Numerical simulation of short-path “drainage-recharge” watercycle and recharge study

In order to maintain the balance of the groundwater system in open-pit coal mines，avoid water resource destruction or adverse environmental impacts caused by drainage and alleviate the contradiction between dewatering and drainage，the method to recharge the water in open-pit coal mines at the same layer within depression cone is discussed.Under the premise of guaranteeing mine safety，the zero discharge of drained water can be achieved through a short-path “drain-recharge” groundwater circulation.Based on the theoretical analysis of water balance，according to the theory of short-path groundwater circulation drainage，this study constructs a drainage-recharge coordinated-mining geological model to simulate groundwater flow under the conditions of draining and recharging through the FEFLOW groundwater numerical simulation software.It analyzes the relationship among the numbers，well space，flow rate and drainage effect of drain wells and recharge wells，and comparatively analyzes the drainage effect of the precipitation hole drainage method and the short-path groundwater circulation drainage method to verify the short-path groundwater circulation drainage method.Using mathematical model，it shows that when the recharge well is completely complementary to the drain well，a short-path groundwater circulation is formed.The paper presents four key issues:the synchronization of draining and recharging，stable draining flow，recharging volume as close as possible to draining volume，and short distance between recharging wells and drain wells，then the simulation calculates that the number of recharge wells that form a short-path groundwater circulation is 1∶1 with the number of drain wells，and that the optimal distance between the recharge wells and the precipitation center is 2 000-3 000 m.The greater the distance，the greater the drain flow drops;the shorter the distance，the shorter the time to reach the maximum recharge flow rate.The limitation that the groundwater recharge in the same layer must be within the depression cone is broken through and the short-path groundwater circulation drainage theory is defined.It proposes an idea that the water-bearing space and the time for water to circulate in the aquifer can be temporarily used in exchange for a safety mining in coal mines，and develops the theory of short-path groundwater circulation drainage.Based on the premise that the groundwater in open-pit coal mines is drained，the study simulates the short-path “drain-recharge” water draining process.It is concluded that when recharge wells are arranged equidistantly in a single ring，the number of recharge wells is equal to the number of drain wells;the distance between recharge wells with better circulation and the dewatering center is given;and by analyzing the relationship among drain flow，recharge flow，the distance between recharge wells and dewatering center，the draining time the formula for the distance of recharge wells closest to the dewatering center is calculated，and the application steps of the drainage method in open-pit coal mines are given.Taking a mine drainage as an example，the result shows that when achieving the same drainage effect as the precipitation hole drainage method，the short-path groundwater circulation drainage method also has many advantages such as keeping the flow of the drainage system stable，making the drainage equipment efficient，and by adjusting location and water level of the recharge wells，it can control the scope and shape of the depression cone.Comparing the two drainage methods through simulations，the short-path “drainage-recharge” method stabilizes the water output of draining wells at 82.5% or more of the maximum water output capacity，and reduces the drainage volume by 33%-98%.