Study on prediction and characteristics of surface subsidence in mining when the bottom aquifer of thick loose layer directly covers thin bedrock

In view of the mechanism and characteristics of surface subsidence in mining with the bottom aquifer of thick loose layer directly covering thin bedrock，the water⁃soil coupling stress and deformation mechanism in the process of hydrophobic consolidation of the bottom aquifer of thick loose layer are analyzed by using mining subsid⁃ ence theory，soil consolidation theory and random medium theory. According to the superposition principle，the predic⁃ tion model of surface subsidence under the combined action of coal seam mining and bottom aquifer dewatering under the geological condition of thick loose layer and thin bedrock is established，and the characteristics and main in⁃ fluencing factors of surface subsidence under such conditions are discussed. The research shows that the proposed min⁃ ing subsidence prediction model with the bottom aquifer of thick loose layer directly covering thin bedrock considers the combined action of coal seam mining and the bottom aquifer hydrophobic consolidation settlement of thick loose bed，which has been verified by engineering application.The hydrophobic consolidation settlement of the bottom aquifer of the thick loose layer increases the settlement of the surface subsidence basin，slows down the conver⁃ gence speed at the edge of the basin，and greatly increases the surface subsidence and horizontal movement range.This results in the phenomenon that the subsidence coefficient is greater than 1 and the boundary point with a surface sub⁃ sidence of 10 mm obviously extends to the outside of the mining area，the surface subsidence curve shows the com⁃ pound characteristics of “small opening deep basin in the middle and large opening shallow basin on both sides”.The surface subsidence range under the combined actions of coal mining and dewatering of bottom aquifer is similar to that caused by dewatering of single bottom aquifer.The boundary angle δ of surface movement basin decreases with the increase of thick loose layer thickness Hls.The maximum waterhead drop value Δhm is positively correlated with the maximum surface subsidence，but has little effect on the subsidence range.The influence radius Rwof bottom aquifer drainage mainly affects the range of surface subsidence.The bottom aquifer thickness M mainly affects the de⁃ gree of surface subsidence. The research results can provide a theoretical basis for scientifically setting up the protec⁃ tion coal pillars for shaft and building (structure)at the industrial square under similar conditions，as well as the envi⁃ ronmental impact assessment of mining subsidence area.