Study on characteristics of coal-rock failure and fracture evolution in mining-induced space of gob-side entry retaining

To address the problem of frequent gas overrun in gob-side entry retaining of protective layers in outburst-prone coal mines, taking the 3207 protective layer working face of Zhongxing Coal Industry as the engineering background, this study adopted a combined method of theoretical analysis, numerical simulation and field measurement to systematically investigate the characteristics of coal-rock failure and fracture evolution in the mining-induced space of gob-side entry retaining in protective layers. The results show that during the mining of the 3207 protective layer working face, the maximum height of the caving zone reaches 7.5 m, the maximum height of the fractured zone is 22.1 m, and the depth of floor fractures is 19.4 m, with both the No.2 and No.4+5 coal seams falling within the effective pressure relief range. During the mining process of backfill-type gob-side entry retaining, the main roof on the entry-retaining side undergoes secondary breakage, where the first breakage occurs on the coal pillar side and the second breakage on the outer side of the backfill wall. Along with the secondary breakage of the main roof, the stress at the backfill wall of gob-side entry retaining shows a trend of first increasing and then decreasing. After the secondary breakage, the influence of roof activities on the retained entry is significantly weakened, and the entry gradually enters a stable state. Disturbed by mining, a large number of fractures connecting with the goaf are developed in the roof and floor of gob-side entry retaining. Under the action of negative pressure, the gas in the goaf can flow into the retained entry through these fractures, which is one of the key causes of frequent gas overrun in gob-side entry retaining. The width of the coal pillar (backfill wall) is the core factor affecting the pressure relief range of the protected layer. Under the coal pillar mining mode, a large-scale un-relieved pressure area exists in the protected layer below the protective coal pillar, which is prone to stress concentration and thus induces coal and gas outburst accidents. In contrast, backfill-type gob-side entry retaining can achieve sufficient pressure relief of the protected layer, with only a small-scale incompletely relieved pressure area at the position of the backfill wall of the protective layer and low stress level, indicating an excellent overall pressure relief effect.