Ground Reaction Curve for Deep Circular Tunnels in Strain-Softening Mohr-Coulomb Rock Masses Considering the Damaged Zone

The drill and blast method is one of the most widely used techniques for excavation of tunnels in rock masses. The most worrying issue in this method is unfavorable damage induced by blast impact into the surrounding rock mass. The blast-induced damaged zone has a possible influence on the tunnel stability. Therefore, the damage in the rock mass should be minimized as much as possible, or if it is inevitable, the design of the tunnel should be performed based on an accurate analysis considering the damaged zone. In this paper, a theoretical and practical method is presented for the ground reaction curve of a circular tunnel excavated in an elastic strain-softening Mohr-Coulomb rock mass considering the blast-induced damaged zone. A cylindrical damaged zone with decreased strength and deformability parameters is taken into account. The results for a typical tunnel obtained using the proposed solution are compared with other well-known analytical methods. It is observed that the proposed approximate solution converges to the exact solution if the stress increments are sufficiently small. The influence of the strain-softening behavior on the results is examined through the establishment of stresses and displacement distributions and ground reaction curves and the discussion for the extent of the plastic zone. The results show that the influence of the damaged zone with reduced properties on the tunnel convergence is significant.