“Shallow preservation-deep storage” method of water resources based on analogous hyperbola model of overburden movement caused by coal mining

In order to solve the mining-induced ecological problems, such as the groundwater level declining and surface vegetation destruction in the mining area, based on water-preserved coal mining and analogous hyperbolic movement of overburden, this paper proposed a differential protection spatial model for different layers of groundwater, named "shallow aquifer preservation-deep aquifer rstorage". Namely, the bed separation zone under the primary key stratum is grouted to control the upper rock layers and protect the shallow aquifer. The damaged aquifer below the primary key stratum is stored in the goaf, which will be reused after self-purified. The new water-preserved coal mining method can effectively integrate the advantages of goaf filling and traditional underground reservoir on water protection, and its applicable overburden conditions are given. According to the analogous hyperbola model of rock strata movement, the theoretical initial and periodical filling holes distance, and the filling ratio of separation zone under primary key stratum is derived. The variation of theoretical filling ratio is analyzed as the key parameters increase, which include goaf angle, full subsidence angle, excavation width and the rock strata thickness between primary key stratum and coal seam. The filling ratio decreases as the goaf angle and the rock strata thickness between primary key stratum and coal seam increase, but increases with an increase of full subsidence angle and excavation width. Furthermore, the field-measured data of grouting under primary key stratum are used to verify the theoretical filling ratio, all of which are in good matches. In addition, numerical simulation is used to analyze the effects of "shallow aquifer preservation-deep aquifer storage", goaf filling, and underground reservoir, as well as their abutment pressure distribution on coal pillars. Grouting under primary key stratum based on theoretical filling ratio can achieve a groundwater differentiated protection. Grouting improves the stability of the water-proof coal pillar of the water storage goaf and the maximum abutment pressure coal pillar can be reduced by approximately 60.8%. The proposed method significantly improves the coal pillars safety of the underground reservoir, and it also can solve the problem of difficult coordination between underground coal mining and filling. Differentiated protection of aquifers is conducive to achieving green, safe, and efficient mining.