Setting up a waterproof coal pillar is an important measure to prevent water inrush from the Weibei mining through fault floor. Based on the plastic slip line field theory, a mechanical model of floor water inrush induced by confined water in the through fault zone was established. The mechanical expressions of confined water pressure and the width of the waterproof coal pillar under the state of limit equilibrium were derived. Combining the laws of floor deformation, failure and fault activation under two kinds of coal pillar width, the safety width of the waterproof coal pillar was determined. Furthermore, the safety threshold is better than the empirical value mentioned in the “coal mine safety regulations.” Following this, grouting transformation was carried out on the K2 sand layer of the cut roadway floor. This provided a theoretical basis and engineering practice for water disaster prevention and the control of the structural floor under similar conditions in the Weibei mining area for future benefit.

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The failure process of a coal body around a borehole has progressive characteristics. Image characteristics can visually characterize the stress and failure characteristics of the coal body around a borehole during progressive failure. To investigate the effect of the moisture content on the progressive failure of the coal body around the borehole, an image test system for the deformation and fracture of coal rock was used, and progressive failure tests of coal body specimens with different moisture content conditions around boreholes were performed. We acquired images of the deformation field during the entire process of specimen failure. Based on the grayscale image theory, the variation in the grayscale characteristic parameters of the progressive failure process was analyzed. The results show that throughout the progressive failure of coal bodies with different moisture contents around a borehole, the main specimen failure can be divided into six stages: compression density, elastic deformation, crack initiation and stable extension, crack nonstable extension, post-peak softening, and post-peak failure. With increasing moisture content, the σcd/σf values of the 20%- and 40%-moisture-content specimens were 5.1% and 11.3% lower than those of the dry specimens, respectively, and the maximum uniaxial compressive strength σf was 5.1% and 17.4% lower than those of the dry specimens, respectively. The number of cracks that developed decreased. The grayscale histogram had a reduced grayscale peak at each stage, and the surface distortion diminished. The declining grayscale mean curve indicates a lagging development of stress concentration zones. The declining grayscale entropy curve indicates that macroscopic cracks form. The rising grayscale standard deviation curve indicates the delayed development of strain localization zones and weakening of specimen damage. The study explains the deformation and failure characteristics of the coal body around the borehole and the variation in grayness.

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