Modeling of spalling failure of in-situ rock mass for quantification of the source mechanics with flat-joint model and moment tensor

Abstract

Spalling failure caused by excavation unloading in tunnels under high in situ stress is a common phenomenon. Here, we attempt to use a three-dimensional flat-joint model (FJM3D) combined with a moment tensor to simulate the process of spalling failure. FJM3D simulation showing the process of micro-crack initiation and propagation in the surrounding rock from a micro-perspective. The nature of the source mechanism is quantitatively assessed by moment tensor analysis. There are a large number of micro-cracks extending deep at the shoulder and foot of the tunnel. This model reproduces similar thin slabs and fracture orientations in Mine-by Experiment (MBE). The excavation disturbed zone (EDZ) characteristics around the MBE show that the tunnel crown and invert are dominated by tensile mechanisms, the tunnel sidewalls are dominated by implosive and shear mechanisms, and the tunnel shoulders are butterfly shaped with tensile and implosive sources. The nature of these sources are not purely tensile/shear cracks, but some isotropic/deviatoric components also play a role. The magnitude and orientation of the principal stress near the excavation face are linked to the advancement of the tunnel face and are related to the direction of micro-cracks initiation and propagation, which sheds new light on the failure mechanics of spalling failure.