Study on visual exploration of fissure field of overlying strata in deep coal seam under subcritical extraction

The detection of the overburden mining fracture distribution and the height of water conducting fracture zone are one of the hot issues in the study of coal mine safety subcritical mining，which plays an important role in mine water disaster prevention and gas management.In order to obtain the distribution characteristics of the overburden fracture field and the development height of the water-conducting fracture zone in deep buried coal seam，taking a mine in Jining Coalfield as a test mine the traditional empirical formula，the leakage observation of drilling flushing fluid and the borehole TV observation were used for theoretical calculation，quantitative detection and qualitative analysis.The results show that under the average dip angle of gentle inclination coal seam of 6°)，deep buried is 538～595 m and strip-type subcritical extraction，the height of water-conducting fracture zone is 66.0 m，which is 23.6 times of mining height； the height of caving zone is 8.45 m，which is 3.02 times of mining height.Overburden mining fractures are mainly the form of NEE syncline fractures，which develops in the collapse zone and the lower part of the fracture zone.The main development range is about 33.55 m above the floor of the gob，which is about 12 times of the mining height，accounting for 50.8% of the height of the water fracture zone.Mining overburden fractures show obvious stratification in the fracture zone.The fracture development strength and width of the fractures in the upper part of the fracture zone are limited.Although it has hydrological significance，it has little effect on the residual deformation on the surface in the gob.The fracture development in the lower part of the fracture zone develops obviously，and there are many separated layers，which have certain degree of impact on underground water seepage and surface residual deformation in the gob.The comparative analysis shows that there are great differences among the three methods in accuracy.The drilling color television observation has the best effect，which can accurately detect the distribution and occurrence of fractures in the rock mass； the drilling flushing fluid leakage observation has the second effect，which can obtain more accurate data； the traditional empirical formula has the worst accuracy，and only approximate ranges can be determined.