Importance of Field Observations for the Numerical Stability Analysis of Rock Slopes: Case Studies from the Sikkim Himalaya

The network of highways plays a vital role in the socioeconomic development of the country. The present study is conducted along the Gangtok–Lachung–Lachen Highway in Sikkim, India. The research aimed to assess the stability of sixteen slope sections along this highway, incorporating geological and geotechnical parameters obtained during field investigations. To classify the rock mass, two different rock mass characterization indexes were calculated based on the field parameters. Kinematic analysis performed in Rocscience Dips software helped to identify potential failure modes. The results indicated that out of the sixteen slopes analysed, ten were susceptible to planar failure, three exhibited wedge failure, while the remaining three slopes were stable. To further investigate, three slopes exhibiting tendency of planar failure were considered based on the site observations to perform detailed numerical modelling using Universal Distinct Element Code (UDEC). Effect of saturation on the joint surface was considered by reducing the basic friction angle in the Barton–Bandis joint model. Surface Wedge software was utilized to assess the factor of safety against wedge failure for the three identified slopes undergoing wedge failure. This approach provides a comprehensive understanding of the stability conditions and failure mechanisms associated with those specific slope sections. The study highlights the importance of field data in selecting the appropriate numerical modelling technique to ensure meaningful results. UDEC has a limitation that it can’t incorporate very closely spaced joints because of convergence issues. And the presence of multiple joint sets drastically reduced the factor of safety (FOS) value in UDEC. By integrating field investigations, kinematic analysis, and numerical modelling, the research aimed to provide valuable insights into the rock slope stability along the Gangtok–Lachung–Lachen Highway.