Study on deformation and failure mechanism of rock mass in deep coal seam floor based on damage degradation of available bearing capacity

According to the research on the deformation and failure mechanism of the deep coal seam floor, the fracture mechanics model considering the sliding deformation of the primary crack and the growth of wing crack were constructed, the theoretical formula of the damage factor considering the crack sliding deformation and the crack growth was derived. The expression of the increase amplitude of the stress of available bearing capacity compared with nominal stress under the influence of damage degradation was established, and the relationship of related factors was analyzed. The results show that the increase in the stress of available bearing capacity compared to the nominal stress under the influence of damage deterioration has a positive correlation with the confined water pressure, crack connectivity, maximum principal stress, crack dip angle and crack half-length. The increase in the stress of available bearing capacity compared to the nominal stress under the influence of damage deterioration has a negative correlation with the crack surface friction coefficient, the minimum principal stress and damage degradation coefficient. This indicates that the increase of the length of the primary crack and the increase of the deviator stress aggravate the damage degree of the rock mass itself and increase the stress of available bearing capacity of the non-damaged rock. Besides the improvement of friction coefficient of crack, the fracture connectivity and water pressure between fractures are decreased by grouting reinforcement, which is helpful to improve the integrity of rock mass, reduce the stress of available bearing capacity of the undamaged rock and improve the stability of rock mass in coal seam floor.