STRESS ANALYSIS AROUND A TUNNEL IN SEMI-INFINITE ROCK MASS

Abstract

Tunnels permit and ensure direct transportation of men and goods through certain obstacles like a mountain, a body of water, dense urban and industrial areas, traffic etc. Thus tunnels may pass under mountains, rivers, straits, denserban industrial areas, buildings and traffic routes, their purpose may be to carry railway, road, pedestrian traffic, to convey water, power, gas, sewage etc., or to provide access to underground plants. When designing an underground structure in- rock or evaluating the stability of an existing structure, we must determine (1) the stresses and/or deformation..in the structure resulting from external or body loads, and (2) the ability of the structure to withstand these stresses or deformations. By excavating the tunnel the stress at every point on the walls of the tunnel are reduced to zero from its initial value i.e. the stress under -Geostatic condition. Due to the excavation of tunnel the stresses are induced in the rock mass surrounding the tunnel. In this study, the concept of equivalent load has been used to determine the stresses around the tunnel by applying equivalent forces on the faces of the tunnel, so that the forces acting due to the weight of the rock mass under geostatic condition is counter balanced by the combined effect of the applied equivalent forces in the semi-infinite weightless elastic mass. The stresses developed at some distance away from the surface of the tunnel in response to the applied equivalent forces will be approximately equal to the stresses that would develop in the rock due to the excavation of the tunnel (as per the Saint Venant's principle). Using the Mohr's failure criteria, the failure condition has been examined in the material surrounding the tunnel so as to evaluate the reinforcement requirement.