Analysis of actual measurement of cracks in overlying rock on roof of gob-side entry retaining during secondary mining

The excavation of underground coal and rock masses will cause the fracture and displacement of the overlying rock layer,which will cause the overflow of gas in the coal seam,the flow of fissure water and even the surface subsidence and other mine disasters. In order to study the fracture breaking law of overlying roof strata in gob-side entry under thick loose layer,the ZXZ20(A) mine drilling imaging device was used to observe the fracture state and fracture development of overlying roof strata in return air lane and headentry of No.12307 working face in Yunjialing Coal Mine. The probe is pushed into the borehole to be measured by the push rod,and the measured position data and the internal structure of the borehole are transmitted to the computer by USB through the display of the imaging device host. In the return airway of the No.12307 working face,the measuring points 1,2 and 3 are arranged along the strike direction,and the measuring points 4,5,and 6 are arranged along the strike direction of the transportation lane. The drilling construction time and observation time of the 6 measuring points are the same. The observation results show that the roof strata above the No.1 measuring point is extremely broken,and the development of roof fissures is increasing from shallow to deep,with obvious separation between 3 and 9 m,and 6 separation cracks can be observed in this interval. There are well-developed cracks in the rock layer from 0 to 3.2 m in the roof of the No.3 measuring point,but lateral cracks can be seen from 3.2 m to 6.6 m above it and seam separation occurs at 6.2 m deeper. The roof strata within 0-2.5 m above the No.4 measuring point are obviously damaged,and the cracks in the strata are criss-crossed. The depth of fissures less than 2.5 m gradually decreased,and only primary fissures existed; no large cracks were found in the observation range of roof strata above the No. 6 measuring point,and the rock formation were relatively well-developed as a whole. The above results show that the main fracture range of roof overburden is within 2 m after the primary mining in the return air roadway,and the fracture degree of roof overburden is intensified due to the secondary mining during the retaining of the roadway,and the deep-level strata are also fractured and separated,especially within 4 m above the roof,and this phenomenon is gradually weakened beyond 4 m. This conclusion provides a reliable design basis for the support of gob-side entry retaining.