Mesostructural Evolution of Coal under Alternating Hydraulic Intrusion Based on Micro-CT Imaging

To investigate the mesostructural evolution of coal under alternating hydraulic intrusion, a multifield coupled seepage device was used to sequentially conduct alternating hydraulic intrusion test and stress loading failure test on the same coal sample. Micro–computed-tomography (CT) scanning was performed to obtain digital images before and after the tests. Three-dimensional models were reconstructed, and the pore-fracture structures were quantitatively characterized, thereby revealing the development and evolution mechanisms of these structures. The results indicate that the alternating hydraulic intrusion test caused certain damage to the coal–rock mass without inducing irreversible deformation. It promoted the development of pore-fracture structures, but the formed pores were mostly isolated with poor connectivity, and the influence diminished with increasing distance from the water injection end. The high hardness of minerals in the coal sample hindered water intrusion. However, under stress loading, internal fractures preferentially initiated and propagated along the weak interfaces between minerals and the coal matrix. The coal sample subjected to alternating hydraulic intrusion was more prone to cracking and fragmentation under stress loading, forming a complex pore-fracture network that played a dominant role in increasing the porosity and fractures of the coal–rock mass.