Multi-field evolution laws of overburden rock in ultra-close multi-seams pressure relief mining and “blocking-reducing-controlling” synergistic control technology of gas

Pressure relief mining is the most economical and effective regional gas control measure under coal seam conditions. However, compared to long-distance coal seam groups, the gas outflow from the first mining face under the condition of pressure relief mining in ultra-close coal seams has the characteristic of sudden increase, and gas exceeding the limit still occurs even under the Y-shaped ventilation mode. For this purpose, based on the typical conditions of the ultra-close multi-seams mine in Shanxi Province, the evolution characteristics of the mining induced fracture field in the bottom rock layer of the ultra-close multi-seams downward pressure relief mining were studied. The “three zones” division method and discrimination criteria for gas pressure relief and migration in the underlying coal rock layer under mining were proposed, and the migration and enrichment laws of gas pressure relief in the ultra-close multi-seams mining field were elucidated. Based on this, the “blocking-reducing-controlling” synergistic control technology of pressure-relief gas was proposed, which includes gas extraction through high-level boreholes to block gas migration, and precise regulation of air volume in the main and auxiliary intake airway Y type ventilation to reduce gas accumulation, and buried pipe extraction in the goaf to control gas flow field. The optimal parameters of high-level boreholes, the optimal air volume ratio of the main and auxiliary intake airways, and the reasonable parameters of buried pipes in the goaf were determined, and industrial test of gas governance was carried out. The results show that downward pressure relief mining causes the bottom rock layer to move toward the goaf, leading to pressure relief expansion of the underlying coal rock layer, development of mining-induced fractures, and pressure relief migration of methane gas, forming damaged gas-interflow zone, floor pressure-relief zone, and confined gas-adsorption zone. The underlying coal seam, which is extremely close to the mining layer, is located within the “damaged gas-interflow zone”, allowing gas to fully decompress and rapidly surge into the mining space along vertical fractures, thereby inducing gas exceeding the limit of the mining layer working face. The maximum gas extraction volume of high-level boreholes reaches 1.34 m³/min, and gas extraction is the most effective when high-level boreholes are 15-38 m away from the working face. When the air volume ratio of the main and auxiliary intake airway is 3∶1, the gas emission rate is the highest. With an average negative pressure of 27 kPa, the amount of gas extracted from the buried pipes in the goaf reached 30 m³/min. The “blocking-reducing-controlling” synergistic control technology of pressure-relief gas in ultra-close multi-seams mining has reduced gas volume fraction in the tailentry and the goaf behind the supports by 73.0% and 64.6%, respectively, without exceeding the coal mine safety regulations in China.