Detection of the self-healing characteristics of mining fractured rock mass after 10 years of underground coal mining:A case study of Wanli Coal Mine Shendong mining area

In the long-term evolution process after underground coal mining,the fractured rock mass is often prone to the phenomenon of self-healing that the water conductivity gradually decreases.It is of great significance to study the generation mechanisms and regular pattern for realizing ecological restoration of the mining-damaged surroundings.In order to reveal the self-healing characteristics of the mining-induced fractured rock mass in Wanli coal mine after 10 years of coal mining,the in-situ drilling detection method is used to observe the overburden fracture characteristics and self-healing degree under two typical conditions of single coal seam mining and multiple coal seam repeated mining.The results show that after 8-12 years of evolution,the self-healing phenomenon of overlying fractured rock mass is obvious,which is characterized by low permeability with small leakage of drilling flushing fluid and slow decline of water level in the borehole.The initial cracking degree of overburden during coal mining directly affects the self-healing effect of rock mass revealed by exploration for many years after mining.Horizontally,the mining fractures markedly develop in the rock mass near the mining boundary,so the self-healing effect of the cracked rock mass near the mining boundary is worse than that in the middle of the panel;and vertically,the self-healing effect in the middle and lower part of the caving zone and the middle and upper part of the fractured zone is the best,and due to the occurrence of thick hard sandstone in the upper part of the caving zone and its transition zone with the fractured zone the self-healing effect is relatively low.The larger the buried depth of the coal seam,the greater the abutment pressure,which is easier to promote the compression of the advanced rock mass,and the decrease of the opening of the obvious cracks near the boundary,which is helpful to promote the self-healing of the cracks.Therefore,the self-healing of the fractured rock mass in the shallow mining coalfields needs a longer time.Compared with single coal seam mining,the secondary activation caused by repeated mining of multiple coal seams will change the existing cracking state of the overlying strata of the upper coal seam,and may also destroy the self-healing process that has occurred in it.The longer the mining interval between the upper and lower coal seams,the slower the self-healing process of the overlying strata as a whole,and the worse the effect.