Influence of face reinforcement and shotcrete support on static conditions of deep tunnels: a thermo-chemo-mechanical study

In the last years, the combination of shotcrete as primary lining and face reinforcement by means of fiber-glass dowels was successfully employed during tunneling in squeezing conditions (e.g., in soft rocks under high overburden stress). In this paper, numerical results obtained from axisymmetric analyses considering the application of the shotcrete lining and the installation of fiber-glass dowels at the tunnel face during the tunnel excavation are presented. The geometric and mechanical characteristics of the support means as well as the time schedule of excavation, shotcreting, and reinforcement stages were selected accordingly to the design and performance of a recently-built Italian tunnel (the Raticosa tunnel). The numerical study focused on the investigation of the behavior of deep tunnels excavated in moderate- to high- squeezing conditions. Accordingly, a viscoplastic material model was chosen for the description of the mechanical behavior of the ground. The behavior of shotcrete was described in the framework of thermo-chemo-mechanics. The obtained results provide insight into the ground-shotcrete interaction, the effect of face reinforcement by means of fiber-glass dowels, and the state of stress in the shotcrete lining. Moreover, the variation of ground properties and the reinforcement density made it possible to evaluate the loading and effectiveness of the employed support means for different geotechnical conditions.