Field and numerical investigations of gateroad system failure induced by hard roofs in a longwall top coal caving face

In longwall mining, hard roofs located immediately above the coal seam should not cave and cause ground control problems around the working longwall face. In this paper, comprehensive field observations and numerical simulations were used to investigate the chain pillar and adjoining gateroad failure during a longwall top coal caving (LTCC) face retreat at the Majialiang coalmine located in Shanxi, China. The results show that hard roof strata did not cave behind the longwall face but overhung to form a cantilever beam above the chain pillar. The roof beam concentrated high stress in the chain pillar and caused the pillar failure. The concentrated stress moved close to the headgate and the solid side of the next panel, causing gateroad damage and rib collapse. Numerical simulations found that hard roof treatment could fill the space near the goaf edge and deliver support pressure to the roof as well as alleviate the stress concentration in the pillar and the solid side of the next longwall panel. Simulations suggested that a pillar size of 50 m or larger was feasible without consideration of hard roof treatment. A 40-m chain pillar with hard roof treatment could also satisfy the demand on the pillar and supply gateroad safety during the service period. Field practices verified the numerical results and suggested that hard roof treatment by hydraulic fracturing is a feasible method for overcoming ground control problems in a situation such as that presented in this paper.