Geoenvironmental and geotechnical assessment of soil slopes in the vicinity of Atal tunnel in Himachal Pradesh, India

In this work, geological, mineralogical, geotechnical and stability analysis using limit equilibrium method (LEM) was carried out around the vicinity of Atal tunnel in Himachal Pradesh, India to access the slope instability. Four representative unstable soil slopes along the Palchan road to the South portal of Atal tunnel were identified. The study area is located in the north-west Himalayas near South portal (S-1, S-2 and S-3) and one slope (S-4) near North portal (NP) of Atal tunnel. It is the only road connecting the city to the tunnel and therefore, the stability assessment of road cut slopes is imperative for safe transportation. South portal is mainly composed of clayey sand and NP consists of poorly graded sand (SP) as thick overburden. Laboratory investigation showed that the soil slopes are sandy and non-cohesive and high dynamic conductivity. Shear strength parameters (c and phi) were also found to be low (Cohesion < 33 KPa and friction angle less than < 25 degrees). Stability assessment of all the four soil slopes was carried out using LEM; however, the detailed analysis was carried out for soil slopes S-3. Factor of safety (FoS) of 0.75 was obtained and reliability index RI (-ve) for soil slope S-3 without reinforcement while FOS got increased to 1.19 and RI (3.61) for reinforced soil condition with nail spacing between 2.5 to 2.75 m. A sensitivity analysis has been performed on geomechanical parameters like cohesion and angle of internal friction for determining the distribution of FoS.