Abstract:
Addressing the coupled challenges of strong mine pressure and gas, induced by large-span fracture of the hard roof during the mining of thick coal seams, a systematic understanding of the disaster mechanism through multi-field coupling of the hard roof is achieved through theoretical analysis, experimental studies, and engineering practice. The study proposes the near and far-field coordinated control technology and the thick coal seam hard roof-gas cooperative control technology. Based on mechanical experiments under different stress paths, the weakening of coal and rock strength and the dynamic expansion of fractures under stepped cyclic loading are elucidated, revealing the progressive evolution characteristics of the hard roof overburden mining fractures in three zones and their fractal dimension “S”-shaped growth pattern. By constructing a “stress-damage-flow” multi-field coupling model, the nonlinear correlation between gas migration and porosity dynamic response within the mining fracture network is investigated, and a coupled control equation for coal deformation and gas flow is proposed. Innovative techniques such as the chain arm sawing top-directed weakening and surface hydraulic fracturing for far-field pre-fracturing are developed, forming the cooperative control technology for thick coal seam hard roof-gas, overcoming the technical bottleneck of low efficiency in traditional single-hazard mitigation methods. Further field practice at the Tashan coal mine demonstrates that, after surface fracturing, the maximum working resistance of the support is reduced by 16.9%, and the cyclic mining step distance is shortened by 15%. The “one well, dual control” technology realizes coordinated control of roof weakening and gas extraction, with the effect of surface drilling gradually weakening after the working face advances beyond 50−100 meters from the boreholes. The combination of vertical surface wells and horizontal fracturing wells for gas extraction can effectively solve the gas problems in the 8204 working face and control the return airway gas concentration within a safe range.