Impacted by large mining height and longwall mining in northwestern China, the surrounding rock mass is severely damaged, which accommodates the differential deflection and fracture of overlying strata. Consequently, the mining-induced space-time evolution of structure and permeability of high-level overburden is considerably obvious, causing the destructive mine water inrush from high-level aquifer to occur frequently. First, physical similar simulation was performed to explore the space-time sequence of separation layers and longitudinal fractures of mining overburden. Second, triaxial compression test was conducted to reveal the mechanics and permeability evolution of high-level overburden under the mining-induced stress path. Then, in-situ monitoring was conducted to explore the relationship between overburden structure and underground water in high-level aquifer. Finally, the model of permeability evolution of mining overburden was built, and the quantitative relationship between advance distance and inflow of water inrush was analyzed. From the physical model, confined separation layers are formed above water-conducting fractured zone in high-level overburden. With further mining, separation layers are connected with the underlying water-conducting fractures, and then close when new separation layers occur in the higher position. Results of triaxial tests show that samples from the area of separation layers developed break first and their permeability increase first, followed by the strata below separation layers, and finally the upper strata of separation layers. Moreover, the established evolution model of overburden permeability divides the behavior of mining overburden into four stages, including the formation of separation layers, connection between separation layers and longitudinal fractures, the closure of separation layers, and the closure of crack network. The deduced formula of overburden permeability, as the function of advance time, agrees well with the tendency of water inflow over mining time. The study improves the understanding towards the evolution relationships of strata structure, rock mass fracture, overburden permeability, and hydraulic hazards in mines under longwall mining.
 

High-level overburden;rock mass mining structure;overburden permeability;separation layers;water inrush