The deformation and instability of roadway surrounding rock reflect the processes of energy accumulation and release. To reveal the instability of roadway surrounding rock, based on the engineering geological conditions of a certain mine, this paper established a nonuniform superimposed stress model of a coal pillar, starting from the energy accumulation and release of the surrounding rock of the floor roadway after coal pillar failure. The essence of the deformation of the lower roadway was revealed, and the following conclusions were drawn: (1) The elastic strain energy accumulated in the lower roadway roof and upper coal pillar is related to not only the physical properties of the coal seam but also the distance between the coal pillar and surrounding rock, the caving height and shape, the burial depth, and the retained pillar width. (2) The elastic strain energy accumulated in the lower roadway roof and the upper coal pillar induced large displacements of the roadway due to the energy release during excavation. (3) It is proposed that the “stress relief degree” and failure morphology are used to identify zones in the rock and coal, and the two zoning modes have a high consistency. (4) The stress distribution in a narrow coal pillar should be calculated in segments. (5) Based on the zoning and energy release characteristics, the following control factors are suggested regarding the coal pillar width and roadway layout: (a) for the coal pillar, avoid the overlap or intersection of the peak values in the limit equilibrium zone and ensure a sufficient elastic zone; (b) arrange the roadway in shear slip Zone B‐2 or the moderate pressure relief Zone B‐2 to reduce the accumulation of elastic strain energy in the surrounding rock.

Coal samples having the same diameter (50 mm) and different length-diameter ratios (l/d), i.e., 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 were tested under dynamic uniaxial impact compression using the Split Hopkinson Pressure Bar (SHPB) experimental system. This study evaluates: (a) The effects of l/d on the energy consumption law and fractal characteristics of coal crushing; (b) The effects of l/d and stress balance on energy dissipation; (c) The effects of l/d and energy consumption density on the fractal characteristics of coal crushing. The findings under different l/d are as follows: (1) The coal samples show similar stress–strain curve shapes in stages including elastic, plastic, and failure stage, which is an “open” shape, the proportion of plastic stage increases, and strain-softening occurs; (2) The dynamic compression dissipation energy and energy consumption ratio of coal shows the same trend, showing two stages with the increase of length-diameter ratio, which increases linearly in the first stage and overall decreases step-by-step; (3) The average particle size increases while fractal dimension of fragmentation decreases linearly, which endorses the decreasing trend of fragmentation degree; (4) It is determined that there is a power relationship between fractal dimension and energy dissipation density; (5) A new index Crushing Density Energy Efficiency (CDEE) is proposed, which can be used to characterize the rock-breaking efficiency of crushing energy consumption under different conditions. This index is inversely proportional to l/d. The research results can provide a basis for the design of top coal caving mining, and the determination of blasting parameters.

In this study, a uniaxial impact compression test was performed on coal samples with length‐to‐diameter (L/D) ratios of 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1 using a Φ 50 mm split Hopkinson pressure bar (SHPB) test system. This study researched the stress uniformity and deformation behavior of coal samples with different (L/D) ratios during dynamic compression, defined the stress equilibrium coefficient ξ, proposed a new method for determining whether a sample meets the stress uniformity hypothesis, and obtained the critical (L/D) ratio of 0.6 and the optimal (L/D) ratio of 0.3 or 0.4 for coal samples to obtain the stress equilibrium. The experimental results showed that the dynamic stress‐strain curve of coal had an elastic stage, a plastic stage, and a failure stage. As the (L/D) ratio increased, the proportion of the elastic stage to the prepeak curve of the samples declined progressively; with an increase in the (L/D) ratio, the peak part of the curve also changed from “sharp” to “stagnated,” while an increase in the plasticity led to strain softening. As the (L/D) ratio of the samples increased, the average strain rate decreased approximately as a power function, and the decreasing trend was gradually reduced from 296.49 s⁻¹ (L/D=0.3) to 102.85 s⁻¹ (L/D=1), with a reduction of approximately 65.31%. With an increase in the L/D ratio, the peak strain gradually decreased exponentially. This study concluded that the SHPB test protocol design is of a certain reference value for low‐density, low‐strength, heterogeneous brittle materials, such as coal.

As mining progresses, complexities arise, leading to potential coal-rock gas dynamic disasters triggered by mining disturbances. These dynamic phenomena are influenced by factors such as loading mode, coal properties, and the presence of gas. To gain a comprehensive understanding of the mechanical properties, deformation, and failure characteristics of gas-containing coal under cyclic loading, we conducted uniaxial compression tests. These tests varied in loading frequencies, amplitudes, and durations. By analyzing the peak stress variation of gas-containing coal and utilizing digital image correlation (DIC) technology, we captured the deformation characteristics of the loaded coal surface. Following the tests, we examined the fragmentation degree of gas-containing coal under different cyclic loading using fractal theory. This involved screening and crushing samples to assess the impact of varying loading on coal fragmentation. The results showed that peak stress is positively correlated with loading frequency and negatively correlated with loading amplitude and the number of cycles. Cyclic loading significantly affects the surface deformation morphology of gas-containing coal, and there is a correlation between the stress level of the coal sample and its surface deformation, with the formation and development of cracks corresponding to the stress level. Fractal theory can analyze the crushing characteristics of materials and quantitatively characterize their degree of crushing, and the fractal dimension is closely related to the mode of cyclic loading and comprehensively reflects various experimental factors. The results of our study aim to provide insights that can guide the prevention and control of coal mine dynamic disasters.

During CO2 sequestration in coal seams, coal stability is affected by CO2 adsorption, three-dimensional in situ stress, and mining-induced disturbances. In this study, three-dimensional impact tests were conducted on coal under different CO2 pressures and confining pressures using a self-developed high-temperature and high-pressure gas-bearing coal–rock dynamic–static combined loading system. X-ray computed tomography (CT) was employed to characterize fracture evolution before and after CO2 exposure. The results demonstrate that CO2 exposure promotes the propagation of primary fractures and the formation of new fractures while reducing the dynamic peak strength and dynamic elastic modulus of coal. In contrast, confining pressure suppresses fracture propagation and enhances the dynamic load-bearing capacity of coal. On this basis, a damage constitutive model for CO2-exposed coal under three-dimensional stress was developed by coupling CO2 adsorption-induced damage with mechanical loading-induced damage. The model effectively describes prepeak deformation, damage evolution, and peak strength and provides a partial representation of the postpeak stress attenuation trend.