Study on damage mechanism and treatment of water sprayed roof in Jurassic stratum roadway

The roof of the Jurassic main coal seam in western China generally contained low level weak rich water layer, which led to the long-term watering of the roof of the coal roadway especially the roof anchor cable hole and the reduction of the roof surrounding rock strength and the roof support effect, and affected the safety of the coal roadway roof. In order to study the damage mechanism and control measures of the water sprayed roof, the No.414106 auxiliary transportation water spraying area of Yangjiacun Coal Mine of Shuangxin Mining in Inner Mongolia was taken as the research object. Through field investigation and roof drilling sampling, it could be seen that there were obvious water conducting cracks in the area 4m above the roof of the roadway in the synclinal area. The water flowing from the anchor cable hole was in a linear water spraying state. The roof surrounding rock had a large degree of deflection, and some anchor cable anchorage sections were separated from the surrounding rock. The mineral composition analysis and water physical test showed that the clay minerals in the roof sandy mudstone contain up to 73% kaolinite, and the softening coefficient is 0.162, which had obvious water softening characteristics. At the initial stage, the roof of the coal roadway in the water spraying area was mainly destroyed by hydrostatic pressure, and the water softening property reduced the mechanical properties of the fracture structural plane, which led to the expansion of the size of the surrounding rock fractures in shear under the action of hydrostatic pressure. In the later stage, the roof surrounding rock was mainly destroyed by hydrodynamic pressure, which was mainly manifested in the deformation and expansion of fracture structural plane, displacement of fracture fillings, piping, etc. The destruction speed of roof surrounding rock was gradually accelerated. The whole process of surrounding rock of water drenching roadway roof from ground pressure appearance deformation and crack softening expansion to piping corrosion failure was analyzed. According to the main forms of the surrounding rock failure of roof drilling (physical softening, seepage failure, suction corrosion expansion failure, scouring deformation failure), it was divided into four different stages. The criteria for determining the development stage of roof failure of roadway drenching water had been formed with the main forms of roof drilling surrounding rock failure, the flow state of the roof anchor cable hole, roof surrounding rock fracture development characteristics and water control reinforcement principles as the key indicators. Combined with the site conditions, the 414106 auxiliary transportation water spraying area was the Ⅲ stage of the development of spraying roof damage. The structural form of anchor cable sealing grouting (drainage) and the principle of “deep hole drainage + shallow water sealing + deep reinforcement + high pressures support” were proposed for the roof of the roadway in the spraying area. An integrated reinforcement scheme of anchoring, sealing and grouting, which combined the reinforcement of high pre-tightened long anchor cables and sealing and grouting on the roof of the roadway in the water-spraying area, had been formulated. According to the field industrial test and rock pressure monitoring, the effect of roof water control and surrounding rock reinforcement was achieved.