Geomechanical properties and permeability of coals from the Foothills and Mountain regions of western Canada

Static and dynamic geomechanical properties and hydraulic permeability were determined for six large blocks of bituminous coal sampled from active mines in the Foothills and Mountain regions of western Canada. Testing showed a nonlinear shear strength envelope and brittle failure during triaxial loading. The dominant failure mode was along distinct shear planes. Effective compressive strengths increased from 8.6 MPa to 80.8 MPa with increasing confining pressure. The Mohr-Coulomb failure criterion showed that the coals had friction angle of 29.8° to 39.8° and cohesion ranging between 3.4 MPa and 8.0 MPa with increasing confining stress. The non-linear Hoek-Brown failure envelope was also fit to the data which provided a better estimation of the strength. Values for static Young's modulus ranged from 1119 MPa to 5070 MPa and Poisson's ratio ranged from 0.26 to 0.48, also varying with the confining stress. Concurrent ultrasonic measurements indicate that the values for dynamic moduli are consistently higher than those obtained from quasi-static measurements. Permeability of the coals tested at simulated in situ stress conditions and parallel to bedding surfaces was highly variable, ranging from 2.09 md in Seam 3, Greenhills Mine, to less than 0.001 md in Seam 10, Elkview Mine. This variation may be rationalized based on variations in maceral composition, stress history, degree of shearing, and the mode of deformation (brittle or ductile) of the coal seams in these tectonically complex regions.