Fractal Characterization of Stress–Energy Response and Progressive Damage in Coal–Rock Mass Before and After Pre-Splitting Blasting of Hard Roof: From Laboratory Fragmentation to Field Fractures
Aug 2026· Fractal and Fractional· 0 citations· 23 references
Abstract
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.
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
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.
Chenglin Tian, Jia-Li Liu, Jian-Lei Liu et al.· Scientific Reports· 0 citations
Understanding the anisotropic mechanical behavior and pore–fracture structure (PFS) evolution of bedded coal under mining disturbance is critical for gas extraction and hazard prevention in deep coal mining. This study employed low-field nuclear magnetic resonance (NMR) and fractal analysis to characterize the PFS of water-saturated coal samples with bedding angles of 0°, 30°, 45°, 60°, and 90°. The pore system was classified into adsorption and seepage pores according to pore size distribution. Real-time triaxial NMR tests were further conducted to reveal the coupled evolution of mechanical responses and PFS under different bedding orientations. Results show that bedding inclination controls pore distribution, connectivity, and structural complexity, while influencing coal strength, deformation, and failure through stress redistribution and bedding-plane activation. The mechanical response and PFS evolution exhibit strong anisotropic coupling during loading. A fractional-order porosity model was established by incorporating bedding orientation, anisotropy, and stress memory based on pore geometry and stress decomposition. Model verification confirms its effectiveness in describing anisotropic porosity and PFS evolution under varying bedding angles. This study provides theoretical support for permeability prediction, stability assessment, and hazard control in deep-bedded coal seams.
Jun Wang, Zixiong Qi, Weiyuan Mou et al.· Fractal and Fractional· 0 citations
Fracture evolution of rock–backfill composites (RBCs) under complex loading–unloading and dynamic disturbances is critical for stope stability in deep backfill mining. Using PFC3D, this study constructs numerical models of RBCs to investigate this process, considering burial depths (500, 1000, 1500, 2000 m), interface angles (IA: 60°, 90°), and cement–tailings ratios (CTR—1:4, 1:8), while replicating true triaxial paths and blasting impacts. Systematic analysis of mesoscopic crack quantity, spatiotemporal distribution, and multiscale fracturing reveals that shear cracks dominate damage, with crack counts evolving in stages as strain increases. With greater depth, the number of propagation stages and growth rate inflection points shift systematically. During mining–filling disturbance, crack quantity negatively correlates with depth but turns positive during late static loading beyond 70% peak stress. Spatial crack distribution is synergistically controlled by IA, CTR, and depth. For IA 60°, shear crack angles spread broadly yet concentrate at 50–70°; for IA 90°, they are near-axial, concentrated at 80–90°. The synergistic process progresses through microscopic initiation, mesoscopic accumulation, and macroscopic instability. In terms of failure modes, IA 60° exhibits shear failure along the cemented interface plus tensile fracturing in rock, while IA 90° shows combined diagonal shear and axial tension. Higher CTR yields more extensive fracture networks in backfill, indicating superior synergistic bearing capacity.
Hongjian Lu, Zhaoyang Ren, Fan Jiang· Minerals· 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
Targeting the rock burst hazard induced by the hard roof key stratum during deep mining at the Mengcun Coal Mine in the Binchang mining area, this study takes the No. 403109 working face as the engineering background and systematically investigates the rockburst prevention mechanism and effectiveness of ground hydraulic fracturing through theoretical analysis, UDEC numerical simulation, and surface microseismic monitoring. The results indicate that fracturing pre-weakens the overlying key stratum, transforming its load-bearing mode from a long-beam rigid support to a segmented flexible support. This significantly reduces the cantilever length, lowers the accumulation of elastic strain energy, and enables flexible load transfer and stress redistribution in the overburden. Numerical simulations reveal that after fracturing, the breakage timing of the key stratum advances, the fragmentation size decreases, and the over-burden movement shifts from stepwise fracturing to sequential caving, with the stress concentration zone substantially narrowed. In the field, a total of 44 fracturing stages were implemented in wells MC-05L and MC-06L, creating a fracture network with an average fracture length of 317 m and an average fracture height of 55 m, achieving an effective stimulated volume ratio of 86.7%. During the mining period, microseismic events exhibited a median energy of only 868.14 J, characterized by high frequency and low energy. The average weighting interval was 13.69 m, the peak coal stress was controlled within 5.0–6.7 MPa, and the loads on roadway bolts and cables remained within safe limits. This study validates the source-control effect of ground hydraulic fracturing on working faces with strong rock burst risks in deep mining, providing a theoretical basis and engineering reference for mines with analogous conditions.
Weixin Zhang, Hailong Xiangli, Hongli Song et al.· Energies· 0 citations
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