Effects of Anisotropy on Strength and Deformability of Anthracite Under High Confinement.

Coal strength and deformability anisotropy are one of the important contents of deep rock mechanics, as well as a crucial aspect influencing the secure and efficient exploitation of deep coalbed methane. However, there are few anisotropic studies on coal. To survey the effects of anisotropy and confinement on strength and deformability of anthracite, a suit of triaxial compression tests up to high confinement were completed on anthracite samples at four confining pressures and five bedding orientations. Then, strength and deformability anisotropy were analyzed. Furthermore, anisotropic failure criteria for the prediction of the strength were evaluated. Finally, the influences of anisotropy and confining pressure on mechanical parameters and failure patterns were systematically discussed. The results show that the strengths of loaded vertically to bedding plane above parallel to bedding plane can be explained by the fracture of different bonds. The Saeidi and modified Hoek–Brown criteria can reflect anisotropy strength better. Empirical linear relationships between the peak axial strain and confining pressure under different bedding orientations are proposed. Young′s modulus of anthracite is independent of bedding orientation under high and ultrahigh stress and remarkably influenced by bedding orientation under low and moderate stress. Furthermore, Young′s modulus with increasing confinement can be distinguished into three stages at all bedding orientations and becomes approximately constant above high pressure except 30°. The fracture pattern is independent of confining pressure and controlled by the shear failure mechanism in the coal matrix at 30°, and is dominated by the bedding plane under arbitrary confining pressure at 45° and 60°. The stress level of 15 MPa is the transition stress value at 90° from tensile fracture to shear fracture. Article Highlights: The influences of bedding orientation and confinement on strength and deformability anisotropy of anthracite under high confining pressure are explored. The reason for the higher strength of loaded vertically to bedding than parallel to bedding is explained from a microscopic perspective. Young′s modulus of anthracite with increasing confinement can be distinguished into three stages and becomes approximately constant above 30 MPa except at the bedding orientation of 30°. Rather than an instantaneous tensile-shear fracture transition, 15 MPa is the transition stress value at the bedding orientation of 90° from tensile fracture to shear fracture. [ABSTRACT FROM AUTHOR]