Investigation of entry-scale model calibration considering shale roof anisotropic brittle failure through FLAC3D models

ABSTRACT: In the Appalachian coal region, coal mine roofs are predominantly composed of laminated shale. Stress-driven instability in these roofs leads to roof falls, presenting significant safety hazards. Common failure mechanisms include spalling and cutter formation, which are linked to the anisotropic brittle nature of shale. This study investigates the calibration of an entry-scale model based on underground measurements, including stress paths from hollow inclusion (HI) cells, roof sagging, and cable loads. The anisotropic properties of shale are incorporated into the model to capture the σ1‚ direction-dependent elastic and failure behaviors. An anisotropic brittle failure criterion is developed and applied within the FLAC3D models of a coal mine entry, accounting for the elastic and strength anisotropy of shale due to bedding planes, brittle failure under low confinement, and shear failure under increased confinement. The model also considers the relationship between principal stresses and the progression of brittle and shear failures. This study aims to simulate the complex mechanical behavior of shale roofs, which exhibit anisotropy due to bedding planes. The simulation results will highlight the impact of anisotropic brittle failure on roof stability.