Mine Strata Pressure Characteristics and Mechanisms in Gob-Side Entry Retention by Roof Cutting under Medium-Thick Coal Seam and Compound Roof Conditions
Coal is among the most important energy sources, and gob-side entry retention by roof cutting (GERRC) is an innovative non-pillar mining technique that can effectively increase coal recovery rates and avoid coal wastage. To investigate the characteristics of mine strata pressure using the GERRC technique, a field case study under conditions involving a medium-thick coal seam and a compound roof was performed, and the mine strata behavior mechanisms were studied by theoretical analysis. Field monitoring shows that the distributions of the weighting step and strength along the longwall working face are asymmetrical. The periodic weighting length on the entry retaining side is longer than that on the other sides of the longwall working face, and the average increase is appropriately 4 m. Compared to the other sides of the longwall, on the entry retaining side, the periodic weighting strength is weaker, the average pressure is reduced by 2.1 MPa, and the peak pressure is reduced by 10.2 MPa. The lateral distance affected by roof cutting along the longwall is approximately 29.75 m, and the closer to the cutting slit, the more significant the roof cutting effect is. The retained entry becomes stable when it is more than 230 m behind the mining face, and the final cross section of the retained entry can meet the reuse demand of the next mining face. Theoretical analysis shows that the roof pressure mechanism in GERRC can be explained using cantilever beam theory. Within the area affected by roof cutting, the thickness of the immediate roof increases, and the suspension plate length of the roof immediately behind the longwall decreases. Then, the gangue pile in the goaf behind the longwall formed by the immediate roof’s collapse and expansion can support the main roof and other overlying strata much better. Therefore, the rotational breaking angle of the main roof is smaller, the periodic weighting step strength increases, and the periodic weighting decreases. According to the structural state of the surrounding rocks during the entire entry retaining process, the retained entry can be divided into coal support, dynamic pressure and stable entry areas
