Geo-Engineering Investigations and Numerical Analysis of Namok Khola Landslide along NH310A, Sikkim, India

In this study, geotechnical, geological, and seismic refraction investigations were carried out along NH 310A in Sikkim, India, to assess slope instability. Namok Khola is one of the most unstable slopes identified along the Gangtok–Chungthang road. The area contains highly deformed mica schist with a thick layer of weathered material comprising sandy silt. Field tests using the Standard Penetration Test (SPT-N) revealed the presence of soil and weathered material up to a depth of 5.5 m. Seismic refraction testing was performed to identify slip surfaces of the landslides, confirming the existence of bedrock at depths ranging from 6 to 9 m below the surface. The seismic velocity within the upper weathered zone ranged from 650 to 700 m/s. Two distinct velocity zones were observed within the bedrock, representing disintegrated and intact bedrocks with velocities ranging from 2000–2100 m/s and 3900–4000 m/s, respectively. Laboratory analysis of samples obtained through borehole drilling validated the results of the seismic refraction tests. Particle flow code (PFC) is used in this study to model the landslide and examine its dynamic properties. The simulation demonstrated that failure happened for about 39 s, with the major sliding event lasting 14 s. The maximum average displacement was 125 m, while the most significant measured displacement was 170 m. Results from the simulation have demonstrated good agreement with the actual landslide characteristics of Namok Khola sliding.