Analysis of non-linear strain-softening behaviour around tunnels

This paper studies the influence of non-linear stress–strain of post-failure on the ground response curve of a circular tunnel excavated in rock mass. Thus, the influence of variations of both the quality of rock mass and the confining stress around the tunnel due to the advancement of the tunnel face are addressed on the value of drop modulus. A new procedure is presented to obtain the softening parameter and its critical value, and the accuracy and practical application of the proposed procedure are shown through examples. The effects of variable drop modulus and the value of the critical softening parameter on the ground response curve are investigated. The results show that, to obtain a ground response curve, the variable drop modulus (non-linear behaviour) in the post-failure region can be approximated by a constant drop modulus (linear behaviour post-failure) if it is calculated by using both quality of rock mass before failure and the average value of confinement stress within the plastic region.