Fundamental Study on Instability of Tunnel in Consideration of Post-Peak Strain Softening Behavior of Rock

Abstract

The main concerns of the present study are, firstly, to formulate the finite element analysis based on the principle of the “stress transfer method”, by which the influnce of the post-peak strain softening behavior of rock, which is one of the intrinsic natures of rock like material, can be appropriately investigated with regard to instability of an underground cavity excavated in rock. Since very little information is available in rock mechanics literature on the post-peak strain softening behav ior, and since no versatile constitutive law is as yet established to be readily employed in a numerical analysis, the analysis is presently conducted by assuming the idealized stress-strain relation and the modified Mohr-Coulomb failure criterion for practical usage. Secondly, the instability of a tunnel possibly caused by the excavation is studied. The analysis reveals a new finding with regard to the mechanism of the instability of a tunnel, such as the falling-off of the tunnel roof and rock burst, which has not been completely manifested so far. As would be anticipated, the shear failure might possibly be developed in a region where the stress concentration arises in relation to the initial stress system in the rock mass and the failure strength of rock. When the strain softening behavior is taken into consideration, the “most critical failure”, (by which the rock loses completely the resitance due to the simultaneous emergence of the mode of shear failure as well as tensile failure), may be subsequently caused as a result of the stress release and the transfer from the previously failed element, the failure zone of which is formed as a cone in shape. This analytical solution is reasonably consistent with the field observation of the instability of a tunnel which takes place under a special stress condition, for instance, a region where a very high horizontal stress remains in the rock formations, as in Ontario, Canada. This is also true in a case where a tunnel is excavated in mountains where a very high overburden pressure might be acting..