Relationships between mining overburden rock and surface subidence and its application to estimate bed separation

Underground mining may result in surface subsidence， infrastructure damage， land degradation and surface water accumulation， and cause the ecological deterioration. In order to control the surface and overburden rock stra⁃ ta movement and reduce the mining damage， the solid filling in goaf and overburden grout injection to bed separation are selected as useful techniques. The location of bed separation can be given by the traditional methods such as key strata theory， numerical simulation， similar tmaterial experiment and theoretical analysis. However， it is difficult to determine the quantitative value of bed separation because of its complicated calculation process， multiple physical and mechanical parameters and the effect of boundary conditions. The research results of mining subsid⁃ ence demonstrate that the overburden and surface move towards to the goaf center along the ray projection， the subsid⁃ ence boundary of overburden and surface can be simplified as straight line according to the synthetic limit angle and the surface subsidence area equals to any overburden strata subsidence area. Based on these research consensus about the characteristics of coal measure strata， the relationship model between mining overburden strata and surface subsidence have been established， which well reveals the evolution of overburden and surface subsidence with the syn⁃ thetic limit angle， width of working face and mining depth. The ratio of mining overburden strata and surface subsid⁃ ence increases with the increasing of burial depth for overburden strata， it reflects the transfer law of mining subsid⁃ ence effect from roof to overburden and surface. When the width⁃depth ratio increases， the ratio of overburden⁃sur⁃ face mining subsidence decreases and the surface subsidence increases. If the synthetic limit angle decreases， the range of surface subsidence increases and the corresponding surface subsidence decreases. The overburden rock strata and surface subsidence can be calculated layer by layer according to the actual strata thickness by borehole. For the sub⁃ sidence inconsistency of two adjoining rock strata， the bed separation can be determined. It must be noted that the inef⁃ fective bed separation should be eliminate because it can’t form a closure filling space in the fractured zone. The maxi⁃ mum bed separation can be then optimized which is suitable for overburden grout injection. This identification and calcu⁃ lation method of bed separation is simple and convenient and helpful to the design of overburden grout injection.