Search Results (105)

To support intelligent and sustainable mine engineering, this geotechnics-based study integrates laboratory testing, three-dimensional numerical simulation, and field monitoring to optimize roadway support and improve resource efficiency. This study investigates the geotechnical behavior of the surrounding rock in coalmine roadways under single-face unloading conditions, aiming to provide theoretical and practical support for surrounding rock control in underground coal mining. Excavation of the roadway creates a free surface, leading to unloading, which makes timely support crucial for preventing instability. True-triaxial single-face unloading tests and mechanical tests on hole-containing coal specimens show that the coal exhibits four characteristic stages, namely fissure compaction (closure), elastic deformation, yielding, and residual strength. Under a confining stress of 4 MPa, the peak strength of Coal Seam No. 3 in the true-triaxial single-face unloading test reached 32.4 MPa, whereas the peak strength of the hole-containing coal specimen was only 17.1 MPa, and failure occurred as instantaneous global instability with an “X”-shaped conjugate shear pattern. Numerical simulations were conducted to optimize the roadway’s surrounding rock control scheme, indicating that increasing the bolt length increases the proportion of the load carried by the rock bolts while reducing the load borne by the cable bolts. In addition, advance abutment pressure increases the forces in the support system and amplifies deformation of the solid rib, coal-pillar rib, and roof; roadway surface convergence is dominated by floor heave. Full article

Fissured rock masses are widespread in deep underground mining engineering, and they are prone to inducing instability and failure during excavation activities. Borehole pressure relief is one of the most effective measures with which to control dynamic disaster in high-stress roadways. After pressure relief, redistribution of stress leads to stress concentration in the far-field surrounding rock (far away from working face), which can be represented by true triaxial compression state. However, current research on the energy evolution behavior of fissured rock masses under far-field conditions remains relatively limited. This study analyzes the energy evolution process, peak energy characteristics, and laws of energy storage and dissipation in fractured sandstone under different fissure dip angles (θ, 30°, 45°, 60°, 90°), with intermediate principal stresses (σ₂, 10, 20, … 120 MPa) and minimum principal stresses (σ₃, 10, 20, … 50 MPa). The results indicate that the curve of dissipated energy ratio versus maximum principal strain becomes more distinctly concave as θ increases under true triaxial compression. The growth rate of the dissipated energy ratio and dissipated energy with maximum principal strain gradually decreases when σ₃ is high, and the fissured sandstone is prone to exhibiting ductile failure, leading to a reduced energy dissipation rate. The peak elastic strain energy of fissured sandstone increases gradually with increasing σ₂ and shows a linear characteristic. The energy storage and dissipation law is nonlinear with increasing peak total energy for the fissured sandstone with different values of θ. However, the law exhibits a linear trend under varying σ₂ and σ₃. This study provides a new approach and insight into the failure characteristics of deep fissured sandstone and aims to offer theoretical guidance for the layout and construction safety of roadways or mining panels in far-field surrounding rock in future engineering practices. Full article

To elucidate the micromechanical origins of the macroscopic anisotropic behavior of granular materials, this study develops a micromechanically based elastoplastic constitutive model for sand. First, anchored in the static equilibrium hypothesis and granular micromechanics theory, a true stress tensor is introduced to characterize the authentic inter-particle contact forces. Serving as a coupled variable of the macroscopic stress and the microscopic fabric tensor, this formulation not only quantifies the directional distribution of the contact network but also enables the mapping of anisotropic yielding and deformation analyses into an equivalent isotropic true stress space. Subsequently, a comprehensive constitutive framework is established by integrating critical state theory, an anisotropic fabric evolution law, and an energy-based stress–dilatancy relationship that explicitly accounts for the evolution mechanism of the microscopic coordination number. The physical interpretation, calibration procedure, and sensitivity analysis of the model parameters are also presented. The predictive capability of the model is rigorously validated against conventional triaxial tests on Ottawa sand, true triaxial numerical simulations, and experimental data for Toyoura sand with inherent anisotropy. The comparisons demonstrate that the model accurately captures not only the stress–strain response and volumetric deformation under conventional loading but also the strength dependency on loading direction and mechanical characteristics under complex stress paths, substantiating the validity and universality of the proposed micromechanical approach. Full article

Supercritical CO₂ (SC-CO₂) fracturing has emerged as a promising technology for the effective stimulation of unconventional tight reservoirs due to its low viscosity, high diffusivity, and environmental advantages. However, existing fracturability evaluation models often oversimplify key parameters and lack validation under realistic dual-well conditions. To address these gaps, we developed a multi-factor coupled evaluation model incorporating well spacing, stress anisotropy, and fluid viscosity and proposed a fracturability index (FI) to quantify the potential for complex fracture development. True triaxial SC-CO₂ fracturing experiments using both single- and dual-well setups were conducted, and 3D fracture networks were analyzed via CT imaging and U-Net segmentation. Results show strong agreement between FI and fracture complexity. Optimal fracturing conditions were identified, providing a practical framework for the design and optimization of SC-CO₂ fracturing in tight reservoirs. Full article

Coarse-grained soil is a commonly used filling material in foundation engineering, and its static shear characteristics are significantly affected by the initial stress state. For coarse-grained soils, clearly defining the drainage conditions and improving the accuracy of pore water pressure measurements are crucial in static shear tests. Based on GDS dynamic and static true triaxial equipment, this paper systematically conducts static shear tests on coarse-grained soil under three-dimensional initial isotropic, three-dimensional initial anisotropic, and plane strain states. The effects of initial mean principal stress, initial generalized shear stress, initial intermediate principal stress coefficient, and water content on the stress–strain relationship, strength, modulus, and friction angle of coarse-grained soil are analyzed. The research shows that under the same initial mean principal stress, the peak strength under a plane strain state is the largest, and that under a three-dimensional initial anisotropic state is the smallest. The peak strength of coarse-grained soil with optimal water content is generally higher than that under a saturated state; under a three-dimensional initial anisotropic state, the peak strength decreases with an increase in the initial generalized shear stress and increases with an increase in the initial intermediate principal stress coefficient. The research results provide a theoretical basis for the analysis of mechanical behavior of coarse-grained soil in foundation engineering. Full article

In deep underground engineering projects, the rock mass is frequently subjected to extreme environments characterized by high geostress and high permeation pressure. This makes the rock mass highly prone to disasters such as collapses, significant deformations, and water seepage. Among these factors, the direction of seepage plays a critical role. In this study, true triaxial tests were performed to investigate the characteristic stress and failure behaviors of mudstone under seepage conditions in different principal stress directions. The test results indicate that, under permeation pressure (σ_p), the characteristic stresses are significantly reduced. After TTS-1 shifts to TTS-2, the permeability of the mudstone decreases significantly. A volumetric dilation hysteresis effect of mudstone was discovered. Furthermore, the increase in β₁ and decrease in β₂ indicate that, after the transition from TTS-1 to TTS-2, the stable crack propagation stage in the mudstone is prolonged, while the unstable crack propagation stage is shortened. In the σ₁–σ₃ plane, after TTS-1 shifts to TTS-2, the change in the included angle between the mudstone fracture surface and the σ₁ direction shows a reverse trend with the increase in σ_p. Full article

The stability and safety of the vertical shaft during construction is an important problem for deep mining engineering because of the high in situ stresses. This paper conducts experimental studies on the difficulty of shaft support during the construction of No. 6 deep shaft at the Huize Mine, Yunnan Province, China. Based on the rule of similarity test, a similar material formula was developed, and standard model samples of the vertical shaft were prepared. Three different support methods were set up, including steel fiber-reinforced concrete support, drilling pressure relief support, and slot filling support. The experiments were conducted by using a true triaxial test system, and the testing process was monitored by a static stress–strain gauge and an acoustic emission system. The experimental results show that the integrity of the borehole pressure relief support shaft is optimal under the in situ stress. As the maximum principal stress increases to the instability and failure of the shaft, the peak load, cumulative number, and energy of acoustic emission events were the highest using the steel fiber concrete support method, and the peak load was the lowest using the borehole pressure relief. The borehole pressure relief transfers the stress around the shaft to the deep part. Although it ensures the integrity of the shaft, it causes internal damage to the shaft, reduces the energy storage of the shaft, and results in the lowest cumulative number and energy of acoustic emission events. After the instability and failure of the shaft, the average block size of the shaft debris is the highest under the borehole pressure relief support along the direction of the maximum principal stress. On the other hand, the mechanical properties of samples with different support methods under dynamic load conditions are studied by applying external low-frequency disturbances, and the test conclusions have been verified through numerical simulation. Field tests have verified that the steel fiber-reinforced concrete lining support can maintain the integrity of the deep shaft wall and ensure safety during mining production. Full article

Excavation unloading in deep rock masses involves a transition from symmetric states of energy storage to asymmetric energy dissipation, in which variations in intermediate principal stress (${\sigma }_2$) play a critical role. To investigate these symmetry-breaking mechanisms, controlled-rate true triaxial unloading experiments were performed on sandstone using a miniature creep-coupled testing system. During unloading of ${\sigma }_3$ at 0.1–0.3 MPa/s, the evolution of elastic, dissipated, and plastic energies was quantitatively evaluated. The results reveal pronounced asymmetric energy responses governed by both ${\sigma }_2$ and the unloading rate. Dissipated energy dominates the entire unloading process, while elastic energy exhibits a non-monotonic trend with increasing ${\sigma }_2$—first rising due to enhanced confinement and then decreasing as premature failure occurs. Higher unloading rates significantly accelerate total, elastic, and dissipated energy conversion and intensify post-peak brittleness. A new metric, plastically released energy, is proposed to quantify the asymmetric energy release from peak to residual state after failure. Its dependence on ${\sigma }_2$ is strongly non-monotonic, increasing under moderate ${\sigma }_2$ but decreasing when ${\sigma }_2$ is sufficiently high to trigger failure during unloading. This behavior captures the essential symmetry-breaking transition between elastic energy accumulation and irreversible plastic dissipation. These findings demonstrate that true triaxial unloading induces energy evolution patterns far from symmetry, controlled jointly by ${\sigma }_2$ and unloading kinetics. The established correlations between ${\sigma }_2$, unloading rate, and plastically released energy enrich the theoretical framework of energy-based symmetry in rock mechanics and offer insights for evaluating excavation-induced instability in deep underground engineering. Full article

To reveal the deformation and failure mechanisms as well as the fracture evolution patterns of water-bearing coal–rock composites under complex stress conditions, this study established a true triaxial stress model for the key load-bearing structure of mined coal pillar dams and developed a true triaxial loading apparatus capable of implementing localized unloading paths. True triaxial loading–unloading tests were conducted on coal–rock composites under different water content conditions, and the internal fracture structures were quantitatively characterized using CT scanning combined with fractal analysis. The results indicate that: (1) under a constant axial stress-unloading confining stress path, failure primarily occurs in the coal component, and the extent of failure significantly increases with the water content of the roof rock. For instance, the total fracture volume in the coal body increased by approximately 66% from the dry to the saturated state, while the lateral strain at peak stress decreased by about 65% over the same range, indicating a transition towards more brittle behavior. (2) CT scanning and three-dimensional reconstruction results reveal that the fracture system exhibits pronounced multi-scale polarization, with significant differences in volume, surface area, and morphological parameters between the main fractures and micropores, reflecting strong heterogeneity and anisotropy; (3) fractal dimension analysis of two-dimensional slices indicates that the fracture structures exhibit fractal characteristics in all directions, with the spatial distribution of fractal dimensions closely related to the loading direction. Overall, the XY-direction fractures exhibit the highest complexity, whereas the XZ and YZ directions show pronounced directional anisotropy. As water content increases, the amplitude of fractal dimension fluctuations rises, reflecting an enhancement in the geometric complexity of the fracture system. Full article

A series of (45 sets) true triaxial shear tests with controlled suction and under conditions of equal-b and equal-p are performed on unsaturated compacted loess. By incorporating the effects of matric suction in unsaturated loess and the influence of middle principal stress conditions, the characteristics of the stress–strain curve evolution, strength failure line and related strength parameters of the unsaturated compacted loess under complex stress paths are analyzed, and the applicability of different strength criteria is analyzed. The results indicated that both the matric suction and the middle principal stress condition have significant effects on the magnitude of shear stress. While matric suction exerts minimal effect on the configuration of the stress–strain curve, the middle principal stress condition markedly affects its shape. A function is established to approximately describe the correlation between the strength-related parameters and the middle principal stress coefficient. A comparison of the failure lines predicted with several strength criteria and the experimental data indicates that the experimental results of unsaturated compacted loess under a controlled suction state are consistent with the results of the predicted failure lines of the Lade–Duncan strength criterion. The findings can provide crucial theoretical support for engineering design and disaster prevention in loess regions. Full article

Engineering unloading activities during deep mineral resource extraction subject the backfill materials to complex true triaxial stress conditions, where their mechanical behavior and damage mechanisms are critical to stope stability. In this article, a true triaxial testing system was employed to conduct unloading tests under different initial intermediate principal stress (σ₂) conditions, aiming to elucidate the influence mechanism of σ₂ on strength, deformation, failure modes, and acoustic emission (AE) characteristics of the backfill, and to establish a corresponding damage constitutive model. The results demonstrate that the σ₂ governs the mechanical response and failure mode of the filling material. Within the tested range, σ₂ nonlinearly enhances both the peak stress, indicating improved load-bearing. As σ₂ increases, acoustic emission activity changes from intermittent to continuous high-intensity ringing counts. The transition from brittle to ductile fracture. Model predictions showed high agreement with experimental data, validating its applicability. This study provides a critical theoretical foundation and modeling framework for assessing the stability of backfill structures under deep well mining conditions and guiding engineering design. Full article

This study employs a three-dimensional numerical simulation based on the discrete–finite element coupled method to investigate the mechanical excavation of a tunnel and its influence on the support structures. The discrete element method accurately reproduces the mechanical cutting procedure during excavation, and the finite element model covers the majority volume of the model for reflecting the response of far-field rock. The main conclusions drawn from this research are as follows: (1) Under true triaxial loading conditions, the influence of the inclined interface between the rock strata on the tunnel’s displacement and stress field is relatively low, and the uniform displacement and stress field are formed around the tunnel. (2) The detailed mechanical excavation and rock-breaking process is simulated, and a secondary crack layer (shear failure dominates) with a thickness of about 30 cm formed on the surrounding rock of the tunnel; secondary tensile and shear cracks present different distributions and orientations, which are caused by the mechanical drilling and cutting processes. (3) Although over 60% of the lengths of the anchor rods in the tunnel-side walls reach the yield strength of the Q235 steel rod, the anchor rod system is relatively safe (lower than the tensile strength) and plays a positive role on rock stability; however, the anchor rods in the tunnel roof are safer because the low deformation (about 50% compared to the rods in the side walls), and only a minority of the anchor rods exceed the yield stress. Full article

Strain localization is a critical phenomenon in geotechnical materials, serving as a precursor to the failure of engineering structures such as slopes, foundations, and tunnels. This paper presents a comprehensive review of the theoretical, experimental, and numerical advances in the study of strain localization. Theoretically, the review spans from classical empirical criteria for shear band inclination to the more rigorous bifurcation theory, which mathematically defines the onset of localization as a loss of uniqueness in the governing equations. Experimentally, various laboratory techniques including direct shear, triaxial, plane strain, and true triaxial tests are discussed, highlighting how they have revealed the influences of microstructure, stress path, and boundary conditions on shear band development. The core of the review focuses on numerical simulations, critically analyzing the limitations of the classical Finite Element Method (FEM) due to mesh dependency. It then elaborates on advanced regularization strategies, encompassing weak discontinuity methods (e.g., Cosserat continuum theory) that introduce an internal length scale to model finite-width shear bands, and strong discontinuity methods (e.g., the Strong Discontinuity Approach, SDA) for simulating discrete cracks. Significant emphasis is placed on innovative coupled approaches, particularly the Cos-SDA model, which integrates the advantages of both weak and strong discontinuity methods to seamlessly simulate the entire progressive failure process from diffuse localization to discrete slip. Furthermore, the application of spectral analysis for evaluating the regularization performance of these numerical methods is examined. Finally, the review concludes by identifying persistent challenges and outlining promising future research directions, including 3D modeling, multi-field coupling, and the integration of data-driven techniques. This synthesis aims to provide a valuable reference for advancing the prediction and management of failure in geotechnical structures. Full article

The fluid flow in underground reservoirs is directly related to resource recovery and hazard prevention. In this study, the evolution of fractured sandstone deformation and permeability under an in situ stress influence is investigated using the true triaxial percolation system. The results show that the strain of fractured sandstone increases logarithmically with the increase in axial stress. The evolution of axial strain is dominated by the maximum principal stress, and the minimum principal stress and the intermediate principal stress affect the strain amplitude. The fracture morphology of low-permeability sandstone affects permeability and strain evolution. Small fractures are more sensitive to the increase in the maximum principal stress, and the response in principal strain to the increase in principal stress is obvious in large fractures. There is a negative exponential relationship between pore pressure and the conductivity of fractures. When pore pressure is 0.3 MPa, the conductivity is the highest; meanwhile, when pore pressure is 1.8 MPa, the conductivity is the lowest. The decreasing range of the conductivity increases with the increase in fracture size. In situ stress significantly affects the evolution of principal strain and related permeability. The permeability decreases with an increase in the minimum and middle principal stresses. Under low pore pressure (0.3–0.6 MPa), the permeability decreases with an increase in the principal stress; meanwhile, under high pore pressure (0.6–1.8 MPa), permeability changes slightly with an increase in the principal stress. The findings provide reference to the engineering practice of underground mining. Full article

Understanding the mechanisms of injection-induced fault slip is critical for managing subsurface energy technologies. This study experimentally investigates the influences of the intermediate principal stress (σ_y), minimum principal stress (σ_x), and injection pressure (P) on fault slip initiation stress and velocity. Experiments were conducted on pre-faulted granite specimens (100 mm cubes) using a true triaxial apparatus, simulating in situ stress conditions. The results reveal a two-stage slip process: an initial stable stage dominated by elastic energy accumulation, followed by a slip stage characterized by rapid energy release and stick–slip oscillations. We found that slip initiation stress increases linearly with both σ_y and σ_x, but decreases linearly with increasing P. A higher σ_y delays slip initiation but can lead to larger stress drops and higher slip velocities upon failure. Conversely, fluid injection weakens the fault by reducing effective normal stress, exhibiting a dual effect: it lowers the stress required for slip and enhances the instantaneous slip velocity after initiation. Our findings provide quantitative, mechanistic insights into fault slip behavior, serving as a critical benchmark for numerical simulations and contributing to improved assessment and mitigation of injection-induced seismicity across various engineering applications. Full article