The permeability of sand is an important factor in determining the movement and occurrence of liquids and gases in sand. The current work studied the influence of soil particle size and gradation on permeability by testing the permeability of different sand samples that consist of defined sand particles in certain ratios. The results of a total of 640 sets of experiments were analyzed and compared with the corresponding theoretical calculations. We found that the sand can be divided into four groups based on particle size: high-permeability particles, medium-permeability particles, low-permeability particles and non-permeable particles; and the critical particle size of sand for permeability is 0.050 mm. Permeation will be lost when non-permeable particles account for >75% of the total in the mixture of high-permeability particles and non-permeable particles. Permeation will be very easily lost when medium- or low-permeability particles are mixed with non-permeable particles. The current findings are of importance for assessing the permeability of sand based on particle size.

Regional grouting treatment is an effective technical means to prevent mine water disasters, and the grouting effect is affected by many factors. In actual grouting engineering, the single constant-rate grouting method is often transformed into a variable-parameter grouting process. However, research on grouting rates has been insufficient. This investigation focused on the issue of “the diffusion law of variable-rate grouting slurry in regional governance”. Methods such as theoretical analysis, numerical simulation, and field verification were used to evaluate the diffusion mechanism of variable-rate fracture grouting. The results indicated that the key parameters of variable-rate grouting, such as slurry diffusion distance and grouting pressure, were affected by the grouting rate. The decrease in the grouting rate reduced the migration speed of the slurry and the grouting pressure. The time for constant-velocity grouting and variable-velocity grouting to reach the same diffusion distance was 60 s and 108 s, respectively, which can be achieved with lower grouting pressure. When the grouting rate was 7.5 L/min and 30 L/min, the maximum grout diffusion distance was 2.81 m and 5.64 m, respectively, which required greater grouting pressure. The slurry diffusion rate decreased with the reduction in the grouting rate. Under the same diffusion distance conditions, variable-rate grouting took longer than constant high-rate grouting. In variable-rate grouting, the grouting pressure decreased stepwise with the grouting rate, with a final pressure drop of 77.4%. In grouting practice, the innovative use of the rate-reducing grouting method can greatly reduce the final grouting pressure under the premise of changing the slurry diffusion distance less, which can not only ensure the stability of surrounding rock but also reduce the cost of high-pressure grouting and the risk of grouting operation. The investigation results can provide scientific guidance for ground grouting renovation projects in deep coal mine water hazard areas.

The deformation behavior of the roof and floor strata during coal seam mining plays a critical role in the development of mining–induced fractures and is an intrinsic factor in the prevention and control of coal and rock dynamic disasters, such as mine water hazards. This study investigates the critical values of the rock failure angle displacement based on the deformation performance of the strata through both laboratory and field experiments. A method for assessing the strata failure condition using the differences in deformation performance is proposed. The results indicated that when the angular displacement was greater than the critical value, the rock layer was destroyed, and the critical value of angular displacement was related to lithology, thickness and cross–sectional area. The critical value of angular displacement of rock strata with high strength was larger, the critical value of angular displacement of sandstone was 0.0552 ° on average, and that of limestone was 0.0818 °. The angular displacement of rock strata with small strength under unit load was larger, resulting in greater deformation. The angular displacement of sandstone under unit load was 5.45 × 10^− 6 ° / N, and that of limestone was 4.41 × 10^− 6 °/ N. The critical value of angular displacement of rock strata with larger thickness and cross–sectional area was smaller. The critical values of angular displacement of sandstone and limestone with 50 mm thickness were 0.0516 ° and 0.0774 °, respectively, and the critical values of angular displacement less than 30 mm thickness were 0.0552 ° and 0.0818 °. The critical values of angular displacement of sandstone and limestone with a cross–sectional area of 150 mm × 50 mm were 0.0483 ° and 0.0720 °, respectively, which were less than the critical values of angular displacement of 120 mm × 40 mm of 0.0516 ° and 0.0774 °. The difference between the angular displacement (θ_i) and the critical value (θ₀) under unit stress leads to the time sequence difference between horizontal and vertical fractures and the ‘anisotropic seepage’ law of groundwater. It was more convenient to use the critical value of angular displacement under unit stress as the criterion for judging the deformation and failure of rock strata than the traditional mechanical criterion. The research results have important practical significance for judging the failure of mining strata, analyzing the movement law of mining groundwater, and calculating the depth of water–conducting fracture zone and floor failure zone.