Mechanism of top-coal cavability improvement by hydraulic fracturing preconditioning

Hydraulic-fracturing (HF) preconditioning is increasingly applied to improve cavability and alleviate strong weighting in hard, extra-thick longwall top-coal caving panels. However, its mechanism remains difficult to verify because fracturing, seepage, damage, and caving processes are strongly interdependent. This study investigates the mechanism of HF-enhanced top-coal cavability using an engineering-scale coupled seepage–mechanical discrete-element framework implemented in PFC2D. A case study was conducted for Panel 1803 of the Hetaoyu Coal Mine. Two scenarios were compared: baseline mining without HF and mining with HF preconditioning. HF was simulated using a domain–pipe pore-network approach in which pore-pressure was applied within the borehole-array region to drive effective-stress reduction and pore-pressure-induced bond breakage, producing a spatially nonuniform weakened zone before mining. The mining process was then simulated stepwise and evaluated using caving morphology, displacement development, force-chain reconfiguration, and global energy evolution. Results show that baseline mining maintains a persistent arching load-transfer structure that suppresses full top-coal caving and exhibits smooth, stepwise strain-energy accumulation with distributed dissipation. In contrast, HF preconditioning disrupts the force-chain backbone, promotes earlier rupture initiation, and triggers an event-driven caving regime characterized by serrated strain-energy evolution, concentrated dissipation growth, and pronounced kinetic-energy peaks at the transition interval (steps 4–5). The combined morphological and energetic evidence indicates that HF improves cavability primarily by weakening and reconfiguring dominant load-transfer pathways, suppressing excessive energy build-up, and enabling major caving to develop more readily during mining. The proposed modeling framework provides a practical, mechanism-oriented tool for interpreting HF effectiveness and supporting the rational design of preconditioning in hard, extra-thick top-coal conditions.