The strong vertical discontinuities pose a fundamental challenge to optimizing stimulated reservoir volume (SRV) in multilayered reservoirs. This research proposes a radial borehole-assisted horizontal well fracturing technology, which is expected to achieve effective vertical stimulation and commingled production across multiple pay zones. Under different geological and engineering conditions, the vertical propagation behavior of hydraulic fractures guided by radial boreholes can be determined by adjusting the interlayered lithologies and radial borehole configurations in experimental samples. Experimental results reveal four fracture network patterns: passivated, cross-layer, skip-layer, and hybrid fractures in the radial borehole fracturing. The radial boreholes perform better fracture guiding performances in the high-brittleness interlayers, which form cross-layer and hybrid fracture networks to improve the growth height. Hydraulic fractures tend to propagate from high-strength to low-strength layers under radial borehole guidance. When radial boreholes interconnect multiple lithology layers, hydraulic fractures initiate preferentially in lower-strength zones rather than remaining confined to borehole root ends. Increased radial borehole length and diameter facilitate fracture skip-layer initiation and cross-layer propagation, while multiple borehole branches enhance fracture penetration across high-strength interlayers. Radial boreholes with inclination angles below 30° enhance fracture height by generating cross-layer and hybrid fracture networks. Furthermore, an inter-borehole phase angle of less than 180° facilitates single-wing fracture cross-layer propagation. Fracture height is primarily governed by radial borehole length, followed by quantity, inclination angle, and diameter. Based on the geometric similarity criteria, the recommended parameters for radial borehole-assisted fracturing in a 5½-inch horizontal well include a length > 15 m, an inclination angle < 30°, and a diameter > 52 mm to ensure effective stimulation across three or more pay zones. Finally, the field-scale numerical model was developed to simulate the optimized radial borehole fracturing and demonstrate the technical superiority. These findings are expected to provide an in-depth understanding of the effective stimulation in multilayered reservoirs.

Mine surveying is an indispensable and crucial basic technical work in the process of mineral resource development. It plays an important role throughout the entire life cycle of a mine, from exploration, design, construction, and production to closure, and is known as the ‘‘eyes of the mine”. With the rapid development of satellite technology, computer science, artificial intelligence, robotics, and spatiotemporal big data, mine surveying science and technology supported by spatial information technology is increasingly playing the role of the ‘‘brain of the mine”. This paper systematically summarizes the characteristics of mining surveying science and technology in contemporary and future mining development. First, based on the requirements of safe, efficient, and green development in modern mining, an analysis is conducted on the innovative practices of intelligent mining methods; secondly, it explains the transformation of regional economic and mining economic integration towards lengthening the industrial chain and scientific and technological innovation. Regarding intelligent mining, this paper discusses three technical dimensions: (1) By establishing a spatiotemporal data model of the mine, real-time perception and remote intelligent control of the production system are realized; (2) Based on the transparent mine three-dimensional geological modelling technology, the accuracy of geological condition prediction and the scientific nature of mining decisions are significantly improved; (3) By integrating multi-source remote sensing data and deep learning algorithms, a high-precision coal and rock identification system is constructed. The study further revealed the innovative application value of mine surveying in the post-mining era, including: diversified utilization of underground space in mining areas (tourism development, geothermal energy storage, pumped storage, etc.), multi-platform remote sensing coordinated ecological restoration monitoring, and optimized land space planning in mining areas. Practice has proved that mine surveying technology is an important technical engine for promoting green transformation and high-quality development in resource-based regions, and has irreplaceable strategic significance for achieving coordinated development of energy, economy, and environment.

Coal serves not only as a crucial energy resource but also as a significant reservoir of critical metal elements, including Lithium (Li), Gallium (Ga), Germanium (Ge), and rare earth elements (REE). This paper provides a systematic review of the enrichment characteristics, occurrence modes, and comprehensive utilization potential of these critical metals in coal. Globally, the distribution of these metal resources exhibits significant regional heterogeneity. While the concentration in most coals falls below industrial cut-off grades, anomalous enrichment in specific coal basins results in Li, Ga, Ge, and REE concentrations far exceeding global averages, highlighting their considerable potential as unconventional metal deposits. The occurrence modes of these metals are diverse: Li is primarily hosted in mineral phases; Ga exists in inorganic, organic, and complex forms; Ge shows a strong association with organic matter; and REE are mainly present in adsorbed/isomorphic forms within clay minerals, while also displaying organic affinity. Direct extraction of metals from raw coal is often cost-prohibitive; effective recovery is therefore more feasible when integrated with coal processing. Metals are further enriched in solid wastes such as coal gangue, fly ash, and bottom ash, from which recovery is more economically and technically viable. Current comprehensive utilization primarily employs integrated mineral processing-hydrometallurgy approaches. Future research should focus on elucidating the precise occurrence forms of metals in coal and solid wastes, optimizing pre-treatment methods, and selecting effective activators and leachants. Advancing the synergistic extraction and green recovery of multiple associated resources from coal and its by-products is essential for achieving high-value, comprehensive utilization of coal-based resources.

To improve the accuracy of rockburst risk evaluation in mining and tunnelling engineering, the influence of intermediate principal stress σ₂ deserves further consideration, which has been neglected in general prediction frameworks. This study employs an integrated approach that combines true-triaxial unloading experiments with three-dimensional grain-based discrete element modeling (PFC3D-GBM) to examine the effects of σ₂ on strainburst systematically and elucidate the underlying mechanisms. Through this dual experimental–numerical methodology, the strainburst characteristics under varying σ₂ are analyzed in detail regarding mechanical responses, failure evolution and patterns, microscope fracture mechanisms, and energy partitioning. The results indicate that elevated σ₂ can enhance the bearing capacity of rock, thereby necessitating a higher stress condition required for strainburst. However, it also enlarges the potential strainburst intensity, manifesting as deeper rockburst pits and more violent ejection of rock fragments. An increasing σ₂ facilitates the microscope transgranular fractures, inhibits intergranular tensile fractures, and raises the kinetic energy conversion ratio slightly. It affects the intensity of strainburst through the following mechanisms, including the increase of energy storage limit, the intensification of Poisson effect for lateral expansion, and the enhancement of the transgranular fracturing mechanism. In practical engineering, the depth and range of support needs to be ensured under high σ₂ conditions, and it is recommended to use prestressing techniques to control the development of significant slabbing.

To address the key scientific challenge of monitoring the dynamic fracturing of surrounding rock in deep roadways, this study systematically investigates the quantitative relationship between stress and charge signals during coal mass loading. By integrating innovative analytical approaches, introducing quantitative evaluation indices, and developing a charge–stress inversion model, and incorporating underground monitoring practices, significant progress has been achieved in elucidating the correlation between stress variations and charge signals throughout the entire coal mass fracturing process. First, in the field of stress–charge correlation analysis, empirical mode decomposition (EMD) was combined with wavelet coherence analysis for the first time, enabling the removal of slow-varying stress trends while retaining high-frequency fluctuations. This approach allowed for the quantitative characterization of the evolution of coherence between stress variations and charge fluctuations across multiple time scales. Second, coherence skewness and the proportion of high-coherence intervals were innovatively introduced to examine the influence of time scale selection on correlation results. On this basis, a criterion for determining the near-optimal observation scale of charge signals was proposed, providing a quantitative reference for time scale selection in similar signal analyses. Finally, by correlating charge signals with coal damage factors and stress states, a charge-based damage evolution equation was established to achieve effective stress inversion. Combined with in situ monitoring of stress and charge in roadway surrounding rock, this approach revealed the correlation characteristics of stress and charge intensity responses during the dynamic fracturing process. The results indicate, first, that charge signals are not significantly correlated with the absolute stress level of coal but are directly associated with stress variations following coal damage and failure, with the amplitude of charge fluctuations increasing alongside stress fluctuations. Second, coherence between stress and charge signals varies markedly across time scales, with excessively small or large scales leading to distortion, and the scale corresponding to the peak proportion of intervals with coherence >0.8 was identified as the near-optimal observation scale. Third, charge signals can effectively characterize coal damage factors, and the established damage evolution equation can effectively invert stress variation trends. Fourth, in underground roadways, zones of dynamic fracturing in surrounding rock are commonly located in areas where stress concentration overlaps with regions of high charge intensity, further confirming the strong consistency between charge and stress variations. These findings improve the theoretical framework of charge signal responses in loaded coal and provide a scientific basis for precise ‘‘stress-charge” monitoring of dynamic disasters, offering practical potential for engineering applications.

Water inrush hazards from the floor strata of longwall workingface are commonly encountered in North China coalfields, which essentially result from the evolution of permeability in the floor rock under complex mining-induced stress conditions. Current research rarely addresses the evolution of rock permeability under such complex stress paths. Describing this evolution using only one stress parameter, such as effective stress, deviatoric stress, axial stress, or confining stress, is highly challenging. In this study, we developed a laboratory loading scheme that simulates mining-induced stress evolution. Hydro-mechanical experiments were conducted to investigate the evolution of rock permeability under mining stress. The mechanism on the change of stress-permeability relationships in mining-disturbed rock is revealed, supporting to the analysis of management strategies for floor water-inrush disasters. The results show that rock permeability evolves through four stages, including rapid decline, gradual fluctuation, sharp increase, and slow attenuation. 1–2 permeability surges occurred during mining-stress loading, closely linked to the emergence and reversal of deviatoric stress in magnitude and direction. With the first permeability surge, the deviatoric stress within the mudstone reached approximately 1.7 MPa, whereas that of the sandstone was about 1 MPa. The second permeability surge in the mudstone corresponded to the secondary rotation of the principal stress direction. CT and ultrasonic tests suggested an increase in microcracks in both rocks during the first permeability surge. However, the deviatoric stress-permeability plot before and after mining indicated that the fracture of mudstone sample changed significantly, while that of the sandstone remained unchanged. The permeability surges observed at different stages are interpreted as resulting from shear-induced reopening of pre-existing fractures and the formation of new shear-failure fractures. A stress-permeability model jointly governed by effective mean stress and deviatoric stress was established. Furthermore, two strategies are proposed for the floor water-inrush disasters prevention, (i) timely backfilling to reduce deviatoric stress, (ii) grouting after the first permeability surge. This work provides insights into stress-seepage behavior in rocks under complex stress evolution and offers new perspectives for identifying potential water inrush pathways in the floor strata of coal seam during longwall mining.

Coal mine underground reservoirs help address the severe water imbalance in ecologically fragile mining regions of western China, but evaluating their storage capacity remains challenging due to the coupled effects of gangue deformation, saturation, and goaf geometry. This study investigates the deformation and void evolution of fragmented gangue with varying lithologies, particle sizes, and water contents through an independent-developed testing system and theoretical model. A planar micro-unit model and a three-dimensional spatial structure model are proposed to quantify the storage coefficient and total reservoir capacity of underground water storage structures. These models incorporate the effects of stratified lithologies, saturation-induced softening, and spatially distributed stress conditions. The methodology is applied to the underground reservoir in Chahasu coal mine, and the results show that under increasing stress, storage coefficients decline exponentially, with pronounced differences between single- and double-lithology structures. The storage coefficient in the spatial model demonstrate greater resilience to stress concentration compared to planar models, and further analysis identifies critical thresholds in roof fracture distances and stress-recovery times affecting long-term storage performance. This research provides a comprehensive framework for evaluating underground reservoir storage potential, offering theoretical support and engineering guidance for the sustainable utilization of mine water.

This study examined non-uniform loading in goaf cantilever rock masses via testing, modeling, and mechanical analysis to solve instantaneous fracture and section buckling from mining abutment pressure. The study investigates the non-uniform load gradient effect on fracture characteristics, including load characteristics, fracture location, fracture distribution, and section roughness. A digital model for fracture interface buckling analysis was developed, elucidating the influence of non-uniform load gradients on Fracture Interface Curvature (FIC), Buckling Rate of Change (BRC), and Buckling Domain Field (BDF). The findings reveal that nonlinear tensile stress concentration and abrupt tensile-compressive-shear strain mutations under non-uniform loading are fundamental mechanisms driving fracture path buckling in cantilever rock mass structures. The buckling process of rock mass under non-uniform load can be divided into two stages: low load gradient and high gradient load. In the stage of low gradient load, the buckling behavior is mainly reflected in the compression-shear fracture of the edge. In the stage of high gradient load, a buckling band along the loading direction is gradually formed in the rock mass. These buckling principles establish a theoretical basis for accurately characterizing bearing fractures, fracture interface instability, and vibration sources within overlying cantilever rock masses in goaf.

Thermal-mechanical damage and deformation at the interface between shotcrete linings and the surrounding rock of tunnels under high-temperature and variable-temperature conditions are critical to the safe construction and operation of tunnel engineering. This study investigated the thermo-mechanical damage behavior of the composite interface between alkali-resistant glass fiber-reinforced concrete (ARGFRC) and granite, focusing on a plateau railway tunnel. Laboratory triaxial tests, laser scanning, XRD analysis, numerical simulations, and theoretical analyses were employed to investigate how different initial curing temperatures and joint roughness coefficient (JRC) influence interfacial damage behavior. The results indicate that an increase in interface roughness exacerbates the structural damage at the interface. At a JRC of 19.9 and a temperature of 70 °C, crack initiation in granite was notably restrained when the confining pressure rose from 7 MPa to 10 MPa. Roughness-induced stress distribution at the interface was notably altered, although this effect became less pronounced under high confining pressure conditions. Additionally, during high-temperature curing, thermal stress concentration at the tips of micro-convex protrusions on the granite surface induced microcracks in the adjacent ARGFRC matrix, followed by deformation. These findings provide practical guidelines for designing concrete support systems to ensure tunnel structural safety in high-altitude regions with harsh thermal environments.

An innovative real-time monitoring method for surrounding rock damage based on microseismic time-lapse double-difference tomography is proposed for delayed dynamic damage identification and insufficient detection of adverse geological conditions in deep-buried tunnel construction. The installation techniques for microseismic sensors were optimized by mounting sensors at bolt ends which significantly improves signal-to-noise ratio (SNR) and anti-interference capability compared to conventional borehole placement. Subsequently, a 3D wave velocity evolution model that incorporates construction-induced disturbances was established, enabling the first visualization of spatiotemporal variations in surrounding rock wave velocity. It finds significant wave velocity reduction near the tunnel face, with roof and floor damage zones extending 40–50 m; wave velocities approaching undisturbed levels at 15 m ahead of the working face and on the laterally undisturbed side; pronounced spatial asymmetry in wave velocity distribution—values on the left side exceed those on the right, with a clear stress concentration or transition zone located 10–15 m; and systematically lower velocities behind the face than in front, indicating asymmetric rock damage development. These results provide essential theoretical support and practical guidance for optimizing dynamic construction strategies, enabling real-time adjustment of support parameters, and establishing safety early warning systems in deep-buried tunnel engineering.

In deep coal mining, surrounding rock is subjected to both high in-situ stress and intense mining disturbances, leading to significant time-dependent behavior. Accurately capturing this behavior is essential for predicting long-term roadway stability, necessitating the development of a reliable constitutive creep model and numerical simulation approach. In this study, creep experiments were conducted on pre-damaged rock with varying initial damage levels to investigate the time-dependent mechanical properties. Based on the experimental results, an accelerated-creep criterion was proposed, and an elastic-viscoplastic creep damage model (EVPCD) was established that simultaneously considers the effects of time-dependent damage and instantaneous damage caused by stress disturbances on rock creep behavior. Subsequently, the effectiveness of the proposed creep model was verified using experimental data, and the secondary development of the EVPCD model was completed based on the FLAC3D platform. Following this, a long-term stability analysis method of deep surrounding rock that accounts for excavation-and mining-induced disturbances was proposed. Using the main roadway of Xutuan Coal Mine as a case study, numerical simulations were carried out to investigate the time-dependent deformation and failure characteristics of the surrounding rock following excavation and mining disturbance. Combined with on-site monitoring of the surrounding rock damage areas, the results indicate that the EVPCD outperforms the CVISC and Nishihara models in predicting the time-dependent behavior of deep surrounding rock.

As underground mining advances to greater depths, cemented paste backfill (CPB) is increasingly subjected to complex thermo-mechanical loading conditions, including multiaxial stress states and elevated temperatures. This study investigates the coupled effects of field-representative vertical self-weight and horizontal rockwall closure stresses, along with in-situ temperatures, on the mechanical behavior and pore water pressure (PWP) evolution of CPB. Experiments were conducted using a novel apparatus capable of controlling multiaxial stress and temperature during curing, replicating in-situ stress paths and thermal profiles typical of deep mine environments. Results show that multiaxial stress enhances CPB strength and stiffness by promoting denser particle packing, reducing porosity, and increasing frictional resistance. Elevated temperatures independently accelerate early-age cement hydration, further improving bond strength and stiffness. When combined, multiaxial stress and elevated temperature produce a synergistic enhancement in unconfined compressive strength (UCS) and elastic modulus, as confirmed by two-way ANOVA and synergy index analysis. PWP responses were also highly sensitive to thermo-mechanical conditions. The evolution of positive and negative PWP was governed by the interplay of thermal expansion, hydration-induced desaturation, and mechanical compaction. Multiaxial stress amplified early positive PWP and delayed its dissipation, whereas elevated temperature accelerated hydration and reduced pore pressure, leading to enhanced suction at later ages. A transient ‘‘stress-induced resaturation” effect was observed under late-stage excessive horizontal stress but was mitigated by elevated temperatures. These findings provide critical insights into the coupled mechanical and hydraulic behavior of CPB under realistic field conditions and offer guidance for optimizing backfill design, binder content, and barricade stability in deep mining applications.