On the stability of tunnels under gravity loading, with post-peak softening of the ground

The effect of gravitational loads on the stability of horizontal circular tunnels excavated in an elasto-plastic (softening) materials is analyzed. Gravity loading in included in the analysis as body forces added to those due to the in situ stresses. The analysis considers the occurrence of a minimum in the ground reaction curve, i.e. the graphical representation of the radial support pressure required for equilibrium, of the excavation as a function of radial deformation at the excavation wall. The analysis is also extended to the face of the tunnel, where the three-dimensional is treated analytically under restrictive assumptions. Results obtained from the numerical analysis are presented; they confirm the results from the limit-depth analysis.