In this paper, we investigate the evolution characteristics of floor failure during pressured mining in extra-thick coal seams. A mechanical expression relating floor failure depth to seam thickness is established based on soil mechanics and mine pressure theory. The findings reveal a linear relationship between seam thickness and floor failure depth; specifically, as the coal seam thickens, the depth of floor failure increases. To simulate the mining process of extra-thick coal seams, FLAC3D numerical simulation software is utilized. We analyze the failure process, failure depth, and the behavior of water barriers at the coal seam floor under the influence of extra-thick coal seam mining from three perspectives: rock displacement evolution in the floor, stress evolution in the floor, and plastic deformation. Based on geological characteristics observed in the Longwanggou mine field, we establish a main control index system for assessing floor water-inrush risk. This system comprises 11 primary control factors: water abundance, permeability, water pressure, complexity of geological structure, structural inter points, thickness of both actual and equivalent water barriers, thickness ratio of brittle-plastic rocks to coal seams, as well as depths related to both coal seams and instances of floor failure. Furthermore, drawing upon grey system theory and fuzzy mathematics within uncertainty mathematics frameworks leads us to propose an innovative approach-the interval grey optimal clustering model-designed specifically for risk assessment concerning potential floor water inrush during pressured mining operations involving extra-thick coal seams. This method of mine water inrush risk assessment is applicable for popularization and implementation in mines with analogous conditions, and it holds practical significance for the prevention of mine water damage.

The process of permeation damage of the filling medium in the fracture is critical to the stability of the fractured rock mass. This study focused on the seepage failure process of filling materials in fractures and faults. To investigate the effects of axial stress and clay content, a series of experimental tests were conducted on internally unstable granular soil specimens with different clay contents under different axial stresses. The variations of flow rate and hydraulic conductivity were recorded and analyzed during the tests, and the typical process of seepage failure was summarized. The flow rate, hydraulic conductivity, and their growth rates were found to be smaller under high axial stress compared to low axial stress, and the flow rate of samples with higher clay content was smaller than those with lower clay content. Initially, the hydraulic conductivity decreased slightly due to clay and fine particle rearrangement, and remained nearly constant when the hydraulic gradient was small. However, as the hydraulic gradient increased, the hydraulic conductivity began to increase in response to the loss of clay and fine particles.