Micro seepage characteristics and macro failure of coal under different hydraulic loading paths

To study the micro seepage characteristics of coal under different hydraulic load paths, a two-media seepage model based on the microstructure of coal is established through CT three-dimensional reconstruction technology. Three equivalent hydraulic loading paths are designed, i.e., constant velocity loading (LP1), conventional pulsating loading (LP2), and fast-loading and slow-unloading loading (LP3). The numerical simulation tests of water injection seepage of coal under different cyclic load paths are carried out. In addition, the self-built water injection device of coal is used to explore the macro damage under three loading paths. The results show that the connected fractures account for 73.49% of the pore-fracture structure and are the main factor influencing coal seepage. The pores with a diameter of 9-23 mu m are the major part of the isolated pores, with the number and volume share exceeding 50%. The pore-fracture structure and loading path within the coal has an important influence on the seepage distribution characteristics of velocities and pressures. The average seepage velocity of LP1 fluctuates greatly along the intrusion direction, while the inhibition effect of LP3 is obvious, and the path of LP3 has a larger span over time than those of LP1 and LP2. Compared with LP1, the loading path of LP3 has the fatigue damage effect of pulsating load, and the damage degree is more severe than that of LP2. This research can provide directions for the study of coal microstructure and the optimization of technical parameters of coal seam water intrusion.