Assessment of the Safety Factor Evolution of the Shotcrete Lining for Different Curing Ages

The behavior of the shotcrete linings during the tunnel construction is complex due to the variability of its mechanical characteristics during the curing time. After the installation of the support structure, the lining is loaded along with the excavation face. A new calculation procedure involving two analytical methods, i.e. the convergence-confinement method and hyperstatic reaction method, have been developed. By means of these two methods, it is possible to assess the evolution of the stress state in the lining, and therefore, also of the safety factor with respect to the failure in compression of the shotcrete. Due to the analysis of the safety factor evolution over time, it is possible to correctly design the lining, to choose the type of sprayed concrete and to define the maximum admissible advance rate of the excavation face, in order not to critically load the lining. In the following paper, after having shown the definition of the safety factor, a parametric analysis is performed, in order to investigate the evolution of the safety factor of the lining for two different rock types, three different shotcrete types and two tunnel advance rates have been considered.