Formation Mechanism and Accurate Identification of Spontaneous Combustion Hazard Zones in Gob-Side Entry Driving Goafs Under Small Coal Pillar Damage

Abstract

Mining-induced damage to a narrow retained coal pillar can establish preferential air leakage pathways between an active goaf and an adjacent previously mined goaf, thereby altering oxygen transport and the spatial distribution of residual coal oxidation regions. Taking the 31119 gob-side entry driving panel of Nalinhe No. 2 Coal Mine as the engineering background, this study combined buried tube monitoring, gob-side directional borehole sampling, and CFD porous medium simulations to investigate the effects of retained coal pillar damage on airflow and oxygen migration. A coal pillar damage–air leakage–oxygen transport model was established, and steady-state simulations were conducted for different prescribed coal pillar damage conditions. Field monitoring showed that the oxygen-defined oxidation zone in the 31119 goaf extended from 37.5 to 83.9 m on the intake side and from 16.2 to 49.0 m on the return side, whereas the corresponding simulated ranges were 35–84 m and 16–51 m, respectively. More importantly, in the adjacent 31120 goaf, the monitored oxidation zone range along the gob-side strike direction was approximately −45 to 14 m, compared with a simulated range of −48 to 13 m. With increasing retained coal pillar damage, fracture connectivity and equivalent permeability increased, promoting deeper oxygen penetration and progressive expansion of the oxidation zone in the adjacent goaf, whereas the three-zone distribution in the 31119 active goaf changed only slightly. These results indicate that damage to the retained coal pillar primarily controls the reactivation of oxygen-accessible oxidation conditions in the adjacent goaf. The proposed approach provides a basis for identifying potential spontaneous combustion–prone regions and for targeted leakage sealing, gas monitoring, and fire prevention measures in gob-side entry driving panels.