Rockburst mechanism caused by the mining-induced drainage of confined water in deep extra-thick aquifer

Extra-thick sandstone confined aquifer is widely spread in the overburden strata in Shaanxi and Inner Mongolia provinces. During coal mining, the water inflow in the working face is severe and the confined water level in the roof aquifer decreases significantly. The monitoring of rockbursts and high-energy seismicity shows that the mining-induced drainage of the confined water in the extra-thick sandstone confined aquifer has a certain effect on inducing rockburst. For a better understanding of rockburst mechanism as a result of the drainage of the roof confined water, the fluid-solid coupling numerical simulations were carried out. The mechanical behavior of rock mass with different pore water pressures and the evolution of stress-energy field in surrounding rock under the condition of mining-induced drainage of the confined water in the overburden aquifer were analyzed, and the effect of the mining-induced drainage of the confined water in aquifer on the stress field in surrounding rock was determined. On this basis, considering the effect of extra-thick strata on the dynamic and static stress in surrounding rock, the rockburst mechanism caused by the mining-induced drainage of the confined water in aquifer was proposed. The results show that the dynamic behaviors under extra-thick confined aquifer, including rockbursts and high energy seismicity, occur during the rapid drop of water level in overburden confined aquifer, and the mining-induced drainage of confined water causes stress disturbance to surrounding rock. Under the condition of mining-induced drainage of the confined water, the pore water pressure of the confined aquifer decreases, the strength and bearing capacity of the sandstone strata increase, and the overburden load is transferred to both sides of the goaf, which results in the increase of the stress and elastic energy in the surrounding rock. The effect of the mining-induced drainage of the confined water in aquifer on the stress-energy field is positively correlated with goaf scale and drainage degree, and negatively correlated with the distance between coal seam and confined aquifer, the thickness and strength of confined aquifer. The large-scale roof cantilever-hinged structures under the extra-thick confined aquifer lead to high static stress and strong dynamic stress in the surrounding rock. Under the effect of the mining-induced drainage of the confined water in the aquifer, the superimposed stress in coal and rock mass exceeds its critical load and the total energy released exceeds the energy consumed by its failure, which induces rockburst. In the process of rockburst, high static stress is the stress basis, and the stress disturbances caused by seismicity and the mining-induced drainage of the confined water are important inducements. In view of the influence of the mining-induced drainage of the confined water and the extra-thick strata structure on the rockburst risk, the confined water plugging of the extra-thick aquifer by ground grouting and optimizing the panel size were put forward. Field application indicates the two measures inferred above can reduce the effect of the mining-induced drainage of the confined water on the stress-energy field, and avoid the formation of the large-scale cantilever-hinged structures, which can reduce the rockburst risk significantly.