Progressive Failure Mechanism and Control of Natural Pillar Groups Using Yielding Expandable Props in Continuous Mining

To address the challenges of high ore loss and brittle splitting of natural pillars in gently dipping and horizontal thin-vein extraction using room-and-pillar mining, a continuous, pillar-free technology that replaces natural pillars with expandable prop group, designated the room and expandable prop mining method, is investigated. In this study, the ground pressure control mechanism exerted by the expandable prop group on the stope roof is theoretically framed. Employing Universal Distinct Element Code (UDEC) numerical modeling in tandem with on-site trials, the deformation, load-bearing response, and ultimate failure of both natural pillars and expandable props, together with the progressive fracturing of the stope roof, during the extraction of gently dipping and horizontal thin veins are monitored. The novelty of this study lies in establishing a unified roof–support characteristic-curve framework for expandable prop groups, which links roof deformation, support resistance, and progressive fracture evolution and enables a direct mechanistic comparison between natural pillar groups and yielding expandable prop groups. Based on the evolution and intersections of the roof and support characteristic curves, the transition from load concentration and progressive failure of natural pillar groups to deformation accommodation and damage redistribution under expandable prop support is quantitatively characterized. The results demonstrate that expandable props permit controlled roof deformation while maintaining stable load-bearing capacity. In contrast to natural pillars, for which progressive damage develops into through-going fractures and domino-type failure, expandable prop groups redistribute roof damage toward the deeper roof and abutments, interrupt fracture coalescence, and effectively suppress progressive collapse. The revealed yielding-support mechanism provides a theoretical basis for expandable prop-group design and the implementation of continuous pillar-free mining.