heavy rainfall, and anthropogenic activities like improper planning and deforestation. In December 2022, a major landslide occurred on the Kokalthorai to Kotagiri road near Uyilatti waterfalls in Kotagiri taluk, Nilgiris district, Tamil Nadu. A study has been initiated through a comprehensive slope stability analysis, and the research methodology incorporates a combination of geotechnical investigations and field surveys. The rock slope stability assessment used geomechanical analysis systems, specifically the Rock Mass Rating (RMR) and Slope Mass Rating (SMR). The type of failure and their Factor of Safety (FOS) for individual discontinuities in rock slopes were calculated using the Hoek and Bray method. The FOS was computed using a Circular Failure Chart (CFC) regarding soil slopes. A detailed field investigation was conducted to collect data required for RMR and SMR calculation. As per Hoek and Bray's Kinematic methods, FOS was calculated. In this study, joint J3 satisfies the plane failure, and joint J2 and J3 are considered for wedge failure analysis. The details are furnished in this paper. Three surface soil samples and one core sample were collected to fulfill the study and calculated ‘c’ and ‘phi’ values using the Tri-axial shear test. The FOS was calculated by the CFC method. The analysis assessed the Factor of Safety (FOS) in three static conditions: dry, partially saturated, and completely saturated. The results indicate that the slopes in two sections (U and M) are unstable (FOS < 1), whereas the third section (L) is stable (FOS = 1.0). Urgent and targeted interventions are necessary to address the instability of the slopes, especially after the rainy season, to mitigate the risk of recurring and frequent failures on the hill.