Ground-shotcrete interaction of NATM tunnels with high overburden

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., in 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 so-called intrinsic material functions. For the description of the mechanical behavior of the ground, a multi-surface plasticity model is employed. The investigation of the ground-shotcrete interaction in tunneling is based on an extensive parametric analyses accounting for each phenomenon in the material model for shotcrete and different mechanical properties of the rock-mass (i.e., stiffness, strength, dilatancy, time-dependent behavior). Moreover, the influence of the constructive schedule of the tunnel is also investigated by adopting different driving speeds, unsupported lengths, and time spans for excavation and shotcreting in the numerical analyses.