Study on the stress and fracture evolution law of surrounding rock during mining of upper and lower protective layers in coal seams

To achieve efficient gas extraction from the pressure relief of the protected layer in the upper and lower protective layers of coal seam groups, numerical simulation methods were used to study the distribution and evolution of surrounding rock stress after the upper and lower protective layer mining. The depressurized gas rich area of upper and lower protective layer mining was identified. The results show that after mining, the lower protective layer and the pressure relief height of the overlying rock on the upper side of the working face are both significantly higher than in the lower side.After mining the upper protective layer again, the pressure relief range of the protected layer is increased.After the mining of the lower protective layer, the pressure relief angle of the protected layer is 63°. The inclined upper and lower boundary pressure relief angles are 87° and 72°, respectively. After mining the upper protective layer, the pressure relief angle of the protected layer is 59°. The inclined upper and lower boundary pressure relief angles are 76° and 79°, respectively. After the protective layer is depressurized, the net amount of gas extraction increases by dozens of times before the depressurization.The depressurized gas extraction has a significant spatiotemporal effect.