The tunnel at depth is stressed by the higher stress levels induced by excavating and is easily failed. Based on the deformation and failure behaviours of rock, it is investigated that the progressive failure of the rock mass around a tunnel at depth. And the analytic formulas are given to evaluate the effects of the softening and dilation of the rock mass on the stability of underground tunnel. Some suggestions are also given to improve the support of underground tunnel at great depth.
The stability of tunnel depends more on the response of the rock mass to the higher stress levels induced by excavating or mining activities. When a tunnel is made at great depth, the redistribution of stress usually results in stress levels high enough to cause progressive failure of the rock mass around tunnel, that is, the stability of tunnel will be lost slowly following excavating. Previous in-situ investigations of the tunnels at great depth have shown the existence and extent of the broken zone around such tunnel. It is necessary to take consideration the progressive failure behavior of rock mass in the design and construction of the tunnel at depth. Unfortunately, only a few of people take this idea for underground tunnel. The design and construction of underground tunnel are according to human experiences or classical theoretical results. An important feature in the design of underground tunnel is to utilise the rock itself as the principal structural support material by minimizing the disturbance of the rock mass during the excavation process and thus employing the minimum amount of artificial support such as steel arches, rings, concrete lining and shotcrete bolts. This is largely dependent upon geological conditions and the rock mass properties encounted in the tunnel. The lack of knowledge concerning the fundamental of rock mass failure has led to the development of numerous emperical and theoretical design formulae (Yuan &Chen 1987, 1988, Wilson 1980, Brown 1983 and Dritz 1984). Such design criteria are theoretical or conceptional and although they have proved to provide successful guidelines in particular circumstances, they currently lack precison and consistency in the variable rock mass and stress enviromments. A clear understanding of the behaviour of broken rock as well as the induced stress enviromment is necessary yet to further advance the effectiveness of such formulae for predictive purpose. The development of design method for underground tunnel support must include the criteria which consider the strength and deformation characteristics of rock mass. Knowledge of the complete progressive failure behaviors of rock mass around tunnel is therefore a prerequisite to the effective design of tunnel at increased depth, particularly in weak ground. A elastic-plastic medium has been made to model the progressive failure of rock mass. The strain softening and linized dilation model arc used to analyse the stress in rock mass around tunnel. An analytic formula is developed to evaluate the effects softening and dilation of the rock mass on the stability of underground tunnel.
