Stability Assessment and Support Design of a Tunnel Excavation in Different Rock Mass Conditions

Presently, India is at the cusp of tunneling revolution as it can be witnessed through several ongoing and upcoming tunneling projects, especially in the Himalayan region. It is a critical phase of any project to carry out stability assessment of a tunnel excavation prior to the actual excavation work considering different rock mass conditions pre-existing in hilly terrains. For this, a detailed numerical analysis has been performed based on the often observed common features of tunneling projects previously and presently being carried out in the Himalayas. The purpose of this study is to emphasize the need of a parametric analysis for tunnel excavations and their support design in phyllite rock mass which is commonly found in the North-Western Himalayas having three different geological surface conditions based on the Geological Strength Index (GSI) values. A D-shaped tunnel with two different overburden heights has been modeled and analyzed using the two-dimensional finite-element method. The deformation at the tunnel opening and global factor of safety values have been computed. Further, the most vulnerable cases have been re-analyzed with a bolting and shotcrete support scheme to check the enhancement in its stability as well as to restrict the crown displacement within the permissible limits.