Research on reasonable cutting roof parameters of gob side entry retaining by roof cutting in thick and hard roof

In the engineering practice of gob side entry retaining by roof cutting, the height and angle of roof cutting are the direct influencing factors, and the reasonable roof cutting parameters allowed by the hard roof with a wide range of roof thickness change have their particularity. Taking the practice of gob side entry retaining by roof cutting in the third panel of the north wing of coal seam No.2 in Tongxin Coal Mine as the engineering background. Based on the fact that difficulty of the thick main roof to collapse and form a large area of suspended roof, resulting in poor roadway retention effect, the reasonable roof cutting parameters varying with the thickness of the hard roof are studied through the combination of field investigation, theoretical modeling research, numerical simulation analysis and industrial test. Detailed field investigation and research found that the fixed cutting height could not adapt to the change of roof thickness. Under the condition of small main roof thickness, the effect of gob side entry retaining was better, and under the condition of large main roof thickness, the deformation of roadway surrounding rock was larger. The mechanical model of gob side entry retaining by roof cutting is established, and the relationship between the tensile stress of uncut part with the cutting height, angle and roof thickness is derived. It is concluded that within the relatively small range of cutting angle, the tensile stress on the uncut roof surface increases with the decrease of cutting angle, and the tensile stress increases more significantly with the increase of cutting height; with the increase of main roof height, the reasonable range of cutting angle allowed by the tensile stress at the uncut roof greater than the ultimate tensile strength of the main roof decrease. From the perspective of engineering practice, the purpose of reducing the length of suspended roof can be achieved; under certain main roof thickness, with the increase of cutting height, the range of cutting angle allowed by the tensile stress at the uncut roof greater than the ultimate tensile strength of the main roof increases, that is, the higher cutting height is, the smaller limit on the cutting angle is; the greater the thickness of main roof, the greater cutting height required for the tensile stress of uncut roof face to reach the same value. When the thickness of main roof is smaller, the change of cutting angle has little effect on the tensile stress of uncut roof face. With the increase of main roof thickness, the influence of cutting angle increases. The numerical simulation analysis is carried out from the four indicators of concave convex degree of roof fracture surface, the development characteristics of roadway roof fissures, the roadway roof subsidence and the completion time steps. It is concluded that the purpose of engineering practice can be achieved by scientifically implementing the partial cutting roof scheme and making the rock stratum of non cutting roof collapse under the action of mine pressure. Based on the above research, the industrial test is carried out in the field, and the on-site monitoring analysis was carried out on the reserved roadway with the roof deformation and the working resistance of support on the roof cutting side as indicators, and the results show that the effect of gob side entry retaining is good.