To identify mechanism of sand particle size effect on the mechanical properties of gypsum cement, 11 grades of sand particles with a size of 0.1–3 mm were used to produce 99 specimens for uniaxial compression and permeability coefficient testing. Based on this, the distribution characteristics of internal stress and horizontal displacement are discussed using the numerical analysis. The results obtained show that the sand particle size effect on the uniaxial compressive strength of similar materials is negatively correlated within the range from −16.51% to 49.79%. SEM observations imply that, in the case of small particle sizes, gypsum crystals develop into denser needle-like structures, while for larger particle sizes, they are mostly loose lamellar structures. Permeability tests indicated that the larger the sand particle size, the greater the permeability, indicating that the internal pore connectivity is better, and the crevices are easier to penetrate when the specimen is compressed. Numerical simulations indicated that the larger the particle size, the larger the extreme deformation value of the specimen in the horizontal direction, and the more uneven the deformation distribution. In addition, specimens with larger particle sizes had a larger total area, where the tensile stress exceeded the ultimate tensile strength, and were more prone to tensile failure.

Microwave heating of water-bearing coal can promote pore water evaporation. The pores are broken under the action of steam pressure, increasing the permeability of the coal. This study is aimed at investigating the mechanism of permeability improvement of water-bearing coal rock by microwave steam explosion. First, a microwave oven was used to irradiate and heat five groups of coal rock with different water contents; the NMR test was then conducted on the heated sample. Second, the internal vapor pressure and temperature changes during the heating process were obtained through the T-connector for samples with different water contents. Finally, a numerical experiment was used to explore the deformation characteristics of pores under three filling conditions. The experimental results showed that the total porosity increased significantly when the water content of coal increased from 0% to 8%, while the permeability increased by nearly 4.78 times. The extreme value of gas pressure inside the sample showed an increasing trend. The gas pressure and temperature were in line with the equation of state for an ideal gas during the rising phase. Numerical experiments showed that the pore boundary shrunk inward under vacuum conditions, and compressive stress appeared at the tip. The saturated humid air and liquid water conditions expanded the pore boundaries outward and caused tensile stress at the tip, with the latter being nearly 2.3 times larger than the former, making it more conducive to the development of pores. The findings of this study can be used as a reference value for the expansion of coalbed methane extraction technology.

Various geotechnical experiments have used gypsum-cemented analog geotechnical materials. However, this material needs a long curing time, and the target strength is not easy to control. Therefore, this research adopted microwave heating as the curing method for this kind of material. Objectively, the authors investigated the variations in the material strength versus heating power and heating time. On this basis, we clarified the influence mechanism of microwaves on the strength of analog materials by analyzing material temperature, moisture content, and microstructure, which eventually led to an experimental control method for rapid strengthening of microwave field-controlled gypsum-cemented analog materials. Consequently, we drew the following conclusions. The stable strength of the material under high-power microwave curing was much lower than that under natural curing, while the material strength under low-power microwave curing was the closest to the material under natural curing.

The weak interlayers in an open-pit blasting bench affect the uniform distribution of explosive energy. To explore the mechanistic influence of a weak interlayer on the effect of blasting, 9 sets of numerical blasting experiments were constructed using the orthogonal experiment method. The degree of bench crushing after blasting, the maximum velocity of the rock mass at 0.05 s, and the displacement of the back of the bench were thus investigated. The analysis revealed that the impact of the thickness of the weak interlayer, its wave impedance, and its location of occurrence on the bench blasting indicated an ordered decreasing effect. Based on this, the evaluation method for the master weak interlayer and the design plan of the specific charge structure were proposed. The evaluated design proposals were verified through both numerical and field tests. The research results will provide a scientific basis to determine a reasonable charge structure of the bench blasting of rock masses containing weak interlayers.

Water in coal pores can generate strong steam explosions when treated with microwave irradiation. In order to improve the sensitivity of coal reservoirs to microwaves, we used varying concentrations of NaCl on the pore solution, which further increased the effect of microwaves on permeability enhancement. In our experiments, we selected 3 ratios of water content and 5 different concentrations of NaCl. The changes of coal water content and permeability before and after irradiation were compared. In addition, changes in coal surface temperature and internal thermal power were analyzed through thermal imaging and random sphere numerical modeling. Results showed that the increase of the proportion of solution mass and the ion concentration in the solution improved the overall dielectric properties of coal rock samples. After microwave irradiation, the average reduction rate of water content in coal rock samples increased with the increase of ion concentration in the solution. Both the average surface temperature of the coal rock samples and the average electromagnetic heating power gradually increased; this increases the temperature rise rate and maximum temperature of coal rock samples under the same irradiation time, which is conducive to better rapid accumulation of steam pressure, thereby improving the pore structure more effectively. Finally, the maximum growth rate of permeability reached 466.18%. This work provides a novel train of thought to enhance coal permeability by using microwave irradiation.