In response to the engineering challenges posed by unclear disaster-causing mechanisms and inadequate control effectiveness for roadway surrounding rock in steeply dipping and extra-thick coal seams, this study focuses on the +400 mB3 roadway in Wudong Coal Mine. A comprehensive research methodology was employed, integrating on-site monitoring, theoretical analysis, numerical simulation, and industrial testing. The investigation examined the evolution and rotation characteristics of the principal stress path under excavation disturbance in steeply dipping and extra-thick coal seams. Additionally, the study elucidated the driving mechanisms behind mining-induced stress evolution, which leads to fracturing in the coal and rock mass, and revealed the mechanism of roadway surrounding rock disasters induced by the coupling effects of the stress field and fracture field. The results indicate that the surrounding rock in the lower section of the roadway is significantly influenced by mining activities, undergoing a stress evolution process characterized by ?1 loading followed byσ3 unloading. The principal stress axis exhibits spatially differential rotation, deviating from its initial orientation. Notably, the degree of stress rotation is most pronounced in the roadway roof, with rotation angles ranging from 16.7° to 20.8°. Due to the impact of mining activities, the stress level within the coal and rock mass reaches its strength threshold, leading to the formation of mining-induced fractures. The angle of stress rotation determines the predominant propagation direction of these fractures, while the presence of the mining-induced fracture field compromises the mechanical integrity and strength of the surrounding rock in the roadway. After the unloading associated with roadway excavation, mining-induced cracks continue to propagate, primarily characterized by Mode I tensile failure. As the inclination angle of these cracks increases, the anti-unloading failure resistance of the coal and rock mass gradually strengthens, resulting in a transition of the failure mode from crack-dominated to matrix material-dominated. The disaster-causing mechanism of the roadway surrounding rock can be summarized as follows: the orientation of stress rotation governs crack propagation, the formation of a crack network weakens the load-bearing structure, and strength degradation leads to large deformation. Under the coupled effects of the stress field and fracture field, the roadway experiences deformation and failure. To address these challenges, a surrounding rock stability control strategy combining “overall reinforcement with key point strengthening” has been developed for roadways in steeply inclined and extra-thick coal seams. Following the implementation of the reinforcement support, the stability of the roadway?s surrounding rock has been significantly enhanced. The findings of this research provide a solid scientific foundation for the control of surrounding rock in steeply inclined and extra-thick coal seams, as well as in similar roadways.

Understanding the mechanical behavior of anchored composite rocks is essential for revealing the deformation and failure mechanisms of bolted layered rock structures. The study examines the mechanical characteristics of sandstone-mudstone composite specimens under different bedding dip angles and cable support types, using acoustic emission (AE) and digital image correlation (DIC) techniques for monitoring. Experimental results indicate that specimens anchored with PR cables exhibit more extensive damage and a higher crack density than those reinforced with NPR cables. Both types of anchored composite specimens display a clear bedding angle effect (15°–45°) on their compressive strength and surface displacement evolution. Specifically, compressive strength decreases progressively with increasing dip angle, and high-displacement zones on the specimen surface expand accordingly. Meanwhile, the failure mode shifts from localized small-scale cracking at 15° to interface slip failure at 45°. Notably, the bedding angle affects the AE characteristics of PR-anchored and NPR-anchored specimens in different ways. In PR-anchored samples, the proportion of shear cracks and high-frequency AE signals increases with dip angle, reflecting a clear sensitivity to bedding orientation. In contrast, NPR-anchored specimens exhibit relatively stable tensile-shear crack proportions and AE frequency distributions regardless of dip angle. These findings provide a theoretical basis for addressing stability challenges in roadways with layered sandstone–mudstone roofs and contribute to the advancement of bolting support technologies for composite rock masses with inclined bedding structures.

To validate the integration of microseismic (MS) monitoring and digital twin (DT) technology in the safety monitoring of high-arch dams, this study proposes a dynamic DT approach driven by MS damage data. First, a data-driven DT system framework was established for modeling MS damage in high-arch dams. A method for constructing the twin model and an adaptive meshing reconstruction technique were introduced. An MS event model was developed using key spatiotemporal and intensity parameters, with model rendering achieved through interpolation algorithms and vertex coloring techniques. A rock mass degradation model was incorporated to dynamically adjust mechanical parameters by accounting for cumulative MS damage. Additionally, a feedback correction mechanism was established to update parameters in real time based on the locations of MS events and their energy release. A dynamic DT simulation method that integrates a development engine, numerical software, and database systems was implemented. This proposed method was applied to a case study of the Dagangshan high-arch dam. During a simulated rise in reservoir water levels from 1 120 m to 1 128 m, the stress concentration zones identified by the model—including the dam heel, the upstream dam face at elevations of 940 to 1 030 m, the downstream dam face at 979 to 1 081 m (both wings and the crown), the downstream arch ends, and the dam toe—demonstrated a high spatial correlation with actual MS activity. The average matching rate reached 92%, with some days achieving 100%. These results confirm the accuracy and effectiveness of the proposed MS data-driven DT simulation method, providing a new technical pathway for the digital construction and intelligent safety monitoring of high-arch dams.

To elucidate the shear deformation and fracture behavior of rocks under the extreme conditions of deep engineering environments, an innovative true triaxial single-shear load transmission apparatus has been developed, accompanied by a corresponding method for true triaxial direct shear. This load transmission device is characterized by three key technologies: (1) A centrally symmetric double shear-double pressure interlocking load transmission structure has been engineered, overcoming the constraint of lateral stress/normal stress ≤ 1. This advancement allows for a more realistic and comprehensive restoration of the stress distribution state during the rock shear fracture process. (2) A hollow hydraulic self-balancing body has been designed, capable of withstanding oil pressures up to 70 MPa and temperatures of 150 ℃. This enhancement reduces the measurement error of shear strength by 52.7% to 75.4% compared to the previous silicone block approach, ensuring stability in load transmission at the flexible end of the shear component and guaranteeing the precise application and measurement of shear forces. (3) An embedded “sealant-cork layer-sealant” sandwich sealing configuration has been adopted, providing multiple layers of protection. This innovation marks a significant breakthrough in high-temperature and high-pressure true triaxial large displacement single-shear technology, accommodating shear displacement/specimen length ratios of up to 10%. High-temperature true triaxial single-shear experiments and acoustic emission monitoring were successfully conducted on granite specimens (50 mm×50 mm×50 mm). These experiments validate the reliability and expandability of the experimental setup. The apparatus features a straightforward structure with significant potential for widespread application, offering promising prospects for advancing studies on the shear mechanical properties of deep rock formations.

The shear strength and stability of rock joints are critical factors in ensuring the safety of surrounding rock in underground engineering. To investigate the shear behavior and destabilization mechanisms of rock joints under the combined effects of roughness, shear rate, and normal stress, we developed rock joint models with varying roughness levels (low-order roughness JRC = 0.42, medium-order roughness JRC = 11.12, and high-order roughness JRC = 18.96) using the particle flow code (PFC2D). Subsequently, we conducted numerical direct shear tests on these models under varying normal stresses (2–6 MPa) and shear rates (50–200 mm/s). By quantitatively analyzing the evolutions of contact area, energy, and acoustic emission (AE) characteristics during joint shearing, we systematically revealed the macro-mechanical responses and associated micro-mechanisms of shear failure in rock joints. The main conclusions are as follows: (1) the peak shear strength of rock joints significantly increases with both JRC and normal stress. However, for the residual shear strength of high-order rough joints (JRC = 18.96) subjected to high normal stress (＞4 MPa), there is a decrease of approximately 30% due to brittle failures at the contact asperities caused by stress concentration. (2) Under constant shear rates, joints exhibit consistent velocity-strengthening behavior, where shear strength increases with an increase in shear rate. Nevertheless, in velocity-stepping tests, an increase in both JRC and normal stress promotes the transition of rock joints from velocity-strengthening to velocity-weakening behaviors. (3) The strain energy accumulated during the shear process of the high-order roughness joint (JRC = 18.96) is 1.8 times that of the low-order roughness joint (JRC = 0.42), and the energy released during the instability stage is also significantly higher in the high-order roughness joint, indicating that rapid energy dissipation serves as a critical driving force for joint instability. (4) The AE characteristic b-value is strongly correlated with velocity-dependent parameters a－b1, indicating that the joint has entered a critical instability state when b＜1 and a－b1＜0. This work establishes an innovative dual-parameter framework for assessing the shear instability of rock joints and proposes a coupled destabilization criterion based on critical decreases in the acoustic emission b-value and the velocity-weakening effect. It elucidates the instability mechanisms of rock joints under complex conditions and potentially provides a theoretical foundation and quantitative criteria for the stability assessment of rock masses in deep underground engineering.

 To address the high false-alarm rate in landslide monitoring and early warning systems, this study aimed to explore more effective methods for rainfall-induced landslide prediction and early warning. By integrating theoretical analysis, field investigations, laboratory testing, and numerical simulation, a dynamic computational algorithm for rock-soil mass softening and stability under rainfall infiltration was developed, focusing on water—the critical factor triggering landslides. This algorithm was established based on a comprehensive analysis of the entire process of rainfall-induced landslides, incorporating considerations of rainfall infiltration boundaries, unsaturated seepage, saturated seepage, rock-soil mass softening, and stability coefficient calculation. The corresponding program was further implemented using FLAC3D?s built-in FISH programming language. Subsequently, through a case study, the developed algorithm and program were validated through analysing pore water pressure, saturation, moisture content, cohesion, internal friction angle, maximum shear strain increment, stability coefficient, landslide location, and extent during rainfall-induced slope failure. Finally, the relationships between monitored data (moisture content, displacement, stress, pore water pressure, and mechanical properties) at three representative locations (ground surface, slip surface, and slip bed) and the stability coefficients, along with their applicability in landslide prediction, were analysed and discussed. The research found that when the moisture content in the middle of the potential sliding surface reaches its peak, the cohesion and internal friction angle at this location drop to the trough value. At this point, the slope stability coefficient reached 1.000, indicating the timing of the landslide. Based on this finding, a new landslide monitoring and early warning method was proposed. This method focuses on monitoring the moisture content in the middle of the potential sliding surface, supplemented by cross-verification through rainfall and surface displacement data. Its feasibility was demonstrated through theoretical analysis and numerical simulations. This research provided a new numerical analysis platform for the study of rain-induced landslides and made beneficial explorations for the precise prediction and early warning of landslides.

To enhance our understanding of the anisotropy in shear fracture resistance of shale, the mode-II fracture toughness of shale with different bedding orientations was measured using the double-edge notched Brazilian disc (DNBD) method. These tests were conducted based on theoretical analyses of shear crack initiation. The initiation and propagation of fractures generated in DNBD specimens were determined, and the mechanism for testing the mode-II fracture toughness of rocks using the DNBD method was investigated. The results indicated that: (1) the high compressive T-stress in DNBD specimens significantly reduces the circumferential stress at the tips of pre-cut notches, thereby inhibiting the competitive initiation of tensile cracks and facilitating self-similar shear fractures. (2) The mode-II fracture toughness of shale increases approximately linearly with the bedding angle, displaying marked anisotropy. The fracture toughness is minimal at the bedding plane, maximal in the matrix, and nearly equal in the direction normal to the bedding compared to the matrix. As an intrinsic material property, the true mode-II fracture toughness, in the absence of normal compressive stress, is considerably lower than the apparent (experimental) value. Moreover, the true mode-II fracture toughness remains relatively constant even when wing-shaped tensile cracks initiate competitively. (3) Similar fracture propagation patterns were observed in DNBD specimens with varying bedding orientations. The findings revealed that complex fracture processes, including the competitive initiation and subcritical propagation of wing-shaped and en-echelon fractures, occurred prior to the shear fracture of the rock bridge. (4) The subcritical propagation of wing-shaped tensile cracks following competitive initiation has been identified as the primary mechanism driving self-similar shear fractures along pre-cut notches in DNBD specimens. This understanding elucidates the mechanism for measuring mode-II fracture toughness in rocks using the DNBD method.

To investigate the key precursor information characteristics and damage state early warning model in the deformation and failure process of fractured rock, a series of uniaxial compression tests were carried out on prefabricated fractured sandstone samples with different geometric configurations. Also, the acoustic emission (AE) and energy mechanism of fractured sandstone were analyzed, and a catastrophe mutation early warning model based on multi-index fusion was established. The failure and mutation process of the sample were analyzed from the perspective of control factor and discriminant function response characteristics. The maximum likelihood estimation (MLE) and Davidon-Fletcher Powell (DFP) algorithm methods were adopted to solve the model. The precursor response coefficient and the precursor response stress ratio were introduced to comparatively analyze the early warning effect of the single-index model based on AE energy variance, AE b-value, elastic energy dissipation ratio, and the multi-index fusion model. The results show that the dissipative energy presents three stages: slow rise, slow decline and sudden rise. Before the sample approaches instability failure, the AE b-value decreases sharply, the AE parameter variance increases abruptly, and the elastic energy dissipation ratio curve changes rapidly from rising to falling. These sudden changes can be regarded as early warning indicators of rock failure. The average precursory response coefficients defined by AE energy variance, AE b-value and elastic energy consumption ratio are 9.19%, 8.26% and 6.72%, respectively. In addition, the corresponding average response stress ratios are 92.54%, 92.70% and 95.84%, respectively. The early warning range of the improved multi-index early warning model is smaller than that of the single index model, and the possibility of false alarms and missing alarms is reduced. The multi-index fusion warning model has good robustness. The early warning interval is within (0.96–0.98)??, the precursory response coefficient is between 5.55% and 10.20%, and the precursory response stress ratio ranges from 93.62% to 97.75%.

Short-term failure forecasting of rocks is a critical scientific issue in predicting major geohazards such as rockbursts, landslides, and earthquakes. This study conducted Mode I and Mode II single-crack failure tests on rock specimens, combined with real-time acoustic emission (AE) monitoring, to investigate the precursors to local instability and their multidimensional damage evolution. A failure forecast model (FFM) based on the time-reversed Omori law, along with a long short-term memory (LSTM) neural network failure forecast model, was proposed to quantitatively predict the single-crack failure time of rocks. The results indicate that the precursory processes manifested as power-law damage acceleration in the time dimension, damage localization in the spatial dimension, and high-energy damage in the energy dimension, which demonstrate physical synchronicity. The traditional FFM, improved FFM, and LSTM neural network failure forecast model all accurately predict the single-crack failure time of rocks, exhibiting good statistical reliability and robustness, with the predictive advantage of the LSTM model being particularly notable. Utilizing the AE event rate and AE amplitude rate as predictive factors can enhance prediction quality. However, the local time series formed by integrating the cracking nature and energy mechanism does not fully represent the complete energy consumption process, resulting in a short early warning time. Additionally, this study shows that the localized zone, which plays a controlling role in rock strength, serves as a reliable precursor source for failure prediction, providing computational technical support for short-term failure forecasting of rocks.

The fracture propagation pattern following hydraulic fracturing in deep coal rock significantly influences the effectiveness of reservoir stimulation. Understanding the characteristics and evolution of hydraulic fractures in deep coal rock under varying injection modes is essential for the efficient development of coalbed methane. To elucidate the fracture evolution patterns in deep coal rock and explore methods for forming fracture networks, large-scale true triaxial hydraulic fracturing physical simulation experiments were conducted. These experiments integrated acoustic emission monitoring with post-fracturing 3D fracture reconstruction technology. Testing was performed on the #8 deep coal seam of the Ordos Basin, utilizing various injection parameters and modes. The findings were corroborated by on-site reservoir reconstruction results in the Ordos deep coal rock area. The results indicate that: (1) the number of fractures and the fracture area ratio in deep coal rock are strongly correlated with the viscosity of the fracturing fluid. Low-viscosity fluids yield the highest fracture count and area ratio, albeit with lower fluid efficiency. As fluid viscosity increases, the fracture count decreases by 91%, the fracture area ratio decreases by 86%, and fracture complexity diminishes. (2) At low viscosities, natural fractures (e.g., cleats and bedding planes) predominantly dictate the direction of hydraulic fractures. With increasing viscosity, the minimum resistance plane, influenced by in-situ stress and rock anisotropy, becomes the primary controlling factor. (3) During variable-rate fracturing processes, a higher injection rate results in increased fracture initiation pressure in rock specimens, reduced fracture initiation time, and heightened intensity of acoustic emission ring-down counts, thereby facilitating the formation of dominant hydraulic fractures. (4) The injection mode of “high viscosity first, low viscosity later” effectively maintains fracture complexity while minimizing fluid loss, enhancing fluid efficiency, and achieving a fluid efficiency 2.06 times that of the low-viscosity-only injection mode. The application of a variable-viscosity injection mode combined with high-rate fracturing technology in the deep coal seams of the Ordos Basin demonstrates significant effectiveness, with post-fracturing daily average production per well reaching 4.4 times that achieved through conventional fracturing techniques.

Deep underground engineering disasters stem from the cross-scale evolution and catastrophic failure of rock masses, with the occurrence and severity of such disasters being contingent upon the dynamics of crack propagation and energy release characteristics during the post-peak stage. To fundamentally elucidate the black-box processes involved in post-peak crack propagation and fracturing mechanisms in loaded rock, this study conducted mode-I fracture tests on quartzite beams under post-peak cyclic loading conditions. Through acoustic emission (AE) analysis, we investigated the spatiotemporal evolution of microcracks and developed a quantitative inversion method for microcracking source parameters based on forward modeling of the entire process, from source generation and wave propagation to signal reception. The evolution patterns of critical microcrack parameters, including type, scale, orientation, and damage, were systematically revealed, with mesoscopic inversion results validated against macroscopic measurements. Key findings include: During post-peak cycling, most microcracking events occur between the pre-peak damage stress and the post-peak stress drop period of the loading stage, with large-magnitude events concentrating in the upper-middle region of the crack. The AE Felicity ratio progressively decreases with the number of cycles, indicating weakening spatiotemporal memory effects. At the mesoscale, individual microcracks predominantly exhibit mixed-mode mechanisms. However, the global volume decomposition of microcracks shows that mode-I tensile components account for over 90% of the accumulated damage volume, where large-magnitude microcracks demonstrate horizontal tensile motion aligned with macroscopic fracture planes, confirming tensile-dominated mechanisms in three-point bending tests. Damage tensor analysis based on microcrack orientation and size reveals that preferentially oriented microcrack clusters dominate macroscopic fracture propagation. The increasing ratio between preferential and random damage components serves as a precursor for fracture progression. Additionally, the scale of microcracks and the energy dissipation range were quantitatively estimated. The average size of microcracks was found to be consistent with grain sizes, while the cumulative opening displacements of microcrack clusters exhibited a linear positive correlation with macroscopic crack opening displacement, thereby confirming the reliability of the inversion results. This research provides a quantitative methodology for representing rock fracture morphology, offering significant implications for understanding rock failure mechanisms and enhancing disaster prevention in rock engineering.

 Accurately understanding the mechanical laws and mechanisms of micro-damage to the near-field surrounding rock in high-level radioactive waste geological disposal repository is of great significance for achieving the disposal encirclement function. Given that the mechanical behavior and response characteristics of rocks are related to stress paths, the mechanical tests were conducted on three typical stress paths, including loading axial pressure-fixing confining pressure, loading axial pressure-unloading confining pressure, and unloading axial pressure-unloading confining pressure. The Beishan granite in the pre-selected area for high-level radioactive waste geological disposal in China was taken as the research object, and the effects of stress path and initial confining pressure on the pre-critical deformation properties and dilatancy behavior of which under high stress levels were systematically studied. Firstly, taking the brittle stress-drop as the critical state, for the controllable and comparable stress path bifurcation stage, a unified quantitative comparison of macro deformation parameter evolution process and characteristics based on deviatoric stress ratio is proposed. It is found that when unloading confining pressure at high stress levels, axial pressure unloading actually exacerbates lateral deformation and dilatancy compared to loading; The secant Poisson?s ratio curves of secants between different paths are dispersed and clustered, with significant differences in evolutionary characteristics, but the confining pressure effect is not obvious; The relationship between the volumetric and radial strain increment follows a quadratic function, but there are significant differences in gradient and curvature of curves under different paths. Secondly, the plastic theory is used to describe the nonlinear deformation characteristics of rocks, and variables such as plastic strain compliance and radial axial plastic strain increment ratio (proportional coefficient) are defined. The results show that the greater the confining pressure, the more significant the inhibitory effect on lateral deformation and dilatancy, and the stronger the ability to withstand plastic deformation. Compared with axial compression loading, unloading delays the failure, making the development of plastic deformation more complete; Although initial confining pressure has a suppressive effect on rock deformation and failure, it has a greater promoting effect on plastic strain sensitivity and development, and has a greater impact on axial deformation. Finally, it is found that the relationship between the volumetric and axial plastic strain increment under each stress path follow an exponential distribution relationship. Based on this, a unified theoretical relationship between the apparent dilatancy angle and dilation angle before the critical point was constructed, and the influence of initial confining pressure was examined through the dilatancy index. The results show that the apparent dilatancy angle and the dilation angle of loading axial pressure-fixing confining pressure are the smallest, and the confining pressure effect of the dilatancy index is the most obvious; The increase gradient of dilation angle of loading axial pressure-unloading confining pressure is the largest, and the dilatancy trend is the most prominent; The apparent dilatancy angle and dilatation angle of unloading axial-unloading confining pressure are the largest, the dilatancy index is least affected by the initial confining pressure, and the dilatancy property is the most significant. The research results reveal the correlation between the pre-critical deformation evolution and dilatancy properties of Beishan granite under high stress level with stress path, indicating that the development of surrounding rock dilatancy micro-damage in multi-directional unloading area need be paid close attention to in the engineering practice and theoretical research of high-level radioactive waste geological disposal.

The safety evaluation of seepage in interlayer rock beneath the underground water reservoir of coal mines is crucial for the secure extraction of lower coal seams. Accurate calculation of the permeability of interlayer rock serves as a fundamental basis for this seepage safety assessment. The transient testing method is frequently employed to measure permeability in low-permeability rocks, relying on a precise description of the decay curve of the seepage pressure difference. However, the spatial distribution of this seepage pressure difference is often overlooked, leading to permeability calculations being influenced by linear Darcy?s laws. Using the interlayer rock of the underground reservoir in the Daliuta coal mine as a prototype, we designed complex disturbance stress paths associated with lower coal seam mining in three stages: in-situ stress, mining stress, and cyclic stress. Multiple seepage tests were conducted at characteristic points throughout the entire process on coal, mudstone, and sandstone under varying hydraulic-mechanical coupling conditions using the transient method. These tests revealed the phenomenon of rebound expansion during the mining stress stage of the interlayer rock. Furthermore, we describe the nonlinearity of the relaxation process of the seepage pressure difference decay curve based on the Forchheimer equation and the Mittag-Leffler function, validating the applicability of the Mittag-Leffler function in characterizing the differential pressure decay process of non-Darcy seepage in rocks. Subsequently, a spatiotemporal theoretical model for the decay process of the seepage pressure difference in interlayer rock under hydraulic-mechanical coupling was established, demonstrating that this decay can be represented by the fluid diffusion equation. An accurate description of the spatiotemporal evolution of seepage pressure difference in rock is achieved through fractional-order theory. We also emphasize that the principle underlying the transient method for measuring permeability is to satisfy Darcy’s law spatially, indicating that transients conform to steady-state tests in space, while seepage exhibits non-Darcy characteristics over time. Finally, based on the hysteresis principle, we derive the discrete format of the one-dimensional fractional-order diffusion equation and complete the discretization of spatiotemporal fractional-order equations for the decay of seepage pressure difference in interlayer rock under hydraulic-mechanical coupling. Utilizing the finite-difference method, we solve and visualize the spatiotemporal distribution surfaces of seepage pressure difference in the interlayer rock, providing a foundation for theoretical solutions regarding the spatial distributions of permeability.

Laser rock-breaking technology presents promising applications for non-blasting excavation of extremely hard rock. To clarify the thermal fragmentation characteristics of continuous-wave (CW) and millisecond-pulsed laser on granite and compare their applicability in rock drilling, we conducted laser ablation experiments on granite. The physicochemical failure processes of granite were analyzed comparatively. We investigated the temperature rise and cooling characteristics of the granite surface using infrared thermography. An instantaneous heat transfer model for the rock surface under laser irradiation was established, incorporating the nonlinear temperature dependence of thermophysical parameters. Through CT scanning and three-dimensional reconstruction, we comparatively analyzed the morphology, structure, and distribution of internal three-dimensional cracks in granite. The results indicate that: (1) the phase transition damage zone in granite consists of a borehole and a molten layer, with approximate ratios of 4:6 under CW irradiation and 7:3 under millisecond-pulsed laser irradiation. The primary phase destruction mechanisms induced by the two laser types are melting and thermal cracking, respectively. (2) The surface temperature evolution over irradiation time can be divided into three stages: rapid rise, fluctuation decline, and sustained decline. Reducing the duty cycle or increasing the repetition frequency enhances the temperature difference during the fluctuation decline stage, resulting in greater thermal stress and intensified thermal cracking reactions. (3) Calculations based on the instantaneous heat transfer model indicate that for CW laser parameters (P = 3 000 W, t = 0.04 s), accounting for the nonlinear temperature dependence of thermophysical parameters yields a calculated surface temperature of 1 296.8 ℃, with a 6.99% error compared to the measured value. (4) The instantaneous temperature rise curve under millisecond-pulsed laser irradiation exhibits distinct “pulse heating-intermittent cooling” phenomena, with the initial heating rate under high-duty-cycle conditions significantly exceeding that of low-duty-cycle conditions. (5) As the laser ablation depth increases, the scale of shear cracks outside the phase transition zone enlarges. The damage capability of the CW laser outside the phase transition zone diminishes with increasing specimen depth, whereas that of the millisecond-pulsed laser strengthens with greater depth.

To investigate the longitudinal dynamic response characteristics of mountain tunnels with wall-type portals, a simplified mass-beam-spring mechanical model was developed. In this model, the portal retaining wall is idealized as a lumped mass, while the tunnel structure is represented as a Timoshenko beam. Based on this model, an analytical solution for the steady-state response of tunnels with wall-type portals was derived using the Green?s function method. The correctness and effectiveness of the analytical solution were validated through numerical experiments, and the influence of key parameters of the theoretical model on the dynamic response of tunnel structures was analyzed. The results indicate that the analytical solution aligns well with the numerical findings, confirming the reliability of the model. When the portal mass is excessively large or its foundation stiffness is inadequate, the portal structure amplifies the displacement of the tunnel near the entrance. Conversely, a lighter portal or a stiffer foundation mitigates this displacement. The influence of the portal on the tunnel increases with the contact stiffness between them. Additionally, poor-quality surrounding rock at the tunnel entrance significantly diminishes seismic resistance. Two primary strategies exist for enhancing the seismic performance of tunnels with wall-type portals: the first involves adjusting the portal?s mass or foundation stiffness to achieve deformation compatibility with the tunnel structure; the second entails implementing seismic mitigation measures to reduce the interaction forces between the portal and the tunnel structure. These findings provide valuable insights for the seismic design of mountain tunnels featuring wall-type portals.

To investigate the anchorage anti-fracture effect of fissured granite, a numerical model representing the crystalline structure of anchored single-fissured granite was constructed using the discrete-continuous coupling (GBM-FDM) method. Uniaxial compression tests were conducted on both unanchored and anchored samples of single-fissured granite with a 45° inclination angle. The failure process of fissured granite under uniaxial compression and the anchorage anti-fracture effect of the anchor system on single-fissured granite were systematically examined from mesoscopic perspectives, focusing on force chain evolution and micro-crack propagation. The results indicated that the failure pattern of the unanchored sample under uniaxial compression exhibited significant propagation along the fissure tip region. In contrast, the internal fractures of the anchored sample were distributed more widely and uniformly compared to the unanchored sample. The force chain distributions within the anchored samples were more uniform, suggesting that the anchor structure enhances the stress distribution during the loading process, thereby effectively improving the load-carrying capacity. Additionally, the number of cracks in the anchored samples was lower, while the mean value of the force chain was higher, and the force chain value required to generate a single crack was greater in anchored samples than in unanchored ones at the same load level.

Granite fractures develop distinct mineral structures at various stages of evolution and experience different shear displacements during tectonic movements, including fault activities and folding. To investigate the coupled effects of shear displacement and mineral structure on the shear properties of granite fractures, direct shear tests were conducted on four different types of split granite fractures at three distinct shear displacements. The study examined the shear mechanical properties, acoustic emission (AE) evolution patterns, and macro-and mesoscopic damage characteristics. Furthermore, the underlying mechanisms by which shear displacements and mineral structures influence the shear behavior of granite fractures were elucidated. Experimental results indicate that (1) with increasing shear displacement, the shear strengths of granite fractures decrease, and the shear dilation curves exhibit a bilinear trend. Coarse-grained granite fractures (CGF) demonstrate higher shear strengths than fine-grained fractures (FGF), and the presence of compact mineral textures and larger quartz grains enhances the shear strength of granite fractures. (2) The stick-slip amplitudes increase with shear displacement, peaking at the critical displacement. FGF generally exhibit larger stick-slip amplitudes than CGF, and a higher quartz content further amplifies this effect. (3) The time required to reach an active AE state decreases with increasing shear displacement. CGF tend to generate low-frequency, high-energy AE events before peak strength, while FGF produce more high-amplitude AE events during stick-slip. The critical displacement thresholds trigger a proliferation of low-magnitude AE activities. (4) Shear damage is primarily characterized by the wear and breakage of secondary asperities at low shear displacements. As shear displacement increases, wear production initially increases before entering a decreasing regime, ultimately stabilizing. When mineral grain sizes are comparable, fractures with higher quartz content and tighter grain boundaries exhibit less microscopic damage. CGF develop larger damage zones but have shorter transition regions compared to FGF. The influence of mineral structures and shear displacement on the shear behavior of granite fractures was clarified, providing experimental support for the analysis of catastrophic failure mechanisms and stability assessments in rock mass engineering.

Performing back compression on the scaling-off-stabilized grout body is a crucial aspect of reinforcing earthen sites affected by scaling through electro-osmotic grouting methods. This study involved on-site experiments to assess scaling-off reinforcement by implementing two distinct back compression modes. It evaluated the normal compression of the grouted body under varying back compression conditions, identified characterization indicators for the reinforcement effect of the back-compression-stabilized grout body, and analyzed the relationship between the work done by back compression and the energy efficiency ratio. Microstructural analysis and principal component analysis were employed to elucidate the reinforcement mechanism of the electro-osmotic grouting method from both qualitative and quantitative perspectives. The research findings indicate that the normal compression of the grouted body under different back compression modes, the characterization indicators of the reinforcement effect of the back-compression-stabilized grout body, and the work done by back compression all increase non-linearly with the rise in back-compression levels. Conversely, the energy efficiency ratio declines non-linearly as back-compression levels increase. Notably, the performance of back compression at a constant force of 25 kPa is found to be optimal in a comprehensive assessment. The synergistic effect of SH solution solidification and back compression enhances the interfacial connections between soil particles, establishes a stable combination with effective force chain transmission, and significantly improves resistance to scaling-off reinforcement.

In the current design code for Mechanically Stabilized Earth (MSE) walls, the influence of the wall facing is not considered in the bearing capacity analysis. Therefore, comparative centrifugal model tests were conducted on MSE walls with full-height rigid facing and modular block facing, in conjunction with the finite element limit analysis (FELA) method to evaluate bearing capacity. The results indicate that the difference in bearing capacity between the wall with full-height rigid facing and that with modular block facing is significant, particularly when the reinforcement strength is low, the vertical spacing is large, and the length is short, with the peak relative difference approaching 20%. Although the influence of wall facing on bearing capacity becomes more pronounced with stronger toe constraints, the difference in bearing capacity among different wall facing types does not exceed 6%. The bearing capacity exhibits an inverted V-shaped distribution as the surcharge load moves away from the wall face. It is recommended to use full-height rigid facing to enhance the bearing capacity for walls with reinforcement lengths greater than half the height of the wall, and surcharge load positions greater than 0.5 times the length of the reinforcement material.

Grasping the strength characteristics of soils under three-dimensional stress states is crucial for assessing the stability of foundations under complex stress conditions. In this paper, true triaxial shear tests under different intermediate principal stress coefficients were carried out on coral sand to investigate the influence laws of intermediate principal stress on the stress-strain relationship, peak-state large principal strain, peak-state friction angle, and stress-dilation relationship of coral sand. The applicability of different strength criteria in describing the variation in the true triaxial shear strength of coral sand is also examined. The results show that under the true triaxial shearing conditions, the stress-strain curves of coral sands are categorized into strain-softening type. The peak-state shear strength of the coral sand increases significantly with the increase of the intermediate principal stress coefficient. At the same time, the deformation in the direction of the intermediate principal stress transitions from expansion to compression behavior with the increase of the intermediate principal stress coefficient. In addition, the peak-state dilation rate of coral sand significantly increases with intermediate principal stress coefficient. By comparison, it is noted that the Lade-Duncan and Matsuoka-Nakai strength criterion describes the three-dimensional strength properties of coral sand with better applicability.