Dilatancy and compaction characteristics of deep high-stressed coal under multiple disturbance behaviors

The mechanical and deformation behavior of deep high-stressed coal seams under ambient disturbance is different from that of shallow coal seams. Based on this, the effects of various disturbance schemes i.e., high-pressurized water injection, depressurized borehole and cyclic loading-unloading, on failure mechanical behavior of gas-bearing coal under high confining pressure constraint are investigated experimentally. The results show that high-stressed coal tends to ductile state transformation without significant post-peak stress drop. Deep high-stressed coal dominated by shear and tensile fracture exhibited post-peak linear dilatancy behavior. Compared with the pure mechanical failure, the strength of coal with horizontal borehole and pressurized water is decreased by 29.1% and 4.1%, respectively, and their degradation mechanisms for coal strength and mechanical properties were different. The former is induced by the continuous increase of vertical deformation, resulting in borehole collapse and compaction zone formation. The latter is induced by pressurized water counteracting the normal stress of fracture surface, which promotes the rapid expansion of coal in horizontal direction, resulting in its dilatancy instability and subsequent shear failure. The dilatancy coefficient β=−1.524<0 at peak stress of coal within horizontal borehole also indicates that it is in a compacted state. The single yield surface model is used to prove the rationality of the post-peak compaction behavior of high-stressed coal within horizontal borehole. The loading-unloading disturbance at constant amplitude plays a cyclic reinforcement effect on the mechanical properties of elastic high-stressed coal, and its strength is increased by 18.0%−27.0% with significant post-peak dilatancy. Due to the greater influence of unloading on coal deformation, the irreversible compaction strain (Δε_H, Δε_v and Δε_h) increases with the increasing loading times, and manifests as strain hardening with the compression trend in all directions, which is contrary to the fatigue damage induced by cyclic loading-unloading on rock. The research results can provide reference for major engineering issues such as deep high-energy coal seam risk solving, three-dimensional fracture network reconstruction and seepage assessment.