Gravity⁃dip effect and strata control in mining of the steeply dipping coal seam

Gravity⁃dip effect is a main factor leading to the deflection and non⁃equilibrium transfer of stress in the me⁃ so⁃layered coal⁃rock mass and the interlayer contact area，the directional offset of dominant fracture in the mesoscopic layered mining model， regional migration of key strata and the variation of rock structure in the macro⁃layered in the mining of steeply dipping coal seam. The research result shows that the gravity⁃dip effects are particularly obvi⁃ ous in the coal seam with dip angle above 35° including 1 the characteristics of non⁃equilibrium stress transmission at the interface of coal⁃rock combined body are obviously and the stress transmission near the interface is deflected and the amount of deflection increases gradually. The failure of coal mass is transformed from compression⁃shear to shear slip nearly parallel to the interface，and the strength and elastic modulus of coal⁃rock mass also decrease accord⁃ ingly. 2 The evolution law of the mining stress path in the stope has been changed，and the damage deformation and failure movement of the roof driven by the stress have obvious regional and time sequence. 3 It leads to the migration and transformation of roof structure，the asymmetric failure of floor slipping，and the partial⁃total failure of the coal pil⁃ lar or coal wall in the section in the steeply dipping coal seam. The instability of the surrounding rock above has a multi⁃scale chain spatiotemporal correlation and the key rock blocks in the stope that cause surrounding rock catas⁃ trophes are changeable，which results in the dissimilation and generalization of the bearing structure of the surrounding rock in the stope. Multi⁃dimension and multi⁃scale are the essential characteristics of strata control in the steeply dip⁃ ping coal seam under the action of gravity⁃dip effect. Breaking through the bottleneck of traditional mining methods and technologies，and developing new complete sets of equipment are effective ways to achieve the safe and effi⁃ cient mining of steeply dipping coal seam.