Search Results (220)

The laminated structure in shale rock plays a crucial role in fluid migration during shale gas exploitation. This study investigated the evolution patterns and mechanisms of shale permeability under the influence of laminae through multiple permeability experiments, including steady-state flow tests, confined pressure permeability tests, thermo-hydro-mechanical coupled permeability tests, fracture permeability tests, and fracturing fluid damage experiments. The results show that the permeability of the shale is significantly controlled by the laminated structure. Pores and micro-cracks develop along the laminated structure direction, which is conducive to the formation of seepage channels in vertical laminated specimens, while parallel laminated specimens are affected by multiple laminated structures, resulting in lower permeability. Increasing confining pressure leads to a rapid decrease in permeability, indicating that the closure of fractures is the key factor controlling the permeability characteristics of shale. Increasing temperature causes shale to further increase permeability under the effects of thermal stress, but the enhancement effect is weakened under high confining pressure. Artificial fractures significantly enhance the permeability of shale under low confining pressure, but the closure of artificial fractures reduces the permeability effect under high confining pressure. The effect of fracturing fluid leads to a decrease in shale permeability, and the damage to shale is more significant under high confining pressure. Full article

Clarifying the evolution of pore-fracture structure (PFS) and the associated seepage mechanisms of deep coal under coupled seepage and mining-induced stress is important for the safe and efficient in situ fluidized mining of deep coal resources. In this study, a nuclear magnetic resonance (NMR) online triaxial testing system was used to conduct coupled seepage–mining-induced stress tests under different seepage pressures, following a mining-induced stress path characterized by increasing axial stress and decreasing confining pressure. Transverse relaxation time (T₂) spectra and nuclear magnetic resonance imaging (NMRI) were combined to characterize the dynamic evolution of PFS in terms of its spatial distribution, pore volume, mean pore size, pore compressibility, and fractal characteristics. The results show that, based on the NMRI characteristics, the deformation and failure process of coal can be divided into three stages: compaction and elastic deformation, PFS propagation, and post-peak failure. Across the tested specimens, higher seepage pressure was associated with an earlier onset of PFS propagation, a lower PFS damage threshold, and enhanced PFS propagation and connectivity. The volumes of adsorption pores (APs), seepage pores and fractures (SPFs), and total pores (TPs) generally increase initially and then decrease during the compaction and elastic deformation stage, increase slowly or remain relatively stable during the PFS propagation stage, and increase sharply at the peak-strength point, with the magnitude of the increase differing among the three tested specimens. The mean pore size, represented by T_2g, initially increases and then gradually stabilizes with increasing strain, followed by a rapid increase at the peak-strength point. For specimen M2, SPF exhibited a substantially stronger compressibility response than AP and TP at the peak-strength point. The fractal dimension of SPF remains relatively stable before the peak strain but decreases sharply at the peak strain, indicating reduced structural complexity and enhanced connectivity of SPF. These observations suggest that, in the tested specimens, SPF expansion and coalescence under coupled seepage and mining-induced stress were associated with the reorganization of the internal seepage pathways of coal. These findings provide an experimental basis for evaluating stress-dependent permeability, gas transport pathways, and seepage-related failure risks in deep coal under mining-induced stress. Full article

The coupled effect of in situ temperature and stress complicates the permeability evolution of coal reservoirs, which restricts the exploration and evaluation of deep coalbed methane (CBM). Two high-rank coal samples were collected from the Sihe (SH) and Zhaozhuang (ZZ) mining areas, and multi-gradient coupled temperature–stress seepage experiments (20–50 °C, 8–32 MPa) as well as supporting triaxial mechanical tests were carried out to investigate the temperature and stress sensitivity of coal permeability. Combined with coal mechanical deformation characteristics, the transition depth mechanism of permeability evolution with burial depth was revealed. Experimental results indicate that coal permeability follows a negative exponential decay trend with increasing effective stress, and the evolution process can be divided into three stages: rapid attenuation, slow decline and stabilization. Temperature rise can weaken the stress attenuation degree of coal permeability under continuous effective stress loading and effectively reduce the stress sensitivity of coal reservoirs. Under constant confining pressure, permeability decreases linearly with rising temperature; the temperature-induced damage effect is prominent at low effective stress, while the regulatory effect of temperature is greatly weakened when fractures are compacted under high effective stress. An exponential function between permeability and burial depth was established based on coupled temperature–stress experimental data, and the critical burial depth of permeability transition depth in the study area was determined to be 550–600 m. The abrupt change interval of elastic modulus against confining pressure is consistent with the burial depth of permeability transition depth, which acts as the key mechanical factor dominating the nonlinear transition of reservoir permeability. This study provides experimental and theoretical support for the development of deep CBM in the study area. The results represent non-adsorbing gas (nitrogen) permeability under the investigated temperature–stress window (20–50 °C, 8–32 MPa) and should not be extrapolated to methane-bearing CBM reservoirs without adsorption–swelling corrections. The transition depth of approximately 550–600 m is a laboratory-derived estimate rather than a field-verified reservoir threshold. Full article

When urban underground interchange tunnels pass beneath a water-rich spoil-disposal landfill, water–mud inrush risk is jointly controlled by artificial fill, geological conditions, tunnel-group construction disturbance, and environmental constraints. This study investigates the section beneath the Bujiuwo spoil-disposal landfill in the North Extension Project of Qiaocheng East Road, Shenzhen, and aims to identify key risk factors and reveal their hierarchical transmission mechanism. Based on engineering investigation data, design documents, construction risk sources, and expert questionnaires, an indicator system containing 18 risk factors was established. The decision-making trial and evaluation laboratory method was used to calculate the influence degree, affected degree, centrality, causality, and centrality-derived weight of each factor. Adversarial interpretive structural modeling was then introduced to extract the topological hierarchy and identify key coupling loops. The results show that structural fissure water pressure, vertical clearance, landfill water level, emergency pumping and drainage capacity, and spoil-fill thickness act as key nodes in the DEMATEL risk-relation network. Two key loops were identified: the landfill water level–structural fissure water pressure loop and the vertical clearance–additional stress of tunnel group–damage rate of anti-seepage membrane loop. A hierarchical control strategy is proposed, including source control, channel blockage, construction disturbance control, monitoring, early warning, and emergency management. From a sustainability perspective, the proposed framework supports groundwater protection, pollution prevention, maintenance of landfill containment integrity, and reduction in construction-induced environmental risks. Full article

Structural fractures and karst cavities are widely developed in deep and ultra-deep carbonate reservoirs, providing preferential pathways for rapid fluid migration while increasing the risk of severe drilling fluid loss. To investigate the lost-circulation mechanism in fractured-vuggy formations, a hydro-mechanically coupled gas–liquid two-phase seepage model was established by considering the multiple-media characteristics of matrix, fractures and cavities, as well as rock deformation and fluid compressibility. We hypothesize that gas–liquid property differences and hydro-mechanical changes in conductivity jointly control drilling fluid loss, with the gas–liquid property contrast exerting the stronger effect under the simulated conditions. In the model, flow in the matrix and fractures is described by Darcy’s law, while high-velocity flow in cavities is characterized using the Forchheimer non-Darcy equation. The coupling between the seepage field and stress field is achieved by incorporating the effective stress relationship, using the Kozeny–Carman porosity–permeability evolution model and the Goodman fracture deformation model. The coupled equations were implemented in COMSOL. Model validation confirms the reliability of the proposed model in predicting drilling fluid loss. The fracture–vug system significantly enhances fluid exchange between the wellbore and formation. Pressure propagates rapidly along fractures and vugs at the early stage and subsequently diffuses into the surrounding matrix, while the loss rate generally decreases with time. After 120 min, hydro-mechanical coupling increased the loss rate from 1.15 × 10⁻³ to 1.23 × 10⁻³ m³/s and the cumulative loss volume from 11.41 to 12.06 m³. Compared with the single-phase model, the gas–liquid two-phase model predicted a 4.82-fold higher loss rate. Fracture aperture, vug size, bottomhole pressure differential, and rock mechanical properties are the principal factors controlling loss intensity and pressure propagation. Through effective stress variations, hydro-mechanical coupling modifies porosity, permeability, and fracture aperture, thereby affecting formation conductivity and dynamic loss behavior. These results provide theoretical guidance for lost-circulation mechanism analysis, risk assessment, and plugging optimization in deep fractured-vuggy carbonate formations. Full article

Rainfall-induced instability of highway slopes with a soil–rock binary structure may be strongly influenced by the hydraulic barrier effect of low-permeability shale. This study investigated the right-side slope along the D-ramp section from DK0+230 to DK0+660 at Deze Interchange on the Zhanhui Expressway, China. A two-dimensional coupled seepage–stress model was developed based on the engineering geological conditions and rainfall records to simulate the slope response under a 72 h extreme rainfall scenario with an intensity of 175.6 mm/d. Field displacement monitoring data were used to validate the modeled deformation pattern under natural conditions. Under natural conditions, the reinforced toe zone remained stable, deformation was concentrated along the interface between the block-stone layer and strongly weathered limestone in the middle and rear portions of the slope, and the factor of safety was 1.1344, indicating a basically stable state. During prolonged rainfall, infiltrating water accumulated near the interface between the strongly weathered shale and the underlying shale owing to the hydraulic barrier effect of the low-permeability shale, forming a continuous transient saturated zone. The plastic zone progressively extended from the upper shallow weak interface to the lower deep interface and eventually became fully connected, while the factor of safety decreased to 0.9886, indicating overall instability. The results reveal a coupled mechanism involving interfacial water accumulation, increased pore-water pressure, the formation of a transient saturated zone, and a shift in the controlling zone of slope deformation and failure from shallow to deeper layers. These findings provide a reference for disaster prevention and mitigation of similar soil–rock binary-structure slopes. Full article

Understanding the anisotropic mechanical behavior and pore–fracture structure (PFS) evolution of bedded coal under mining disturbance is critical for gas extraction and hazard prevention in deep coal mining. This study employed low-field nuclear magnetic resonance (NMR) and fractal analysis to characterize the PFS of water-saturated coal samples with bedding angles of 0°, 30°, 45°, 60°, and 90°. The pore system was classified into adsorption and seepage pores according to pore size distribution. Real-time triaxial NMR tests were further conducted to reveal the coupled evolution of mechanical responses and PFS under different bedding orientations. Results show that bedding inclination controls pore distribution, connectivity, and structural complexity, while influencing coal strength, deformation, and failure through stress redistribution and bedding-plane activation. The mechanical response and PFS evolution exhibit strong anisotropic coupling during loading. A fractional-order porosity model was established by incorporating bedding orientation, anisotropy, and stress memory based on pore geometry and stress decomposition. Model verification confirms its effectiveness in describing anisotropic porosity and PFS evolution under varying bedding angles. This study provides theoretical support for permeability prediction, stability assessment, and hazard control in deep-bedded coal seams. Full article

Waterfront multi-pit excavation in highly permeable gravel deposits can induce complex pile deformation because excavation unloading, groundwater drawdown, and river-stage disturbance act simultaneously. This problem is particularly important for foundation pits constructed near existing pile-supported buildings, yet the combined effects of excavation sequence, pit spacing and excavation depth under river-connected gravel aquifers remain insufficiently quantified. This study fills the research gap on the seepage–stress-coupled deformation mechanism of adjacent building piles under multi-pit excavation in highly permeable gravel strata, and quantifies the spatial superposition effect of excavation disturbance. In this study, a three-dimensional seepage–stress-coupled finite-element model was established for the Yidu Green Intelligent Shipbuilding Industrial Park project on the right bank of the Yangtze River. The model was validated against field monitoring data from the slipway pit excavation, and comparisons show that the relative errors of pile horizontal displacement and ground settlement between simulation and measurement are both less than 8%, verifying the reliability of the numerical model. The validated model was then used to evaluate single-pit excavation, different multi-pit excavation sequences, pit group spacing, excavation-depth ratio and steel sheet pile parameters. The results show that pile deformation is controlled not only by the excavation of an individual pit, but also by the interaction between pit groups located on opposite sides of the building. Simultaneous excavation reduced the peak horizontal displacement of the adjacent building pile by 45.7% compared with single excavation of the slipway pit and by 31.6% compared with the slipway-first sequence. For pits on the same side of the building, a far-to-near excavation sequence produced the smallest pile displacement and settlement. The inter-pit ground deformation changed from heave-dominated to settlement-dominated when the spacing increased to approximately 90–100 m. The research results can provide reference for deformation control and safety assessment of adjacent buildings during multi-pit excavation in similar highly permeable gravel areas. These findings indicate that coordinated excavation sequence and spacing control can effectively reduce deformation risks in waterfront multi-pit projects, although the proposed thresholds should be verified for different layouts, geological conditions and hydrogeological conditions. Full article

This study addresses stability degradation of cemented unclassified tailings backfill (CTB) under seepage-stress coupling in deep water-rich metal mines. CTB specimens with diverse mix proportions were prepared to explore their mechanical and permeability responses under varying seepage water pressures, and a response surface regression model was built to quantify multi-factor interactive effects on permeability. Results reveal that increased seepage pressure degrades CTB mechanical performance, while the degradation rate gradually declines. Higher cement-tailings ratios amplify the weakening effect of seepage pressure on elastic modulus. The permeability-strain evolution curve of CTB resembles its σ-ε, and the strain at peak permeability k_max always exceeds peak stress strain. The interaction between cement-tailings ratio and seepage water pressure dominates the variation in k_max and k_min. This work deepens the understanding of CTB seepage-mechanical behaviors and offers experimental references for proportion design and stability assessment of CTB in water-rich underground mines. Full article

Long-term agricultural irrigation in the loess platform region of Northwest China has raised the groundwater level and triggered numerous irrigation-induced loess landslides. The widely developed fissures in loess provide preferential pathways for irrigation water infiltration and serve as key factors that control the hydrological evolution and stability of landslides. The Jiaojiayatou landslide in the Heifangtai platform, Gansu Province, was selected as the study case. A coupled saturated-unsaturated seepage–stress numerical model incorporating fissure structures was established to systematically investigate the effects of fissure depth, location, and number on the seepage field evolution, stability, and deformation characteristics of loess landslides under long-term irrigation. The results show that fissures significantly accelerate the advance of the wetting front, enlarge the high-water-content zone, increase pore water pressure, and reduce the factor of safety. Among these parameters, the effect of fissure depth is the most significant: for fissure depths of 5 m and 10 m, the simulated average annual rise in groundwater level is 0.63 m/a and 1.21 m/a, respectively. When the fissure depth increases to 15 m, irrigation water directly recharges the groundwater, leading to landslide instability (factor of safety drops to 0.97). The displacement at the slope shoulder increases by 54% compared with that in the no-fissure case, and the displacement pattern shifts from predominantly horizontal sliding to vertical settlement. Furthermore, the closer the fissure is to the platform edge and the greater the number of fissures, the lower the stability becomes and the larger the soil displacement at the slope shoulder. Full article

Mitigating catastrophic groundwater inrush hazards in deep underground engineering remains a challenge due to extreme hydrostatic pressures and complex hydrogeological conditions. To address this issue, this paper proposes an optimized analytical design methodology for three-dimensional (3D) seepage-control curtains in deep shafts. Structurally, the 3D system is decoupled into independent horizontal and vertical components. The required thickness of each component is conceptualized as the sum of a blasting-influenced zone and a stress-influenced zone. To ensure hydro-mechanical safety, the stress-influenced thickness is rigorously determined by evaluating both a seepage-based theoretical model and an elasto-plastic mechanical model, with the maximum value adopted as the design criterion. When applied to the 545 m deep main shaft of the Lianhuashan Phosphate Mine, the proposed dual-control model yields optimized thicknesses of 14 m and 20 m for the vertical and horizontal curtains, respectively, achieving a robust anti-heave safety factor of 1.90 under a confined water pressure of 4.46 MPa. The analytical design is successfully benchmarked and validated using 3D hydro-mechanical coupled numerical simulations based on Biot’s poroelasticity theory. Furthermore, multi-parameter sensitivity analyses reveal critical mechanical singularities, defining the strict physical capacity limits of the system. The theoretical derivations demonstrate a distinct functional divergence: the vertical curtain acts as a seepage-governed bottleneck highly sensitive to hydro-mechanical variations, whereas the horizontal curtain functions as a highly redundant, strength-governed structural plug. This comprehensive methodology provides essential theoretical insights and quantitative guidelines for effective groundwater hazard mitigation in deep-earth environments. Full article

Underground gas storage converted from depleted oil and gas reservoirs requires reliable long-term sealing of caprocks, faults, and other geological barriers during cyclic injection and withdrawal. Conventional evaluations mainly focus on static geological parameters, whereas the effects of stress evolution during operation are often insufficiently addressed. In contrast, four-dimensional geomechanical simulations based on fluid–solid coupling can capture the dynamic evolution of geological sealing behavior under injection and withdrawal conditions. The review summarizes progress in heterogeneous geomechanical model construction, stress-field evolution under cyclic loading, dynamic sealing assessment of caprocks and faults, and determination of safe operating pressure limits. Geological sealing evaluation has evolved from a static assessment based on geological characteristics to a dynamic assessment controlled by mechanical criteria. Geostress inversion has developed from three-dimensional heterogeneous mechanical models to four-dimensional geomechanical dynamic coupling analyses that account for seepage, stress, temperature, and other factors. Safe pressure evaluation has also progressed from conventional gravity-driven storage construction to the assessment of critical pressure evolution during the safe operation stage. Existing studies indicate that heterogeneous parameter characterization, coupled flow-stress simulation, and dynamic pressure management strongly affect the reliability of sealing evaluation in reservoir-type UGS. The results further show that pressure history, stress redistribution, and creep effects should be considered together when assessing long-term storage safety. The engineering cases listed at the end of this paper verify some of the research findings. The results presented above are of great significance for the construction and safe operation of reservoir-type gas storage facilities. Full article

The development of deep oil and gas reservoirs has become a key focus in the exploration and exploitation of hydrocarbon resources. To elucidate the mechanisms governing the flow evolution of reservoir sandstone under deep, high-triaxial stress conditions, this study takes the deep reservoir sandstone of a specific block as its subject. Using a true triaxial rock mechanics–seepage coupled testing system to simulate the in situ high-triaxial stress environment. Synchronous testing of the sandstone’s mechanical and seepage properties was conducted, and a staged permeability model for the elastic and damaged zones of the sandstone was developed and experimentally validated. The results indicate that, under deep, high-triaxial stress conditions, the total stress–strain behaviour of sandstone is divided into compaction, elastic deformation, yield and post-failure stages. The mechanical behaviour exhibits a ‘compression–unloading–failure’ pattern, whilst permeability follows a U-shaped evolution characterised by an ‘exponential decline–sudden increase–stabilisation’ trend; furthermore, the influence of damage on flow properties is irreversible. The intermediate principal stress exerts a significant strengthening effect on the mechanical properties of sandstone; as it increases, the peak strength and residual strength of the sandstone rise, whilst post-peak brittleness decreases and ductility increases. The intermediate principal stress is a key factor regulating the evolution of permeability. During the elastic stage, the initial permeability of the sandstone decreases as the stress increases, and the rate of exponential decay accelerates; during the damage stage, the post-peak permeability decreases as the stress increases, and the irreversible closure of pores caused by true triaxial stress makes it difficult for the post-peak permeability to recover to its initial value. The coefficient of determination for the permeability model fitted to the elastic stage is greater than 0.9, whilst that for the damage stage ranges from 0.76 to 0.85. These models effectively characterise the quantitative relationship between permeability and strain under different stress conditions, demonstrating good reliability and applicability. The research findings provide experimental evidence for elucidating the coupled relationship between mechanics and flow in sandstone under deep, high-triaxial stress conditions, as well as for predicting flow behaviour in deep reservoirs and designing development schemes. Full article

Under complex terrains and extreme environmental conditions such as high wind speeds, prestressed rock anchor foundations for onshore wind turbines are subjected to multiple coupled adverse effects during long-term service, including cyclic wind loading, temperature variation, groundwater intrusion, and rock mass weathering. These factors significantly affect structural performance and service life through corrosion and material degradation processes, while conventional design methods mainly focus on ultimate bearing capacity and lack a systematic consideration of corrosion-induced deterioration mechanisms and long-term performance evolution. Without changing the theoretical framework of current design codes, this study introduces a durability-oriented design concept and explicitly incorporates corrosion effects and material degradation into the analytical system of prestressed rock anchor foundations. First, from the perspective of anchor force evolution, a time-dependent analysis method for long-term prestress loss is established, considering the coupled effects of steel corrosion, material relaxation, and cyclic loading. Second, for the mechanical behavior of group anchor systems, a shear capacity model is proposed that accounts for rock mass strength degradation and grout–rock interface deterioration. Meanwhile, the coupling relationship between foundation void development and groundwater seepage is analyzed, revealing its critical role in the corrosion evolution process. On this basis, a coordinated design method for foundation dimensions and prestress parameters is developed to satisfy both load-bearing capacity and durability requirements. Finite element analysis is further conducted to verify the stress and deformation characteristics of the foundation–rock–anchor system under nonlinear conditions. Engineering case studies demonstrate that the proposed method not only meets bearing capacity requirements, but also effectively suppresses void development, reduces corrosion risk, delays structural performance degradation, and improves long-term service reliability. The findings provide a theoretical basis and engineering reference for the durability design and lifecycle performance optimization of prestressed rock anchor foundations for onshore wind power structures in extreme environments. Furthermore, the study underscores the critical role of advanced anti-corrosion coatings and surface protection systems in mitigating the coupled corrosion-degradation mechanisms, aligning with the scope of this Special Issue on corrosion protection and durability of infrastructure in harsh environments. Full article

Ensuring the sustainability of deep coal mining requires a comprehensive understanding of hydrogeological risks, particularly fault-induced water inrush, which threatens human safety, depletes freshwater resources, and causes irreversible ecological damage. This study addresses the sustainability gap in managing heterogeneous fault fracture zones by conducting coupled loading–seepage experiments on representative limestone, sandstone, coal, and coal–rock mixtures from the Zhaogu No. 2 Mine. Results demonstrate that seepage behavior follows the Forchheimer non-linear regime (E = 0.2–0.95), deviating significantly from Darcy’s law. We quantified that effective stress induces particle crushing and rearrangement, leading to a drastic porosity reduction (up to 97.52% in coal). Crucially, lithology dictates permeability evolution: coal and mixtures exhibit exponential decay, whereas sandstone and limestone follow quadratic functions. The fractal dimension of particles correlates negatively with permeability (R² > 0.95). These findings provide a quantitative framework for predicting water inrush channels, enabling proactive strategies to prevent catastrophic groundwater loss and ensure the long-term viability of mining operations. This research supports SDG 6 (Clean Water) and SDG 12 (Responsible Consumption and Production) by offering scientific guidance for balancing resource extraction with hydrogeological integrity. Full article