Mechanism of parallel fractures around deep underground excavations

Abstract

The typical parallel fractures of spalling or slabbing is often encountered in deep underground excavation. In recent years, another type of parallel fractures in large scale called zonal disintegration is captured by borehole camera in deep surrounding rock mass. The mechanism of these two types of failure and their correlations are discussed through 3D numerical testing. Results show that the biaxial stress parallel to the free surface confined by zero or low confinement stress contributes to the parallel fracturing of spalling or slabbing. Zonal disintegration can be motivated from spalling or slabbing failure to deeper area under high multi-axial stress. Parallel fracturing can be taken as an inherent character of rock like material of heterogeneity under high biaxial or multi-axial stress conditions. Failure patterns of surrounding rock mass are influenced by the multi-axial stress state. A larger horizontal stress along tunnel axis contributes to more intact fracture rings of zonal disintegration. The spacing and number of parallel fractures of zonal disintegration are directly affected by the curvature of the free surface.