Stress distribution characteristics and oriented fracturing mechanism of ultra-thick overlying strata in extra-thick coal seams.

To address the problem of strong mine pressure caused by ultra-thick overlying strata (UTOS), this paper analyzes the distribution law of mining stress field in different layers of UTOS, and the expression of mining stress concentration coefficient of UTOS is given. The law of hydraulic fracture propagation in stress concentration area and original stress area of stope roof is expounded. The stress-oriented fracturing mechanism of UTOS is revealed. The results show that: (1) Along the advacned direction, the stress concentration value in the UTOS increases first and then decreases, exhibiting a "core" distribution pattern. As the buried depth of the roof increases, the range of the stress concentration core gradually decreases; (2) When hydraulic fracturing is carried out in the stress concentration area and the original stress area respectively, vertical hydraulic fractures and horizontal hydraulic fractures will be formed in the roof respectively; (3) The fracture propagation pressure in the stress concentration area is significantly greater than that in the original stress area. After the stress-oriented fracturing of the UTOS, the support resistance of the working face is significantly reduced, which is beneficial to the mine pressure management of the working face.