Instability mechanism and prevention technology of roadway in close distance and extra thick coal seam under goaf

During the mining process of the close distance coal seams under goaf,mining roadway in the lower seam is affected by the coal pillars left over in the goaf of the upper seam and the adjacent working face’s dynamic pressure.In view of the serious deformation and failure phenomenon of roof fall,the inward extrusion of two roadsides and floor heave in 13311 air return roadway of top coal caving face of the extra-thick coal seam of Sunjiagou coal mine,the field measurement,theoretical analysis and numerical simulation are adopted to study the mechanism of roadway’s instabili-ty and the main influencing factors. The results show that the main factors affecting the deformation and instability of 13311 return air roadway include the driving under the influence of mining dynamic pressure in adjacent working face, the influence of the unreasonable layout of roadway and the support parameters of roadway. When the roadway is ar-ranged vertically with the upper coal seam,the support intensity of the roadway is low and the roadway is driven under the influence of mining dynamic pressure in adjacent working face,the lower coal mining roadway is easy to lose stabil-ity. To improve the stability of surrounding rock of 13313 return air roadway and effectively control the deformation of roadway,according to the physical and mechanical properties and stress characteristics of surrounding rock of test road-way,a targeted solution was put forward. Firstly,the layout of roadways is improved,and the lower coal seam mining roadways are arranged under the goaf,and the distance between the lower roadway and the remaining pillars in the goaf of the upper coal seam is not less than 20 m. Secondly,the width of coal pillar in roadway protection is increased,and the width of coal pillar in roadway protection section is increased to more than 20 m,so as to reduce the influence of dynamic pressure of adjacent working face. Finally,the high pre-stressing full anchor cable is used to strengthen the support,so as to improve the integrity and bearing capacity of the bolt anchorage section. Industrial test was carried out in 13313 return air roadway and the roadway pressure observation was carried out. The results show that after being af-fected by the mining dynamic pressure of the adjacent 13311 working face,the coal pillars in the section are integral and have good bearing capacity. The force of anchor cable reaches 250-300 kN,which is about 50% of its breaking force. The force of anchor cable increases steadily,and the deformation of roadway separation and surrounding rock is well controlled. The displacement of roof and floor of 13313 return air roadway is about 400 mm,and the displacement of two sides is about 300 mm,the deformation of roadway surrounding rock is effectively controlled,which ensures the overall stability of the roadway and achieves a good support effect. However,with the roadway layout,during the lower 13313 working face mining,due to the influence of the concentrated stress of the coal pillar in No. 11 coal seam above the working face,higher requirements were put forward for the management of its roof and coal wall,and great attention should be paid to it at that time.