Understanding the creep deformation behavior and the evolution of pore-fracture structures (FPSs) in coal under cyclic loading–unloading is crucial for safe extraction and efficient methane utilization. Coal samples were subjected to cyclic loading–unloading creep experiments using online Nuclear Magnetic Resonance (NMR) and Nuclear Magnetic Resonance Imaging (NMRI) techniques. The results revealed the significance of instantaneous plastic and viscoplastic strains during creep, with creep failure modes analyzed using NMRI data and macroscopic fracture distribution. Viscoplastic strain was identified as a key indicator of accelerated failure, and NMRI revealed a transition from splitting–shear to V-shaped shear failure under increasing confining pressure. From a microscopic perspective, real-time T₂ spectra monitoring tracked the evolution of FPS at different loading levels, and the geometric mean of the pore structure (T_2g) quantitatively described the co-evolution of various pore types. A generalized model was developed to describe coal creep under cyclic loading–unloading, integrating microscopic and macroscopic deformation features and refined using fractal theory. These findings provide theoretical insights and practical guidance for coal extraction and methane management under cyclic loading–unloading creep.

Brittleness Index (BI), while not universally standardized, is one of the most critical parameters in the assessment of rock failure behavior, drillability, and excavation efficiency in mining; however, its applicability to extraterrestrial environments remains poorly constrained for the In-Situ Resource Utilization (ISRU) mission on Mars. This study qualitatively and quantitatively investigated the mechanistic attributes (compressive strength, σ_c; tensile strength, σ_t) and BI of simulated Martian rocks under distinct conditions. Martian analog rock specimens, developed using Mars Global Simulant, were subjected to controlled bulk-scale mechanical tests (uniaxial compression and Brazilian disc tests) to determine σ_c and σ_t, supported by SEM-EDS analyses, to validate mineralogical similarity with Martian samples. Results revealed that the simulated Martian rocks are representative of Jezero crater lithologies and exhibit behavior characterized by linear elastic deformation followed by abrupt failure under stress. A gravity-modified brittleness index (BI_M1, BI_M2, BI_M3, and BI_M4) was proposed, which yielded lower brittleness thresholds consistent with a mechanically weaker Martian lithosphere. Further, the Martian-specific brittleness classification indicated that the analog Martian rocks fall predominantly within low-to-moderately brittle categories (0.1<BI_M4<9.3) under Martian conditions, suggesting favorable drillability and relatively low energy requirements for excavation. These findings offer novel insights into the feasibility of predicting Martian excavation performance for ISRU.

Understanding aggregate effects within fault fracture zones is crucial for assessing fault reactivation risks in grouted deep coal mines. This study investigates the compressive-shear damage mechanisms of grouted concrete specimens with varying aggregate contents (20%, 40%, 60%) and particle sizes (1–5 mm, 6–10 mm). Uniaxial compressive-shear tests were monitored synchronously using digital image correlation (DIC) and acoustic emission (AE). Additionally, a calibrated PFC2D discrete element model, incorporating matrix, aggregate, and interfacial transition zones, elucidated the mesoscale mechanisms. Results indicate: (1) peak shear load decreases linearly (R² >0.86) with increasing aggregate content, dropping 64.3% from 20% to 60%, driving a transition from brittle to plastic failure; (2) AE b-value evolution tracks progressive damage, while damage rate k exhibits contrasting size-dependent trends; (3) macroscopic failure modes are synergistically controlled by aggregate size and content; and (4) numerical simulations validate the mesoscale mechanical origins of this brittle-to-plastic transition. These findings reveal the micro-mechanical mechanisms of anisotropic failure and re-strengthening in grouted fault materials, offering vital geological insights into stress evolution and instability precursors during fault reactivation.

The dynamic evolution of in-situ stress during coalbed methane drainage governs coal seam stability, and instability-induced damage critically impacts well productivity. Triaxial deformation and methane adsorption-induced expansion experiments were conducted to investigate the stability evolution mechanisms and controlling factors in mid-deep intact and fractured seams. Results show that permeability surges at the peak stress, then declines sharply and eventually stabilizes. Adsorption pressure correlates positively with adsorption-induced strain, with the maximum volumetric strain of 1.756%. Fractured zones are more prone to instability failure than intact coal seams. Instability failure occurs under normal faulting stress regimes with moderate-to-strong desorption capacity and strike-slip stress regimes with strong desorption capacity. Seams under reverse faulting regimes remain stable. Stress difference, critical desorption pressure, elastic modulus, internal friction angle, cohesion, and fault friction coefficient are inversely related to coal seam stability. In late-stage production, failure of faulted zones in the #2 and #4 coal seams occurred at pore pressures of 1.17 and 1.70 MPa, respectively. These failures induced five abrupt bottom-hole pressure drops, each lasting 26–54 min, severely compromising well productivity. Pore pressure inversion yields fracture-wellbore distances of 15–111 m.

Under close-distance coal seam (CDCS) mining conditions, surrounding rock failure in gradient offset roadways exhibits pronounced zonal heterogeneity and complex evolutionary behavior. However, existing studies lack a systematic understanding of zonal failure mechanisms and effective full-length, quantitative identification methods, which limits the precise matching between support strategies and surrounding rock failure characteristics. To address this issue, a mechanical model for principal stress distribution in the goaf floor is established, clarifying the spatial variation of principal stresses under different offset distances and providing explicit boundary conditions for plastic zone analysis. Considering the implicit nature of the plastic zone boundary equation and the difficulty of analytical integration, a quantitative calculation framework combining polar coordinate discretization and numerical integration is proposed to determine the plastic zone area and maximum failure depth. Based on this framework, the coupled control mechanism of the principal stress ratio (g) and the orientation of the maximum principal stress (a) on zonal failure evolution is quantitatively revealed. The results indicate that continuous variations in offset distance induce the coupled evolution of g and a, which govern the expansion scale, failure depth, and deflection characteristics of the plastic zone. Furthermore, a geophysical-borehole joint inversion method (GBJIM) is proposed for refined identification of surrounding rock failure zones. The method achieves relative inversion errors of 0.59%–3.37%, satisfying engineering accuracy requirements, and enables continuous, full-length characterization of roadway surrounding rock failure. The inversion results reveal significant spatial variability in failure depth, which undergoes a rapid decrease, followed by an increase, a gradual reduction, and eventual stabilization with varying offset distance, and show good agreement with numerical simulation results. Based on the obtained precise zoning results, a zonal support optimization strategy for the full-length roadway is developed and validated through field application.

Achieving carbon neutrality and large-scale industrial waste utilization requires low-carbon mine backfill materials. This study investigates a strategy to enhance cement-fly ash based composites using CO₂ nanobubble water. Normal cement-fly ash based backfill and CO₂ nanobubble-modified cement-fly ash based backfill were compared through mechanical and microstructural analyses, including uniaxial compression, mercury intrusion porosimetry, scanning electron microscopy and thermogravimetric analysis. The results demonstrate that CO₂ nanobubbles effectively mitigate the strength degradation induced by high fly ash replacement. Compared with normal backfill, the uniaxial compressive strength and elastic modulus of modified samples increased by 6.5%–13.4% and 14.8%–59.1%, respectively, enabling high-volume fly ash utilization without compromising mechanical integrity. Microstructural analyses reveal that CO₂ nanobubble water promotes hydration and in-situ carbonation reactions, leading to the formation of uniformly distributed C-S-H gels and calcium carbonate crystals that refine the pore structure and reduce total porosity by approximately 20%. Thermogravimetric results further confirm that CO₂ nanobubble significantly enhance carbonation efficiency, with the maximum carbonation degree reaching 13.07% at a fly ash content of 60%. Balancing performance and cost, the optimal fly ash content is identified within 20%–60%, providing a green pathway for mining waste valorization.

Accurately determining the effective fracture toughness (K_eff) of rock-concrete (R-C) bi-materials, governed by interface inclination and ambient temperature, is a prerequisite for assessing their structural stability. This study developed a hybrid NRBO-XGBoost prediction model using the Newton-Raphson-Based Optimizer (NRBO) to tune the hyperparameters of Extreme Gradient Boosting (XGBoost) model. The established model was developed based on 154 datasets obtained from laboratory tests and numerical simulations with the cracked straight-through Brazilian disc (CSTBD) specimens, including twelve input parameters. The NRBO-XGBoost model for K_eff prediction was investigated and compared with seven more models. Furthermore, the Shapley Additive exPlanations (SHAP) method was employed to quantify the contributions of inputs to K_eff to improve the interpretability of the developed model. Finally, new data were used to validate the model. Evaluation results demonstrate that metaheuristic optimization algorithms significantly enhance the performance of XGBoost, with NRBO-XGBoost performing the best. The models rank from highest to lowest prediction performance as follows: NRBO-XGBoost, WOA-XGBoost, PSO-XGBoost, XGBoost, RF, CatBoost, LightGBM, and AdaBoost. The interpretable analysis shows that the interface inclination angle exerts the dominant influence. The validation results demonstrate that NRBO-XGBoost achieves high predictive accuracy on a new dataset, showing promising implications for practical applications.

In composite coal seams, the gas transfer relationship of structural coal has a significant impact on gas extraction. Regarding the influence mechanism of multi-scale mass transfer (MSMT) of tectonic coal in composite coal seams on gas extraction, the paper has constructed a corresponding multi-physics field coupling model, which systematically investigates gas migration patterns in composite coal seams and the dominant control mechanisms during coal mining. Results indicate that gas extraction efficiency in tectonic coal exhibits significant sensitivity to key parameters, including Poisson’s ratio, matrix elastic modulus, and extraction negative pressure. This reveals the synergistic extraction mechanism in composite coalbeds: highly permeable primary coal dominates pressure gradient formation during initial extraction, effectively driving gas desorption and cross-scale migration in adjacent tectonic coal. However, the low permeability of tectonic coal causes slow pressure decay, becoming a critical bottleneck constraining overall extraction efficiency. Based on this mechanism, a phased differentiated technical approach was proposed that involves hydraulic fracturing to enhance permeability in primary coal combined with CO₂ displacement and stepwise negative pressure extraction for tectonic coal. These findings provide crucial guidance for optimizing extraction design and achieving efficient, safe mining in composite coal seams.

It is challenging to remove feldspar from spodumene at low temperature due to the poor collecting ability and selectivity of conventional collector. In this work, Glycolic acid ethoxylate oleyl ether (GAEOE) was evaluated as a low-temperature resistant collector in the flotation separation of spodumene and feldspar. The flotation performances and the interfacial interaction mechanisms were comprehensively investigated. Micro-flotation experiments showed that GAEOE could realize the efficient separation between spodumene and feldspar at low temperature, producing high-quality concentrate with Li₂O grade and recovery reached 6.02% and 81.22%, respectively. Contact angle, induction time and bubble-mineral interaction force measurements proved that GAEOE significantly increased the adhesion force and attachment probability between air bubbles and spodumene, while it exerted negligible influence on feldspar. Zeta potentials and in situ microcalorimetry tests confirmed the stronger affinity of GAEOE toward Ca-activated spodumene than feldspar. Adsorption capacity and turbidity measurements revealed that GAEOE exhibited more stable adsorption with low sensitivity to temperature changes than sodium oleate (NaOL), due to the better solubility and superior dispersion stability at low temperature. XPS and DFT analysis revealed a stable bidentate configuration involving synergistic interactions between the O atoms of C–O and C＝O groups within GAEOE and Al as well as Ca sites on Ca-activated spodumene surface. These results suggested GAEOE was a promising alternative for conventional fatty acid collectors for spodumene flotation in temperature-sensitive environment.

To address the lack of a unified energy mapping framework for energy release and damage evolution in rocks, uniaxial compression tests were conducted on sandstone with typical pore configurations. Using acoustic emission (AE) and digital image correlation (DIC) techniques to characterize the energy release characteristics and strain evolution laws during rock fracture process. Based on the principle of energy conservation, the energy conversion efficiency α is defined to correlate AE energy with stored strain energy, and an energy type damage factor D is introduced to quantify the damage accumulation process. The results indicate that when D < 0.4, the damage evolution is relatively stable; When D ≥ 0.4, the damage rapidly intensifies, indicating the failure of the specimen. The study also found a significant time lag between AE energy release and macroscopic deformation, which is consistent with the law in engineering practice that microseismic activity precedes monitoring displacement response. Furthermore, a Lemaitre damage model incorporating α was established, which can effectively characterize the nonlinear evolution process of sandstone from elastic deformation to complete failure. Finally, an energy based rockburst warning index based on α and D was discussed, providing technical path for identifying high-risk areas and preventing rockburst disasters in mining engineering.

Static cracking demolition agent (SCDA) is an environmentally friendly and cost-effective material with broad potential for building demolition and rock fracturing engineering. Under expansive loading induced by SCDA, the directional fracture mechanisms of symmetrically slotted boreholes remain inadequately understood. Acoustic emission (AE) monitoring, digital image correlation (DIC), and numerical simulations were combined to systematically investigate the regulating mechanism of slot length on fracture behavior. The results indicate that as the slot length increases, the fracture modes transition from competitive propagation of multiple cracks to rapid through-going propagation of the main cracks along the slot direction. The competitive propagation of multiple cracks disperses the expansive energy among the propagating cracks, whereas rapid through-going propagation along the slot concentrates energy release in the slot direction. A critical slot length is identified at which energy release is most intense, causing the cumulative AE energy to first increase and then decrease with increasing slot length. The expansive pressure required for through-going of the main crack decreases progressively as the slot length increases. Once the main crack causes pressure release, the propagation of secondary cracks is suppressed due to insufficient driving force. These findings provide a theoretical basis for achieving controllable SCDA-induced directional fracturing.