A regularized fracture-based continuum model for simulating tunneling-induced rock mass collapse

Tunneling in brittle rock masses under high stress may be accompanied by fracture-induced collapses, that is a complete loss of the structural load carrying capacity. Predicting the location and extent of the collapses is crucial for ensuring construction safety and designing support systems. This study proposes a novel fracture-mechanicsbased continuum model for simulating excavation-induced collapses in highly stressed rock masses. It captures the excavation damaged zone caused by microcrack initiation as well as the transition to a highly damaged zone formed by macroscopic fracture propagation. The modeling is unique in that the simulated cracks are regularized to alleviate the mesh dependency. For large-scale problems the mesh size selection depends only on the characteristic length l0 of the smeared fracture band. It is shown that the correct scaling of l0 in tunnel-scale problems enables relatively coarse meshes to be adopted, significantly reducing computational time. Additionally, a novel method for introducing structural discontinuities into an excavated rock mass is presented, enabling the simulation of interactions between pre-existing geological discontinuities and excavation-induced fractures. Two tunnel excavation case studies are presented, demonstrating the model's capability of simulating fracture propagation, final collapse zones and corresponding rock mass deformation induced by excavation.