Study on the zoning characteristics and fracture rotation behavior of overburden strata in steeply inclined coal seams

In order to study the mechanical response behavior of overlying strata in steeply inclined coal seams，the stress distribution patterns and energy accumulation characteristics of the overburden were analyzed through theoretical analysis，numerical simulation and field measurements. The evolution law of destruction in the overlying strata was established，elucidating the interaction mechanisms of the overburden structures in different regions during the rotation process. Additionally，he fracture rotation behavior of the overlying rock strata in steeply inclined coal seams was discussed. The results show that the bending moment<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>M</mml:mi><mml:mi>x</mml:mi></mml:msub></mml:mrow></mml:math></inline-formula>in the short side direction of the roof in steeply inclined coal seams is large，with the maximum principal bending moment in the middle area exhibiting a “trapezoidal-heart-shaped”distribution. Conversely，the maximum principal bending moments on both long sides display an“oblique trapezoidal”characteristic. Notably，the elastic energy is released substantially in the middle upper parts of the overlying strata，and the overall energy characteristics of the roof show an evolutionary trend of release in the middle upper parts，oblique extension at the upper end，and dip expansion at the lower part as the advancement distance increases. The advanced elastic energy in the middle upper parts of the roof is relatively concentrated，reaching a peak value. When the mining distance reaches 150 m，the peak value of the advanced elastic energy of the middle upper parts is 2.7 times than that of the upper area. With the increase of mining distance，the advanced elastic energy of the roof in steeply inclined coal seams experiences sequentially accumulation in the middle lower areas and rapid growth in the middle and upper areas. The fractured overlying rock structures squeeze each other and undergo rotation along the dip，with the compressive stress on the fracture surface of the overburden structure increasing with the increase of the dip angle. The slip parameter of the upper fracture surface is relatively small，and it is easy to experience slip instability during the rotation process. In contrast，the middle fracture surface remains stable，with the slip parameter directly proportional to the rotation angle. The lower fracture surface exhibits significant stress concentration but remains stable due to the filling effect. An increase in overburden load will aggravate the instability of the fractured rock strata，while the extension of gangue filling enhances the stability of the overburden structure. This study provides a theoretical basis for analyzing the migration characteristics of overburden in steeply inclined coal seams，and it is of great significance for the stability control of surrounding rock in steeply inclined coal seams.