Elucidation of ground-lining interaction in NATM tunneling accounting for chemo-mechanical couplings in shotcrete behavior

According to the New Austrian Tunneling Method (NATM), after excavation of a cross-section of a tunnel, shotcrete is applied onto the tunnel walls, constituting a thin, flexible, and closed shell. The NATM is characterized by a strong interaction of the hardening shotcrete shell and the creeping ground. In this paper, this interaction is investigated by means of axisymmetric analyses. The assumption of axisymmetry with respect to the tunnel axis provides a good approximation for the conditions met in deep tunnels, i.e., tunnels characterized by high overburden. Moreover, it accounts for the three-dimensional nature of the excavation process. So far, axisymmetric analyses reported in the open literature are restricted to the assumption of elastic material behavior of shotcrete. The increase of stiffness during hydration is, if at all, controlled by means of empirical material functions. In this paper, the mechanical behavior of shotcrete is modeled in the framework of thermochemomechanics. The effect of the hydration of shotcrete on strength, stiffness, and chemical shrinkage is considered. Moreover, microcracking as well as creep are accounted for. All material parameters are related to the degree of hydration by the so-called intrinsic material functions. For the description of the mechanical behavior of the ground, a viscoplasticity material model is employed. The performed simulations of the excavation process focus on the significance of each phenomenon accounted for in the material model for shotcrete. Moreover, the influence of the driving speed and the creep behavior of the ground on the ground-shotcrete compound structure is investigated.