Accurately predicting the dynamic evolution of permeability during CO₂ injection into shale reservoirs is crucial for carbon sequestration and enhanced shale gas recovery. However, traditional permeability models often fail to comprehensively describe the full-range evolution of permeability throughout the entire CO₂ injection process in shale—from the low-pressure gaseous state to the supercritical state. To address this limitation, this study develops a shale permeability evolution model based on a dual-elastic system comprising both the matrix and fractures, determined by component permeability weighting. By incorporating key factors such as mechanical degradation of the matrix, secondary adsorption, and strain hysteresis effects, we establish a governing equation for permeability evolution under multi-effect coupling. Utilizing an overlapping dual-elastic medium structure, we perform parallel cross-coupling numerical solutions, achieving an accurate representation of the nonlinear permeability evolution during full-pressure CO₂ injection. Furthermore, a decoupled analysis of influencing effects reveals that the degradation of mechanical parameters of the matrix material due to CO₂ defines the boundary thresholds for permeability fluctuation ranges. The asynchronous response between mechanical strain and adsorption strain significantly amplifies differences across evolutionary stages, leading to clearly distinguishable phase transitions. Additionally, the strain hysteresis effect prolongs the duration of evolution. Gas adsorption and mechanical responses jointly regulate the transition points between evolutionary stages, with the secondary adsorption-induced swelling strain particularly enhancing phase differentiation throughout the evolution process. This study also provides an in-depth analysis of the fundamental framework of fluid-solid coupled permeability modeling and explores the characteristics of different numerical simulation methods. The findings not only deepen the understanding of shale permeability evolution during CO₂ injection but also offer valuable insights for theoretical modeling and numerical simulation of permeability in geological fluid sequestration.

Ice avalanches are a primary trigger for glacier-related disaster chains in high-mountain regions. Understanding how boundary conditions influence the dynamics and deposition of ice avalanche debris flows is crucial for deciphering the evolution of such disaster chains. This study systematically investigates the motion and depositional behavior of ice avalanche debris flows under varying mass, elevation differences, slope gradients, and toe constraints, utilizing a chute-based experimental setup within a low-temperature laboratory. Key parameters, including flow velocity, basal force, and deposition morphology, are analyzed throughout the debris flow movement. Results indicate that elevation differences and mass govern the dynamic energy transfer within the flows. Specifically, elevation differences control depositional dispersion by regulating peak flow velocity, while mass influences travel duration, resulting in a positive correlation between run-out length and deposit thickness. Furthermore, topographic conditions significantly affect energy dissipation during deposition. An increased slope gradient in the run-out zone reduces basal resistance, thereby expanding the depositional area and enhancing particle scattering at the flow front. A wider slope toe promotes lateral spreading, increasing travel distance and shifting the mass center, which transforms deposit morphology from tongue-shaped to fan-shaped. Finally, theoretical analysis confirms that run-out distance is dictated by the efficiency of kinetic energy transfer among particles and their interaction with the substrate, exhibiting a positive correlation with both particle energy-transfer efficiency and fluctuations in basal stress.

Multiple drying-wetting, freeze-thaw, and drying-wetting-freeze-thaw cycle tests were conducted on intact expansive soil. This was followed by conventional shrinkage tests and controlled suction desorption tests on saturated samples under each cycle condition to investigate the differences in the effects of drying-wetting, freeze-thaw, and combined drying-wetting-freeze-thaw cycles on the soil-water characteristics and shrinkage behavior of expansive soil. The results indicate that the yield suction (s_y) and shrinkage limit suction (s_SL) divide the desorption process of saturated expansive soil into three zones: (1) when s<s_y, the soil is in the elastic zone, (2) when s_y≤s≤s_SL, the soil is in the elastoplastic zone and (3) when s>s_SL, the soil is in the shrinkage limit zone. Following drying-wetting (DW) cycles, the expansive soil exhibited the highest critical suction, water retention capacity, air entry value, yield suction, and shrinkage limit suction, with freeze-thaw (FT) cycles yielding intermediate values, while drying-wetting-freeze-thaw (DW-FT) cycles resulted in the lowest values. Under various cycling conditions, the e-S_r curves during desorption can be approximately divided into three segments: a gentle segment, a steep descending segment, and a vertical segment. The shrinkage deformation was essentially completed after the steep descending segment. The degree of saturation (or void ratio) decreased (or increased) with the number of cycles and eventually stabilized. Notably, the first cycle caused the most significant reduction (or increase), with the degree of saturation (or void ratio) stabilizing after three cycles. The dry shrinkage degree of saturated expansive soil was greatest after DW cycles, followed by DW-FT cycles, and smallest after FT cycles. A model for the Soil Shrinkage Characteristic Curve (SSCC) and Soil-Water Characteristic Curve (SWCC) of saturated expansive soil, incorporating the effects of DW, FT, and DW-FT cycles, was proposed, and the fitting results demonstrated good agreement with the experimental data.

To elucidate the precursory tilt deformation patterns of tension-fractured hazardous rock masses under gravitational loading, this study conceptualizes the collapse process as subcritical propagation under stress corrosion, utilizing a bending Mode-I fracture model. A time-dependent evolution equation for tilt deformation is derived, and the theoretical characteristics of tilting behavior are examined. Based on the principles of micro-electro-mechanical system (MEMS) gravity accelerometry, a method for monitoring the cumulative tilt angle along the primary tilting direction is established using spatial vector angles. A physical model test simulating the collapse of such rock masses under predominantly gravitational loading is designed and conducted, with the resulting tilt deformation behavior analyzed. Additionally, high-low temperature tests are performed to calibrate MEMS tilt sensor drift, and automated field monitoring is implemented to capture time-series variation patterns of tilt angles during collapse events. Comprehensive analysis indicates that precursory tilt deformation transitions from a constant-rate phase to an accelerating phase. However, due to subcritical crack propagation within a heterogeneous medium, localized step-like fluctuations occur during the constant-rate stage, while trend alterations manifest during acceleration. A power-law relationship is identified between the tilt rate and its acceleration prior to collapse. Based on this relationship, a collapse time prediction equation utilizing the inverse of the tilt rate is proposed, and the predictive efficacy of both linear and nonlinear formulations is evaluated. These findings support the application of tilt-sensing technology in monitoring and early warning systems for rock collapse.

In-situ permeability evaluation of rock masses at the engineering scale is crucial for deep energy extraction and subsurface energy material sequestration, including CO₂, hydrogen, and nuclear waste storage. Compared to laboratory-scale tests, this approach more accurately reflects the seepage behavior of rocks under their original in-situ conditions. This study proposes a theoretical method for calculating the in-situ gas permeability of rock masses at the engineering scale, systematically analyzing key parameters that influence permeability results. Special emphasis is placed on determining and conducting sensitivity analyses of the effective testing radius. Through simulated engineering-scale permeability tests under in-situ conditions and comparative analysis with core-scale results, it is observed that permeability values differ by no more than a factor of three. This discrepancy is primarily attributed to the confining pressure of 0.8 MPa applied during core-scale tests and the presence of interconnected pores and microcracks induced by local air bubbles during the casting process. Based on these findings, a series of pilot field tests were conducted in a deep underground laboratory and in both coal and sandstone roadways of a coal mine in Shandong Province, utilizing a self-developed portable in-situ gas permeability testing system. The results demonstrate that the proposed method and integrated system exhibit strong adaptability, stability, and repeatability across diverse engineering scenarios, thereby facilitating effective evaluation of rock mass permeability and grouting effectiveness. This research offers a novel technical pathway and theoretical foundation for the in-situ assessment of reservoir exploitability and the sealing performance of barrier systems in deep subsurface energy material sequestration projects.

The mechanical interaction between glaciers and the underlying bedrock is a primary factor influencing ice avalanche disasters. However, research on the mechanical properties of the ice-rock interface remains limited. To further investigate the key mechanisms involved in the initiation of ice avalanches in high-altitude cold regions and to elucidate the main controlling factors and their underlying principles, this study designed and developed a small high-speed centrifuge device suitable for conducting debonding tests at the ice-rock interface. Systematic tests on the bonding strength of the ice-rock interface were carried out under various conditions. The main findings are as follows: (1) The centrifuge device demonstrates high testing efficiency and low data dispersion, facilitating strength tests of the ice-rock interface under multiple conditions, including tension, pure shear, and compressive shear. (2) The bonding strength of the ice-rock interface is closely related to temperature, rock surface roughness, and rock lithology. Lower temperatures lead to greater bonding strength, exhibiting an overall linear relationship. The bonding strength shows a nonlinear positive correlation with rock surface roughness; however, when roughness exceeds a certain threshold, the formation of interface cavities inhibits further increases in bonding strength. Rock lithology affects bonding strength with ice through factors such as porosity and mineral hydrophilicity. (3) A computational model for the bonding strength of the ice-rock interface was established, clarifying the quantitative relationships among bonding strength, temperature, roughness, and normal pressure. This study provides a novel experimental method for analyzing the mechanical properties of the ice-rock interface, and the results offer a quantitative basis for understanding the mechanisms of disaster and assessing the risks of ice avalanches in high-altitude cold regions.

Current research on tunnels crossing active faults primarily focuses on individual tunnel cases, while the group tunnel effect in tunnel groups has not been systematically addressed. The influence of high internal water pressure on deformation mechanisms is rarely considered. This study employs physical model tests and numerical analysis under high internal pressure to investigate the fault resistance of tunnel groups. The results demonstrate the following: (1) Corrugated expansion joints significantly enhance fault resistance, delaying and reducing peak longitudinal strain (with maximum tensile strain reduced by 69% and compressive strain by 48%) and converting shear failure into coordinated deformation. (2) Group effects intensify the fracturing of surrounding rock during dislocation, resulting in a complex “Y-shaped intersecting crack system.” (3) The sides of adjacent tunnels exhibit higher strain responses than the outer sides (with peak compressive strain at 87% and longitudinal tensile strain at 35%), indicating tunnel-rock-tunnel interaction. (4) Earth pressure between tunnels increases abnormally due to group effects, while the pressure on the outer sides remains largely unaffected. (5) The mechanical response of the lining (axial and shear force) strengthens with smaller tunnel spacing but diminishes and stabilizes as spacing increases. This study reveals the failure mechanisms of high-pressure hydraulic tunnel groups, providing insights for fault-resistant designs in seismic zones.

Conventional finite element methods for large-scale numerical simulations are often constrained by high computational demands and extended runtimes. To enhance efficiency, we developed a predictive model based on a backpropagation (BP) neural network. A three-dimensional finite element model of a buried pipeline with corrosion defects crossing a reverse fault was established using ABAQUS. We systematically analyzed the effects of four key parameters—corrosion depth-to-thickness ratio, diameter-to-thickness ratio, internal pressure, and burial depth—on the seismic response of the pipeline. In this parametric study, fault displacement and the four key parameters served as inputs to the BP neural network, with the pipeline’s axial peak compressive strain as the output. The model was trained and validated using training, validation, and test datasets. Results indicate that increasing the corrosion depth-to-thickness ratio, diameter-to-thickness ratio, internal pressure, or burial depth reduces the fault displacement necessary for the lower section of the pipeline to reach its strain limit. Failure modes differ between unpressurized and pressurized pipelines, exhibiting inward local buckling and outward bulging, respectively, at stress concentration zones. The four parameters are highly correlated with the compressive strain response, with correlations transitioning from linear to nonlinear as fault displacement increases. The trained BP neural network achieves maximum prediction errors of 13.60% on the validation set and 12.84% on the test set, both below 15%, demonstrating robust accuracy and generalization in predicting the seismic response of in-service buried pipelines across reverse faults.

Geothermal energy, a renewable resource with immense potential, has garnered significant attention. In deep geothermal reservoirs, artificially stimulated fracture networks serve as critical pathways for heat extraction. Consequently, the permeability and spatial distribution of these fractures directly impact heat extraction efficiency. This study conducted shear-seepage experiments on a single rough granite fracture under varying confining pressures, shear displacements, and fracture roughnesses. A nonlinear relationship between permeability and the aforementioned three factors was established based on the experimental results. This relationship was then integrated into the THM coupled framework TOUGH2MP-FLAC^3D to assess the long-term performance of Enhanced Geothermal Systems (EGS) under varying fracture networks, fracture densities, and horizontal stress ratio conditions. The findings reveal that fracture permeability exhibits an exponential negative correlation with confining pressure, a logarithmic positive correlation with shear displacement, and a quadratic correlation with fracture roughness. Increased fracture density significantly enhances thermal performance; as fracture densities increase from 0.1 to 0.25, the thermal breakthrough time extends by up to 6.4 years, the EGS lifespan increases by up to 13 years, and total heat production rises by approximately 22.5%. Horizontal stress anisotropy negatively affects thermal performance, while higher fracture density effectively mitigates the reduction in heat extraction caused by stress anisotropy. This work provides a theoretical foundation for hydraulic fracturing during the stimulation of hot dry rock reservoirs.

This paper investigates the effect of anisotropic stress states on the small strain stiffness of red mudstone fill material (RMF). A comprehensive experimental program was conducted, including 18 triaxial-bender element tests, 4 isotropic consolidation tests, and 6 stress-controlled loading-unloading tests. The results indicate that the normalized strength is well characterized by the nonlinear strength envelope. Under isotropic stress conditions, the small strain stiffness increases with mean stress, which can be described by a power equation. During conventional triaxial shear, small strain stiffness increases at low axial strains. When the axial strain exceeds 2%, the damage point can be identified, at which point small strain stiffness decreases by more than 25% with further axial strain. A power model has been employed to characterize the small strain stiffness and shear stress at both the damage point and peak point. Unloading at stress states below the damage point results in an increase in small strain stiffness. Conversely, due to irreversible structural disturbance, unloading at stress states above the damage point leads to a progressive reduction in small strain stiffness. The difference in small strain stiffness at various unloading points can exceed 30%. Therefore, the coupled effects of stress history and stress path should be considered for accurate determination of small strain stiffness, as the conventional monotonic model is not applicable in such coupled scenarios.