Background The method of supporting roadways using small coal pillars has been widely applied in China. As a result, the stability of goaf-side coal pillars affects mining safety. Resulting from multiple disturbance effects, coal pillars tend to contain various types of damage fractures. The confined water environment in goaves with water accumulation might cause the instability failure of damaged goaf-side coal pillars.

Methods To investigate the impacts of the damage fracture structure on coal stability, this study introduced the concept of relatively stable initial damage fractures. Based on the theory of damage mechanics, this study constructed hydro-mechanical-damage (HMD) coupling models for goaf-side coal pillars. Using the COMSOL finite element analysis software, this study explored the seepage failure process of goaf-side coal pillars under unilateral water pressure.

Results and Conclusions The results indicate that the mechanical parameters of initial damage fractures significantly influenced the occurrence time of secondary damage to coals, with critical thresholds existing between them. Damage fractures significantly affected the stability of the goaf-side coal pillars. Specifically, under unilateral water pressure, the goaf-side coal pillars exhibited asymmetric failure morphologies, and the seepage velocities at local water-inrush points increased by two orders of magnitude compared to pre-damage states. The fracture structure affected the instability process of coals. As the fracture angle increased from 30° to 60°, the occurrence time of coal instability decreased from 7.0 h to 4.1 h. The model with 0° fractures experienced instability the most quickly (3.9 h) due to the special stress orientation. From the perspective of macroscopic damage development, all models underwent four damage stages: (1) the secondary damage development at tips of initial damage fractures, (2) the damage penetration through boundaries, (3) the large-scale propagation of damage zones, and (4) the formation of pathways for local water inrushes at boundaries. In contrast, the evolutionary characteristics of the quantity of damage units reveal two damage stages: damage development and rapid propagation. The fracture structure affected the damage development rate primarily during the damage development stage, while the duration of the rapid propagation stage differed insignificantly across varying models. The results of this study provide a reference for the stability assessment and instability process identification of water-resisting coals in goaves.