Mechanical properties and failure mechanism of cemented broken coal under triaxial confined unloading: Insight of experiment and DEM modelling

Grouting is widely used to reinforce fractured coal pillars in deep coal mines. However, cemented broken coal (CBC) formed from grout-coal mixtures is often subjected to mining-induced unloading disturbances. In this study, a series of triaxial unloading tests were conducted on CBC specimens with different coal-to-grout (C/G) ratios to investigate their mechanical responses and damage evolution under unloading conditions. The results show that CBC specimens exhibit brittle failure and lateral dilatancy along the unloading path. Increasing unloading confining pressure enhances the energy storage capacity and aggravates internal damage accumulation. As the C/G ratio increases, internal defects become more numerous, strength and elastic modulus decrease, and the damage mode shifts from concentrated abrupt failure to dispersed progressive damage. On this basis, a damage model incorporating both unloading damage and coal content was established, which reasonably captures stiffness degradation and post-peak softening during the unloading process. In addition, a simulation framework integrating the proposed damage model parameters with the discrete element method (DEM) was developed. The simulation results indicate that, as the C/G ratio increases, the load-bearing skeleton transitions from a continuous grout phase to a weak coal-dominated skeleton, accompanied by enhanced interfacial tensile debonding and shear sliding and more dispersed energy dissipation. Unloading triggers force-chain reorganization and aggravates stress concentrations at crack tips. These findings provide a mechanical basis and microscale interpretation for stability evaluation and instability-risk control of grouted cemented bodies in deep, complex stress environments.