Multi-method investigation of overlying strata stability and hazard mitigation in ultra-shallow-buried isolated mining faces with burned-out zones

This study addressed complex engineering challenges of the 30,101 isolated island working face in Hejiata Coal Mine, Shaanxi Province, characterized by three interrelated geological-mining conditions: ultra-shallow burial (78 m), well-developed surface water (dominated by the Wulanmulun River), and porous thermally altered rocks from adjacent coal spontaneous combustion. Contrary to conventional assumptions, the 78 m shallow 3-1 coal seam had high burst risk, confirmed by uniaxial compression tests showing high elastic modulus and abrupt post-peak brittle failure. To evaluate its stability and hazards, a multi-method approach was used: FLAC3D for stress/displacement/plastic zone simulation, UDEC for discontinuous fracture analysis, and field bolt load monitoring for real-time data. Results showed overlying strata formed a typical “S”-shaped structure during mining, with stress concentration factor rising from 1.63 (20 m advance) to a stable ∼3.02 after 180 m. UDEC simulations revealed water-conducting fractures connected surface water, burned-out zone pores and the mine, posing water inrush risk. The 15 m coal pillar retained a ∼3 m elastic core at 220 m advance. Field monitoring showed 30,101 CBE had 40%–50% higher roof displacement and peak bolt loads (192 kN) than RAE, with greater stress on the coal pillar side. Targeted measures (optimized CBE support, comprehensive water prevention-drainage) were proposed. This study provides technical guidance for safe mining of similar ultra-shallow, high-risk isolated island working faces.