Energy Evolution of Anthracite Considering Anisotropy Under High Confining Pressure: An Experimental Investigation

The investigation of the energy evolution of coal under the coupling of the bedding plane and confining pressure is critical to engineering failure analysis. However, there are few anisotropic geo-mechanical studies on coal, especially in terms of the energy evolution anisotropy. To survey the effects of bedding plane and confining pressure on energy evolution, a suit of triaxial compression experiments up to high confining pressure (45 MPa) were conducted on anisotropic coal samples with five different bedding orientations. First, five peak energy parameters were calculated. Then, a function model among peak energy parameters, bedding plane and confining pressure was established. Furthermore, bedding plane and confining pressure effects in energy allocation and energy conversion rate were systematically discussed. Finally, the destructive potential index was established based on the energy conversion rate. The results show that peak energy parameters change as a trigonometric function with increasing bedding orientation. Peak total energy, peak elastic energy and peak dissipated energy grow as a quadratic function with increasing confining pressure. The energy evolution can be divided into three stages: the energy initial stage, energy hardening stage and energy softening stage based on the energy storage coefficient. High confining pressure shortens the energy hardening stage and prolongs the energy softening stage, and this conversion trend is independent of the bedding plane. Maximum energy storage rate changes as a trigonometric function with enhancing bedding orientation. The destructive potential index presents a decreasing trend with enhancing confining pressure, and this trend can be described by the exponential function. Energy storage coefficient and maximum energy storage rate can be used as the failure precursors.