From experimental modeling of shotcrete to numerical simulations of tunneling

Shotcrete shells constitute an essential part of modern tunnel construction according to the New Austrian Tunneling Method (NATM). Immediately after excavation of a tunnel segment, layers of shotcrete are applied, forming a supporting shell which is loaded already at a very early age of the material. In the past decades, huge effort was spent on the improvement of wet-mix shotcrete compositions, which nowadays are characterized by a fast setting and hydration process, ductile behavior in compression, and good adhesion on rock surfaces even for thick layers applied overhead—making shotcrete an ideal securing measure in tunneling. Shotcrete exhibits a highly nonlinear and time-dependent material behavior, in particular due to the hydration-dependent evolution of stiffness and strength. In the present review, an advanced constitutive model for shotcrete representing the time-dependent and nonlinear material behavior is discussed. It accounts for the time-dependent evolution of material properties due to hydration of shotcrete, hardening and softening material behavior, the latter resulting from cracking of shotcrete, based on the combination of the theories of plasticity and continuum damage mechanics, as well as nonlinear creep and shrinkage. Since softening material behavior results in pathological mesh dependence in finite element simulations, a sophisticated extension of the material model for obtaining mesh objective results is presented, based on the implicit gradient-enhanced continuum. For calibrating the material model, a comprehensive experimental program was conducted, for which specimens were sampled directly on site during the construction of the Brenner Base Tunnel. It focused on the evolution of stiffness and strength, the creep and shrinkage behavior, as well as the specific mode I fracture energy. Finally, the calibrated shotcrete model is applied in a finite element study of the construction of an emergency stretch of the Brenner Base Tunnel. For this specific stretch, a single, permanent single shotcrete lining is used throughout the complete structural life time, which motivates for a detailed analysis of the load-bearing capacity and the assessment of the degree of utilization.