By coring and sampling coal from the roadway roof at different depths, 0.7 m, 2.5 m, and 4.4 m, the strength parameters and mode I fracture characteristics of the coal were tested. The results show that with increasing borehole depth, the compressive strength and elastic modulus of coal increase by 79.25% and 112.26% at 4.4 m compared with those at 0.7 m, respectively. In contrast, the tensile strength and Poisson’s ratio decrease by 45.40% and 28.57%, respectively. The mode I fracture toughness K IC decreases from 0.2448 MPa·m. 5 to 0.1437 MPa·m. 5 . K IC is positively correlated with tensile strength (R 2 = 0.9984) and Poisson’s ratio (R 2 = 0.9566), and shows a strong negative correlation with the brittleness index (R 2 = 0.8916). A method for accurately determining the critical crack tip opening displacement W 0 is defined. The measured W 0 values of coal at depths of 0.7 m, 2.5 m, and 4.4 m are 0.0148 mm, 0.0135 mm, and 0.0114 mm, respectively. For shallow coal (0.7 m), the load at the crack initiation point is only 64.05% of the peak load. The length of the FPZ increases from 5.21 mm at crack initiation to 5.73 mm at the peak load. And the actual crack propagation length accounts for 43.78% of the distance from the crack tip to the specimen tip. It exhibited clear plastic fracture characteristics and a nonlinear energy-dissipation process. For deep coal (4.4 m), the load at the crack initiation point reaches 98.07% of the peak load, with almost no stable propagation stage, displaying typical brittle fracture. This study reveals the key laws that the real crack initiation point of quasi-brittle coal occurs before the peak load and that linear elastic fracture theory does not apply to highly plastic coal. It provided an important theoretical basis for analyzing the fracture behavior of coal at different positions of the roadway surrounding rock and for controlling the surrounding rock stability.
Ma et al. (Mon,) studied this question.