Investigation on the Ground Pressure Induced by Hard Roof Fracturing at Different Layers during Extra Thick Coal Seam Mining

Table 1

Statistics of the strong ground pressure on the working face.

Data	Mining distance (m)	Phenomenal description
2015.11.13	110.2	Strong ground pressure 10 m in advance of the working face, the floor heave was 0.3–0.4 m, rib heave was serious: 0.4 m; the resistance of supports #45 to #67 was high.
2015.11.21	168.8	A stronger ground pressure happened, the advanced range of influence was 10–20 m, roof subsidence and floor heave was severe, ten hydraulic props were dumped, roof subsidence 0.3–0.4 m, floor heave 0.45 m, the resistance of supports #23 to #35 and #56 to #71 were high.
2015.12.30	214.5	A stronger ground pressure occurred 12–35 m in advance of the roadway, thirty hydraulic props were dumped; the roadway deformation in horizontal and vertical direction was 0.5 and 0.8 m, respectively. The resistances of supports in the working face were all higher and the safety valves opened at supports #45 to #67.
2016.1.14	450.0	Strong pressure happened 10 m in advance of the working face, an obvious deformation occurred in the roadway, one hydraulic prop was bent 8 m in advance working face; the resistance of supports increased obviously.
2016.2.13	635.0	Ground pressure occurred, the pressure was concentrated 10 m in advance, the floor heave was 0.2–0.3 m, and roof subsidence was not obvious.
2016.3.18	736.0	Ground pressure occurred. At 15 m in advance, the floor heave was 0.2–0.4 m, rib heave and roof subsidence were not obvious, and five props inclined.