Equivalent water resisting overburden thickness for water conservation mining:Conception,method and application

Water conservation mining of coal resources is one of the core technologies to realize green mining,and the feasibility study of water conservation mining is the basic problem of scientific mine design. From the point of view of the overall overburden water resistance,an equivalent water resisting overburden thickness,indexed by equivalent conductivity,was conceptualized for assessing on the feasibility of water conservation mining,which takes the consideration of the overburden hydraulic characteristics and the groundwater system dynamic response. Based on the Darcy’s law,the Kozeny-Caman equation,and the relation of influence function of overburden subsidence,the equivalent concept was developed by using the layered combination and zonation equivalence. The equivalent vertical hy-draulic conductivity for a single stratum was developed through a parallel calculation of all micro units in horizontal direction. The equivalent vertical hydraulic conductivity for the overall overburden was presented through a series calculation of all strata,which is functioned with mining height,mined out length,subsidence and initial hydro physical parameters. Taking the balance between the runoff rate through the overburden and the recharge rate of the focused aquifer as the critical condition,the relation of the maximum critical hydraulic conductivity of the overburden with mining height and overburden thickness was determined. Thus,a method for calculating equivalent water resisting overburden thickness was developed based on the assumption that the hydraulic conductivity of the overburden is inversely proportional to its equivalent water resistance thickness. The feasibility of water conservation mining could be evaluated on the basis of the allowed maximum critical hydraulic conductivity for maintaining the stability of the aquifer water level. The method was verified in the case of water conservation mining in Ili No.4 Coal Mine based on the field monitoring on the water level. The study results provide a new method for the prediction and evaluation,method selection and parameter optimization of water conservation mining.