ABSTRACT
This paper addresses first of all the stability of a fully unlined circular tunnel. Attention is focused on a homogenous Mohr-Coulomb material under drained conditions. The unlined tunnel is assumed to be supported by an internal pressure and a formula for the collapse pressure is derived. The soil friction angle is shown to be extremely important, as otherwise there is very little arching. For highly frictional material, an arching ground ring is shown to develop around the tunnel. This confirms common ideas on tunnelling, but the novel finding is the extreme influence of the friction angle as well as the role of the cohesive strength. Comparison is made with data for lined tunnels as published in a separate paper. Finally attention is focused on the accuracy of FEM for limit loads. To this end upper and lower bound solutions by other authors are considered. These data testify directly the accuracy of present solutions.
