Variations in coal-seam thickness affect the load-bearing structure and electrical response of coal–rock composites, contributing to dynamic disasters in coal-thickness variation zones. To investigate failure mechanisms and resistivity precursors, conductive coal–rock analogue materials with similar mechanical properties were prepared. Composite specimens with rock-to-coal thickness ratios of 1:1, 1:2, and 1:3 were tested under uniaxial compression with synchronous resistivity monitoring, and PFC3D simulations were used to analyze crack evolution and damage accumulation. As the coal proportion increased, peak strength decreased from 18.73 to 15.62 MPa, and failure strain decreased from 2.45 to 2.20%. The total input energy and elastic energy storage capacity also declined, indicating weakened load-bearing and energy storage capacities and earlier damage localization. During loading, resistivity evolves through compaction‑induced reduction, elastic stabilization, pre‑peak rise and failure fluctuation. The failure-stage resistivity increased by 1.27–1.33 times, and the pre-peak resistivity rise preceded the macroscopic stress drop, providing a precursor window for instability identification. PFC3D results reproduced the experimental responses and failure modes, showing that increased coal thickness promoted earlier crack clustering and coalescence, with shear cracks dominating near peak stress. The normalized damage variable based on broken particle bonds corresponded well with resistivity evolution. A resistivity increment index was developed to quantify the transition from stable fluctuation to sustained increase, enabling identification of the precursor stage from stable crack growth to accelerated unstable coalescence. These findings provide a basis for failure-mechanism analysis and resistivity-based warning in coal-thickness variation zones.
Overburden rotation and mining disturbance promote deep coal fracture and spalling, potentially inducing dynamic hazards, while the coal–rock thickness ratio largely governs the composite’s dynamic response and failure severity. Based on the geological conditions of a typical deep coal mine, SHPB tests were performed on coal–sandstone composite specimens with four systematically varied coal-to-rock ratios (C:R = 1:0, 2:1, 1:1, and 1:2). High-speed photography and the digital speckle correlation method (DIC) were integrated to capture displacement, strain, energy, and fragmentation fields throughout progressive dynamic compression. Experimental data revealed four findings: (1) Crack development follows a sequential evolution process of initiation, propagation, and failure. Higher impact air pressure accelerates crack development and coalescence, resulting in a higher degree of specimen fragmentation and fewer large blocks. Within the tested range of coal–rock ratios, an increase in the rock proportion accelerates coal fracture, which is attributed to the significantly higher density and hardness of rock compared to coal. (2) Energy evolution consistently follows three stages: absorption, accumulation, and dissipation. Under identical impact pressure, a higher rock ratio elevates equivalent stiffness and wave impedance, leading to monotonic increases in peak stress, peak strain, absorbed energy, and dissipated energy. (3) The fragmentation degree exhibits a pronounced dependence on impact pressure. Specifically, for the pure coal specimen (C:R = 1:0), when the impact pressure increases from 0.3 MPa to 0.7 MPa, the mass percentage of coarse debris (>30 mm) drops from 73.37% to 18.57%, whereas that of fine particles (<4 mm) rises from 15.69% to 35.24%. (4) Under identical impact conditions, a higher rock proportion leads to increasing trends in all measured indicators, including peak stress, strain, and energy accumulation and dissipation, which are consistent with the superior mechanical properties of the rock. Based on these observations, it can be inferred that the wave impedance mismatch and stiffness ratio at the coal–rock interface play a key role in controlling stress wave transmission/reflection and strain incompatibility; however, the individual contribution of each factor warrants further dedicated investigation.
Jiaxin Dang, Jianwei Li, Min Tu et al.· Fractal and Fractional· 0 citations
During underground excavation, structural instability and failure of coal-rock masses are major causes of engineering accidents. This study focuses on the coal mass ahead of the excavation face and divides into a multi-layer composite structure to systematically analyze the mechanical response characteristics of the combined coal mass under static and dynamic loading. Drop hammer impact tests were conducted to investigate the crack propagation and failure morphology of the combined coal mass. The results show that, under static loading, single-layer coal mainly exhibited X-shaped shear failure, whereas the combined coal mass showed V-shaped layered spalling failure. Under dynamic loading, compressive failure dominated in the central region, while shear failure mainly occurred at the four corners, and the damage was more pronounced along the minor axis direction. Two crack patterns were observed on the coal surface, simple radial cracks and complex intersecting radial and circumferential cracks. Under the combined effects of rectangular geometry and impact loading, the radial cracks of the specimen show a structural tendency to preferentially propagate along the minor axis direction. Under impact loading, the combined coal mass exhibited an overall spindle-shaped failure morphology in the axial direction. In the damage zone, the superposition of tensile stress and energy accumulation made the coal mass more susceptible to failure, resulting in a greater number of radial and circumferential cracks. These findings provide a scientific theoretical basis for revealing the incubation mechanisms of coal-rock dynamic disasters and optimizing disaster prevention and control measures under mining-induced disturbance conditions.
Feng Li, Boyin Xu, Bolong Wang et al.· Engineering Research Express· 0 citations
The load-bearing and damage characteristics of coal seams vary under different geological conditions. To investigate the failure characteristics of coal bodies under hard roof conditions, this study is set against the backdrop of the Zhangji Coal Mine's 11,129 work face. By employing a combination of physical experiments, numerical analysis, theoretical calculations, and field validation, we explore the load-bearing capacity of coal bodies and their damage characteristics. The research findings indicate the following: (1) During the coal seam extraction process, the thick hard sandstone roof has a large span and exhibits minimal deformation, thus providing support for the overlying weak rock strata within a certain range. Following the implementation of pre-splitting blasting, the initial fracture step distance of the roof was reduced to 45 m, while the periodic fracture distance decreased to between 10 and 30 m. (2) The coal rock exhibits energy accumulation and dissipation at the moment of bearing stress or fracture. When the roof initially collapses, the peak stress within the coal body reaches 33.1 MPa, with a stress concentration coefficient of 1.7. The compressive displacement of the coal body within the stress concentration zone ranges from 2.8 to 12.1 cm. Energy accumulates within the coal body, reaching a maximum value of 124 kJ/m
3
, while the energy of the load-bearing layer above the goaf accumulates between 62.3 and 103 kJ/m
3
. (3) Based on the mechanical equations governing the load-bearing state of coal rock, a quantitative analysis was conducted on how four indicators—fracture step distance, distance from the rock layer to the work face, elastic modulus of the rock layer, and thickness of the rock layer—affect the coal body under dynamic and static load conditions, revealing the primary influencing factors. (4) Considering the geological conditions of the 11,129 work face, a pre-splitting blasting depressurization scheme was proposed using a fan-shaped hole grouping arrangement to reduce the fracture step distance of the hard rock layer. Upon the initial fracture of the roof, the support pressure in the lower section of the work face was consistently between 18.8 and 22.7 MPa. When the work face advanced through the pre-splitting blasting area (exceeding 700 m), the pressure in the middle support increased from 14.7 to 18.3 MPa to between 30.0 and 32.3 MPa, with a maximum recorded support pressure of 35.3 MPa. Field data indicate that the pre-splitting blasting scheme for the work face achieved the desired results. The study outcomes provide insight into the overall failure modes of coal rock bodies under the geological conditions of directly overlying hard roofs, thereby offering references for further investigation into the load-bearing characteristics of coal bodies and the calculation of crack propagation.
Jiaxin Dang, Jianwei Li, Min Tu et al.· Geomechanics and Geophysics...· 0 citations
The heterogeneous occurrence of coal-seam thickness represents a common geological characteristic in underground mining. Variations in coal thickness can directly alter the instability-failure behavior of coal–rock systems, thereby triggering various dynamic disasters. Therefore, revealing failure and disaster-inducing mechanisms of coal–rock systems dominated by coal-thickness effects is critical for deep mining engineering design as well as dynamic disaster prevention and control. To this end, uniaxial compression tests combined with acoustic emission (AE) monitoring were performed on coal–rock combinations with different coal thicknesses. The evolution laws of characteristic strengths (uniaxial compressive strength, initiation strength, and damage strength) versus coal thickness were systematically analyzed. Using full-process spatial localization of internal damage derived from absolute AE energy, an instability evolution model for coal–rock combinations was established. Furthermore, intrinsic disaster-inducing mechanisms governing coal–rock system instability under coal-thickness regulation were summarized, with corresponding engineering prevention-control suggestions put forward. The results show that: (1) UCS, initiation strength, and damage strength of specimens exhibit a nonlinear negative correlation with coal thickness. Initiation strength and damage strength account for approximately 50% and 75% of UCS, respectively; (2) Increasing coal thickness weakens the confinement effect of upper- and lower-sandstone, which shifts the dominant failure zone gradually from coal–rock interfaces to coal interiors. Meanwhile, internal energy accumulation-release processes of combinations present staged evolution characteristics; (3) Different coal thicknesses produce distinct disaster-evolution paths for coal–rock systems. Larger coal thickness corresponds to higher risks of high-energy dynamic disasters. Accordingly, a differentiated hierarchical prevention strategy of “thin protection, medium pressure relief, and thick control” was proposed. These findings provide a theoretical basis for mine engineering design and dynamic disaster prevention-control under dominant coal-thickness effects.
Baochen Wang, Yanwei Duan, Kai Ren et al.· Processes· 0 citations
Investigating the mechanics-permeability similarities between natural-like and natural gas-bearing coal–rock specimens provides a theoretical basis for using natural-like specimens as substitutes for natural ones in laboratory simulations of the incubation of coal–rock gas composite dynamic disasters. Based on the similarity between coal and rock in uniaxial compressive strength ratio, natural-like coal–rock specimens were prepared; their mechanics-permeability responses were analyzed through uniaxial and triaxial tests. Both specimen types underwent brittle failure under uniaxial compression, with compressive strength falling between those of coal and rock components but closer to that of coal. Under loading axial stress (LAS), bearing capacity was directly proportional to confining stress (σ3) for both specimen types—at instability failure, both axial and radial strains increased with σ3, whereas the axial-to-radial strain ratio decreased; under unloading confining stress (UCS), both specimen types showed reductions in compressive strength and axial strain at peak strength, along with an increase in radial strain, reflecting pronounced dilatancy. Overall, the natural coal–rock specimen (NRCS) and natural-like coal–rock specimen (NLRCS) exhibited similar mechanics-permeability patterns under both uniaxial and triaxial tests, suggesting that natural-like specimens can serve as substitutes for natural ones in laboratory simulations.
Hard roof strata in deep coal mines commonly cause rib failure and roof collapse, restricting extraction efficiency. This study employs theoretical analysis, numerical simulation, and field experiments to investigate the fractal evolution of damage in coal–rock mass under loading and blasting disturbances, with the aim of quantifying progressive failure and optimizing roof control. Key findings include: (1) The fractal dimension D of fragment size distribution increases monotonically with loading rate, with fine-particle proportion rising from 48.5% to 52.3%, indicating more thorough fragmentation at higher rates. (2) Load intensity, elastic modulus, and seam thickness govern coal bearing capacity and energy accumulation, with D serving as a quantitative damage indicator. (3) Pre-splitting blasting shifts the stress peak away from the working face, with shear fractures dominating the fracture network and tensile fractures playing a secondary role. (4) Field application at Zhangji Coal Mine (9 coal seam, 7–23.5 m sandstone roof) confirms the effectiveness of segmented fan-shaped borehole pre-splitting blasting in controlling roof behavior; fractal dimension derived from borehole images quantifies fracture propagation. Dynamic adjustment of blasting parameters based on geological core samples is recommended to enhance fracture network complexity and improve roof control efficiency under varying hard rock conditions.
Jiaxin Dang, Jianwei Li, Min Tu et al.· Fractal and Fractional· 0 citations
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