Slope Stability Assessment of Half Tunnel near Thopan area on National Highway-05 in Kinnaur District, NW Himalaya, India using Empirical, Kinematic and Limit Equilibrium methods

Half tunnels occurring as ‘overhangs’ within steep slopes of massive and hard rock mass have advantages over full tunnels or open excavations as they are economical and take short time in construction. Because of their sporadic occurrence along NH-05 the stability analysis of half tunnels in these areas is undone and some are still unexplored. These half tunnels are excavated through a highly jointed/fractured rock slopes which may be the threat to people’s lives and can disrupt the transportation in any time if any reinforcement is not given. The detached rock blocks from these half tunnels and surrounding rocky slopes took many local peoples and tourists lives in the past. These half tunnels in the Himalayan regions have existed since many years despite any reinforcement given to them. The stability analysis of these fractured and jointed rock mass associated with half tunnels are needs to be carried out and requires proper remedial measures and reinforcements to avoid any mishap in future. Therefore, in this regard, the present study endeavors the slope stability assessment of one such half tunnel, a stretch of ca.1km located near Thopan on NH- 05 in Kinnaur district, Himachal Pradesh, India. Basic Rock Mass Rating (RMR basic) has been used to classify and evaluate rock mass exposed in this half tunnel. Total six slopes have been chosen for stability assessment. Rock mass classification done by Basic Rock Mass Rating (RMR_basic) categorizes all six rock slopes into class II (good rock).  Continuous Slope Mass Rating (CSMR) was used to evaluate the stability of these six slopes in which 5 slopes fall into the Class V category and one slope fall into Class III category. The kinematic analysis demonstrates that wedge failure is the most common and likely failure type amongst the three failures (wedge, planar, and toppling) in the jointed rock slopes of half tunnel. The Factor of Safety (FoS) was also calculated for all the six slopes having the lowest CSMR values in the wedge failure case. All these 6 slopes are unstable showing FoS values less than one.

 Keywords: Slope Stability, NW Himalaya, Half Tunnels, Rock slopes, RMR, CSMR, Kinematic Analysis, FoS, Swedge model.