Simplified behavior models of tunnel faces supported by shotcrete and bolts.

Ground-support interaction in the vicinity of a tunnel face is a typical three-dimensional problem, nevertheless the tunnel support design is usually based on simplified plane strain analyses. The results of such analyses strongly depend on the choice of ground stress release at the time of lining installation. The paper focuses on tunnels excavated in difficult ground conditions, where the application of shotcrete and face reinforcement by fiber-glass bolts is widely used as a primary measure to reduce ground deformation and ensure safe working conditions. The results of an exhaustive parametric study based on 3D axisymmetric models are presented, taking into account the effect of shotcrete hardening and density of reinforcement. An Italian case history of a tunnel driven in difficult conditions is also reviewed. Finally on the basis of the results, a strategy is proposed in order to enhance the capability of two-dimensional analyses to give a more realistic prediction of stress conditions in lining and reinforcement systems.