Goaf Gas Ignition Due to Hard and Thick Rock Stratum Fracture Friction Effects: A Case Study

When thick and hard sandstone strata overly high gassy coal seam, sometimes gas in the goaf will be ignited or exploded owing to the roof instability. Taking Xiakuotan coal mine as the engineering background, the roof samples were collected and the friction tests were conducted, the results of which were used to analyse the source of the gas ignition owing to the rock friction effect. In the paper, the roof is simplified as a bilateral fixed support with two opposite edges—one fixed and one simply supported—along the inclined direction of the coal seam. The occurrence condition of slide instability is studied base on O–X fracture theory and yield line analysis method. The relationship between the working-face boundary-support status and the roof slide and instability is analyzed. During the experimental process of hard sandstone friction, a high-temperature friction surface is more likely to ignite the gas than spark bundles and high-temperature rock dust, and it’s the main ignition source in the case of such gas incidents. During the roof strata initial broken process, the ratio of the broken rock size to rock thickness (d_i/h) is usually greater than 2.6, and in the periodic broken process, the ratio of d_i/h is usually less than 2. The top plate periodic failure is more prone to slip-off instability than the initial breakage. As compared with the roof strata fixed bearing side, the rock strata thickness required for the slide and instability on the simply supported side is smaller, and the roof strata on the simply supported side is easier to slide and destabilized. If the upper section adjacent to the working face has been excavated, the upper part of the slope is a danger zone for gas ignition owing to rock stratum friction effect.