STABILITY ANALYSIS OF UNSUPPORTED TUNNELS IN WEAK ROCK MASSES

Abstract

Tunnel design in weak rock mass has been a challenging task for engineer‘s worldwide. Due to the change in geology and type of structure of rock from place to place, it seems rather difficult to reach on a set of defined rules to design tunnels in these conditions. Many times it is impossible to predict the stand-up times of tunnels and in many cases various unsupported tunnels have been found to cave in immediately after excavation. This is dangerous for the manpower and the timeline of the project. Though a lot of work has been done to use rock classification as a system for tunnel design but still large gaps are found. This study deals with finding the stability of tunnels in weak rock masses using finite element method and lower bound approach of analysis. The stability of tunnel is evaluated in terms of a dimensionless number defined as stability factor (SF). The variation of stability factor with the changes in the ratio of tunnel cover to diameter of the tunnel, GSI and mi of rock masses and disturbance factor during excavation has been established for both static and seismic loadings.