Determination method and application of bed separation grouting quantity and injection-production ratio based on subsidence control orientation

Separation layer grouting and filling is one of the used methods of coal mining under buildings, water bodies and railways, grouting quantity and injection-production ratio are key parameters to ensure the effect of separation layer grouting and filling. To calculate grouting quantity and injection-production ratio, a method for determining minimum grouting quantity and injection-production ratio of bed separation grouting based on subsidence control guidance is proposed. This method establishes relationship between grouting quantity, injection-production ratio, and surface movement deformation parameters. Firstly, maximum allowable deformation value is determined by the protection level of surface buildings (structures). Secondly, calculate and determine maximum allowable mining height and remaining mining height of the coal seam. Then, establish a prediction model for the volume of the separation space, calculate the transmission coefficient of mining space, and combine it with maximum allowable mining height and remaining mining height to determine minimum volume of the hydraulic entity for grouting and filling the separation space. Finally, derive formulas for calculating grouting quantity and injection-production ratio. The method of determining minimum grouting quantity and injection-production ratio was applied to separation layer grouting and filling mining under gas station of 3801 working face of Huoerxinhe Coal Industry. The theoretical calculation determines minimum grouting quantity to be 1.6807 million m³ and injection- production ratio to be 0.53. Considering the impact of mining near 3802 and 3803 working faces, a numerical simulation experiment was conducted on surface subsidence under condition of separation layer grouting and filling injection-production ratio of 0.53 for 3801 working face to verify the accuracy of minimum injection-production ratio. The results show that during the mining process of the working face, maximum inclined deformation of gas station area is 2.03 mm/m and maximum curvature deformation is 0.075×10⁻³ m⁻¹. After mining of the working face, maximum subsidence of gas station area is 844.02 mm, maximum inclined deformation is 2.13 mm/m, and maximum curvature deformation is 0.034×10⁻³ m⁻¹. The surface movement and deformation meets protection requirements of buildings (structures) in gas station area. During the engineering practice of grouting and filling mining in bed separation of 3801 working face, actual grouting quantity is 2.176 9 million m³, and injection-production ratio is 0.68, which is greater than theoretical calculation value and limit injection-production ratio. This may be due to the narrow width of the isolation coal pillar between the adjacent working face, causing some of the slurry to migrate to the remaining separation space above the 3802 working face. Based on observation data of surface rock movement in 3801 working face (not monitored in 3802 and 3803 working faces), during the mining process of the working face, maximum horizontal deformation in gas station area is 1.43 mm/m, and maximum inclined deformation in north-south and east-west directions are 1.157 mm/m and 1.341 mm/m, respectively. The gas station has been in operation all along. 167 days after the end of working face mining, the subsidence of gas station area is 262.7 mm, and horizontal deformations in the north-south and east-west directions are 0.55 mm/m and 0.58 mm/m, respectively. The inclined deformations in north-south and east-west directions are 0.898 mm/m and 0.997 mm/m, respectively. The gas station is safe and stable, and the accuracy of this method is verified by comprehensive numerical simulation and engineering practice, providing theoretical support for the implementation of grouting and filling control of surface subsidence.