Influence of primary interface characteristics on mechanical properties and damage evolution of coal-rock combination

Indirect fracturing technology of coal seam roofs is a crucial method to enhance coalbed methane extraction efficiency in broken soft coal seams. The cracks formed significantly increase permeability by extending from the rock-coal interface into the coal seam. The structural features of the coal-rock interface are pivotal in determining the path of crack propagation, highlighting the importance of studying the mechanical properties and damage evolution in coal-rock combinations. Industrial CT, three-dimensional (3D) reconstruction, and 3D printing technology are utilized to create physical models with authentic interface structure features. Subsequently, coalrock combination samples exhibiting smooth and original interface characteristics are produced through a layered pouring method. Uniaxial compression experiments are conducted on various interface feature combinations to examine the impact of primary interface structure characteristics on the mechanical properties and damage evolution of the combinations. Realtime monitoring is performed using acoustic emission (AE) equipment. The results show that the compressive strength of coal-rock combinations treated with glue and pouring is significantly higher than those treated with vaseline, while the axial strain shows an opposite trend. The presence of an interface constraint effect weakens the strength of the rock near the coal-rock interface, while enhancing the strength of the coal. The damage in coal-rock combinations initially occurs in the middle and lower parts of the coal, then progresses towards the interface and rock. Finally, a uniaxial compression model considering the interface constraint effect explains and theoretically demonstrates the phenomenon observed in the experiment, where more complex coal-rock interface structures result in higher compressive capacities of samples. It provides insights for further investigation into fracture trans-media expansion and permeability enhancement during the indirect fracturing of coal seam roofs.