Entry-scale investigation on shale roof model calibration considering anisotropic brittle failure in U.S. coal seams

Abstract

In U.S. underground coal mines, immediate/main roofs are usually composed of laminated shale, which exhibits strong anisotropic brittle failure behavior due to the presence of bedding planes. Existing numerical models often fail to accurately capture this behavior, as they neglect the dependence of strength and elastic properties on the orientation of bedding planes and lack comprehensive calibration against underground measurements. To address these limitations, this study investigates five key U.S. coal seams—Lower Kittanning, Pittsburgh, Pocahontas No. 3, Blue Creek, and Sunnyside—by developing and applying an anisotropic brittle failure criterion within a FLAC3D entry-scale model. The model explicitly accounts for strength and Young’s modulus anisotropy, cohesion-weakening friction-strengthening (CWFS) behavior, and dilatational response based on plastic shear strain. A systematic calibration procedure is proposed to ensure realistic representation of field conditions: (1) Calibration of vertical and horizontal stresses; (2) Roof sag and cable loads calibration; and (3) Verification of anisotropic brittle failure characteristics. The model simulates vertical stresses from 5 MPa to 48 MPa and horizontal stresses from 6 MPa to 42 MPa, covering a broad range of geological conditions across U.S. coal seams. The calibrated model provides a more accurate representation of stress distribution, roof sag, cable loads, and failure characteristics in shale roofs compared to conventional approaches. Furthermore, the simulation results presented anisotropic brittle failure characteristics under four types of mining geological conditions: (1) Highly laminated shale roofs. (2) High horizontal stress conditions. (3) Deep mines with a three-pillar system. (4) Deep mines with a one-pillar system. This research enhances the understanding of anisotropic brittle failure characteristics in laminated shale roofs at an entry-scale. An entry-scale FLAC3D model incorporating strength and Young’s modulus anisotropy, CWFS behavior, and dilatational response is developed and calibrated across five U.S. coal seams. The model simulates vertical stresses from 5 MPa to 48 MPa and horizontal stresses from 6 MPa to 42 MPa, covering a broad range of geological conditions. Anisotropic brittle failure characteristics are analyzed across four types of mining geological conditions: laminated shale roofs, high horizontal stress, deep mines with three-pillar, and one-pillar systems. An entry-scale FLAC3D model incorporating strength and Young’s modulus anisotropy, CWFS behavior, and dilatational response is developed and calibrated across five U.S. coal seams. The model simulates vertical stresses from 5 MPa to 48 MPa and horizontal stresses from 6 MPa to 42 MPa, covering a broad range of geological conditions. Anisotropic brittle failure characteristics are analyzed across four types of mining geological conditions: laminated shale roofs, high horizontal stress, deep mines with three-pillar, and one-pillar systems.