Search Results (1,058)

This study investigated the mechanical response and instability precursors of highly porous coral blocks from the South China Sea under complex stress paths through conventional triaxial compression tests, two types of triaxial unloading tests, and synchronous acoustic emission (AE) monitoring. The effects of confining pressure, unloading path, and unloading stage on strength, deformation, dilatancy, and failure behavior were examined. The coefficients of variation of dry density, saturated density, porosity, and P-wave velocity were 5.07%, 3.56%, 3.72%, and 5.77%, respectively, indicating relatively limited variability in the measured physical properties, although the influence of specimen heterogeneity cannot be fully excluded. Within the 0–2 MPa confining-pressure range, peak strength increased from 8.81 to 16.85 MPa, whereas axial strain at peak strength changed from 0.33% at 0 MPa to 0.63% at 1 MPa and then decreased to 0.40% at 2 MPa, indicating strong strength sensitivity but a nonmonotonic deformation response. During unloading, all specimens exhibited a transition from compaction to dilatancy. At unloading rates of 0.2 and 0.5 MPa/min, the absolute value of the volumetric strain evolution slope was higher under the increasing-axial-pressure unloading path than under the constant-axial-pressure unloading path, indicating that the path-related difference in dilatancy appears more pronounced under the present test conditions. AE activity increased progressively near peak stress during conventional compression, whereas unloading-induced AE events concentrated near macroscopic failure. Lateral strain anomalies generally preceded AE bursts, suggesting that lateral deformation appears to provide a more sensitive early-warning indicator under the present test conditions. Full article

To comparatively evaluate the shear behavior of controlled low-strength materials (CLSM) prepared from different local soil sources, three representative soils from the Yellow River alluvial plain, namely, silt, silty clay, and sand, were used to prepare CLSM with a cement–slag–fly ash–gypsum blended cementitious binder. Triaxial shear tests and scanning electron microscopy (SEM) observations were conducted to compare the failure modes, stress–strain responses, strength characteristics, and hardened microstructures of the three CLSM types under different binder contents and confining pressures. The specimens generally exhibited inclined shear planes, conjugate shear planes, vertical cracks, and plastic bulging. Their stress–strain responses could generally be divided into four stages: linear elastic deformation, plastic yielding, strain softening, and residual stabilization. Within the tested binder-content ranges, the peak strength generally followed the order of sand-based CLSM > silt-based CLSM > silty clay-based CLSM. On average, the residual strength retained approximately 75% of the peak strength. The failure stress states of the tested CLSM could be reasonably represented by the Mohr–Coulomb criterion within the investigated confining-pressure range, and preliminary empirical relationships were established within the tested ranges to estimate peak strength, residual strength, and shear strength parameters. SEM observations suggested that C–S–H-like gel and needle-like products appeared to fill pores and form cemented connections between soil particles, providing a possible qualitative interpretation of the macroscopic strength differences among the three CLSM types. These findings provide a basis for shear strength evaluation and the mix design of CLSM prepared from Yellow River alluvial soils. Full article

Surface roughness is ubiquitous in hard rock discontinuities at different scales and plays a critical role in governing frictional behavior and rupture dynamics. In this study, triaxial shear tests were conducted on sawcut smooth fractures and tension-induced rough fractures to investigate frictional behavior, roughness evolution, and rupture dynamics with increasing shear cycles. The results demonstrate that rough fractures exhibit higher shear strength and more intense stick-slip behavior than smooth fractures, but show strength weakening and reduced stress drops with shear cycle. In contrast, smooth fractures display relatively stable strength and stress drops. These differences in frictional behavior are governed by roughness evolution. Although roughness decreases in both fracture types after shearing, rough fractures experience degradation nearly an order of magnitude greater than that of smooth fractures. The initial stick-slip event on rough fractures generates the largest stress drop and apparent breakdown work. In addition, analyses of stress drop and energy dissipation reveal that friction drops for different types of fracture are concentrated within the range of 0.01 to 0.3. These findings highlight the critical role of roughness evolution in fault stability and provide valuable insights for seismic hazard assessment in deep underground engineering. Full article

The ultra-deep carbonate reservoirs of the Fuman Oilfield in the Tarim Basin are characterized by intense fracture development. The coupled effects of high in-situ stress and fracture structures significantly deteriorate the mechanical properties of the rock mass, thereby constraining wellbore stability evaluation and safe drilling and completion operations. Existing studies have primarily focused on medium- to low-confining-pressure conditions and isolated fracture parameters, making it difficult to characterize the mechanical response of fractured rock masses under the high-stress conditions of ultra-deep reservoirs. To address this issue, limestone from the Yingshan Formation of the target reservoir was selected as the research object, and fractured specimens with varying fracture angles, widths, and densities were prepared. Uniaxial compression tests and triaxial compression tests under high confining pressures of 90 MPa and 120 MPa were conducted to systematically reveal the evolution of rock strength, deformation parameters, shear strength parameters, and failure modes under the coupled influence of fracture geometric parameters and confining pressure. On this basis, a confining-pressure–fracture coupled damage prediction model was established, and wellbore stability around the reservoir was analyzed using Finite Difference Method. The results indicate that fracture angle causes the peak strength and Young’s modulus to first decrease and then increase, with an inclination angle near 45° representing the most unfavorable fracture orientation. Increases in fracture width and density lead to continuous degradation of strength and stiffness. Although high confining pressure can close fractures and enhance load-bearing capacity, it cannot eliminate the controlling influence of fractures on failure pathways. Sensitivity analysis shows that the Young’s modulus and Poisson’s ratio are most sensitive to fracture width; cohesion is mainly governed by fracture width and density; and the internal friction angle is most sensitive to fracture density. Numerical simulations of wellbore stability further confirm that medium-inclination, large-aperture, and high-density fractures significantly increase the risk of wellbore instability. The findings provide experimental and theoretical support for mechanical-parameter correction, wellbore stability assessment, and construction-risk control in ultra-deep fractured carbonate reservoirs. Full article

Multilithologic interbedded reservoirs commonly consist of frequent alternations of fine-grained rocks, carbonate rocks, and soluble evaporite interlayers. The contrasts in mechanical properties and fluid–rock interactions tend to induce hydraulic-fracture deflection, height containment, and complex cross-interface propagation. To elucidate fracture initiation and cross-layer connectivity, a self-developed true-triaxial hydraulic fracturing simulation system was used to systematically investigate the effects of lithologic configuration, fracturing-fluid viscosity, injection rate, interface position, and injection-fluid type on fracture morphology and cross-interface behavior. Integrated analyses were performed by jointly interpreting injection-pressure responses and three-dimensional fracture reconstructions. The interactions between hydraulic fractures and lithologic interfaces/natural fractures can be categorized into three modes: (i) deflection with restricted growth, (ii) penetration without activation, and (iii) penetration with synchronous activation. Under water-based fluids, soluble evaporite interlayers predominantly develop dissolution-induced conductive pathways, which reduce stress concentration at the fracture tip and weaken interface strength, thereby promoting activation of interfaces or natural fractures. Moderately increasing viscosity and injection rate enhances cross-layer connectivity while lowering the probability of passive activation of interfaces/natural fractures; however, excessively high injection rates may induce fluid diversion and increase the likelihood of complex fracture growth. The injection-fluid type exerts a pronounced control on breakdown pressure and connectivity patterns: supercritical CO₂ yields the lowest initiation pressure, water-based fluids the highest, and alcohol-based fluids an intermediate response. In the pressure curves, attenuation of propagation pressure corresponds to enhanced cross-layer penetration, whereas a sustained pressure increase indicates dominant diversion or restricted propagation. These findings provide experimental support for parameter optimization and fracture-control design in multilithologic interbedded reservoirs in Southwest China and analogous geological settings. Full article

To reveal the damage–seepage coupling mechanism of delayed floor water inrush induced by small fault activation under mining-induced stress, a cubic cement mortar specimen containing a persistent small fault was prepared based on similarity theory. Systematic triaxial loading–seepage tests were conducted under different fault fracture zone particle gradations, fracture zone widths, and fault angles, with simultaneous monitoring of stress–strain behavior, acoustic emission (AE) characteristics, and seepage flow evolution. The results show that: ① The peak strength decreases with increasing fracture zone width, but increases with increasing Talbot gradation coefficient (a fractal grading method) and fault angle. The failure mode transitions from shear-dominated to tension–shear composite failure. The spatial localization of AE events corresponds well with macroscopic fracture surfaces, and the AE source amplitude is positively correlated with compressive strength. ② The seepage flow exhibits a nonlinear evolution pattern of “compaction stabilization—stepwise rise—plateau stabilization” during loading. In the early loading stage, compaction of the fracture zone causes a slight decrease in flow. Approaching peak strength, the initiation and propagation of through-going fractures create interconnected seepage channels, leading to a stepwise jump in flow. In the post-peak stage, accompanied by fine particle erosion and framework reconfiguration, the flow tends to stabilize. A larger fracture zone width, smaller gradation coefficient, and smaller fault angle result in a more significant post-peak seepage surge, with the maximum flow rate reaching 3.6 times that of the specimen with a 2 mm wide fracture zone. ③ Grey relational analysis indicates that the fault angle is the most sensitive factor affecting the risk of delayed water inrush (correlation degree 0.788), followed by particle gradation and fracture zone width. The study demonstrates that under monotonic loading conditions, the damage evolution and seepage response of small faults are jointly controlled by their geometric parameters and internal structure, with the fractal grading method effectively quantifying the role of particle gradation. The findings provide a theoretical basis for risk assessment of delayed water inrush from small faults in working faces above confined aquifers. Full article

Deep and ultra-deep carbonate reservoirs commonly experience fracture closure and conductivity reduction under high-temperature and high-stress conditions. In this study, triaxial creep tests were conducted on unacid-etched and acid-etched carbonate cores under different stress levels to investigate their time-dependent deformation behavior and the influence of acid etching on rock rheology. The results indicate that carbonate rocks exhibit pronounced creep behavior, including instantaneous elastic deformation, primary creep, and steady-state creep. Acid etching significantly altered the creep characteristics and rheological parameters of carbonate rocks, leading to distinct time-dependent deformation responses compared with the unacid-etched core. The Burgers constitutive model was employed to characterize the creep behavior, and all fitting correlation coefficients exceeded 0.9. Finite element simulations based on the fitted parameters successfully reproduced the experimental creep curves, verifying the reliability of the constitutive model. This study provides a theoretical and numerical basis for evaluating the long-term deformation behavior of acid-etched carbonate rocks and its implications for fracture closure and conductivity evolution. Full article

Seasonal freeze–thaw cycling progressively rearranges pores and propagates microcracks in silty clay, reducing the reliability of cold-region earthworks. This study evaluated a bio–polymer stabilization strategy combining microbially induced carbonate precipitation (MICP) with anionic polyacrylamide (APAM) to improve mechanical performance and freeze–thaw durability. Six groups were prepared at identical moisture and compaction conditions: water, APAM, and four MICP–APAM groups with bacterial optical densities (OD600) of 0.8, 1.0, 1.2, and 1.4. Unconfined compressive strength, unconsolidated-undrained triaxial compression, ultrasonic pulse velocity, and SEM, TG/DTG, XRD, and FTIR analyses were conducted before and after freeze–thaw cycling. The M1.0-APAM group showed the best overall performance, with UCS values of 1.35 MPa before cycling and 0.89 MPa after nine cycles, together with high shear resistance and ultrasonic velocity. Lower bacterial concentration provided insufficient cementation, whereas higher concentrations promoted non-uniform carbonate deposition, pore heterogeneity, and local stress concentration. Microstructural evidence indicated that OD600 ≈ 1.0 produced a relatively homogeneous network of fine carbonate clusters and polymer-associated films, with calcite formation supported by TG/DTG and XRD. The results show that MICP–APAM treatment enhances silty clay primarily through coordinated mineralization uniformity, pore refinement, and polymer bridging, providing a sustainable stabilization option for seasonally frozen soils. Full article

Salt rock exhibits pronounced viscoelastic creep, continuously imposing radial extrusion loads on casing and threatening long-term well integrity. Field observations in the Missan Oilfield, Iraq, show that casing damage is concentrated near salt–non-salt interfaces, where lithologic contrasts intensify stress redistribution and mechanical coupling. This study integrates triaxial creep experiments, a calibrated modified Burgers model, UMAT implementation, and three-dimensional finite element simulations to investigate casing stress evolution and failure mechanisms. The calibrated model reproduces salt rock creep with a maximum relative strain error of 16.8%. Results show that post-cementing salt creep amplifies non-uniform radial loading at the interface, causing progressive casing stress concentration. At low inclination, the interface–casing intersection evolves into an elliptical annulus; the circumferential variation in equivalent wall thickness and stress-peak migration jointly weaken local stress concentration. However, when the inclination angle reaches approximately 45° at β = 0°, the peak Mises stress begins to exceed that under the vertical-well condition. When α ≥ 65°, the peak stress no longer decreases monotonically with azimuth but exhibits a decrease–increase trend. This indicates that eccentric loading and the additional bending moment dominate the transition from radial extrusion to coupled bending–shear–extrusion loading. A casing stress risk map and grade-selection chart are developed to support casing design in salt-interbedded formations. Full article

Although foamed lightweight soil is widely used for its light weight and high strength, its insufficient dynamic performance under cyclic loading and the poorly understood reinforcement mechanism have become key bottlenecks restricting its optimized application. To investigate the dynamic characteristics and influencing factors of geogrid-reinforced foamed lightweight soil (GRFLS), laboratory dynamic triaxial tests were conducted using a DJSZ-100D dynamic–static triaxial testing system. The effects of the number of geogrid layers and wet density on the dynamic mechanical properties were examined, with analysis focused on failure patterns, backbone curves, dynamic strength, dynamic shear modulus, and damping ratio. The results indicate that the inclusion of geogrids effectively restrained the propagation of longitudinal cracks in the foamed lightweight soil. The hyperbolic backbone curves were well characterized by the Hardin–Drnevich model. An increase in wet density significantly enhanced the dynamic strength, and an optimal number of two reinforcement layers was identified based on the reinforced strength–stress ratio. The dynamic elastic modulus and damping ratio of GRFLS increased with growing dynamic strain. Compared with the unreinforced condition, the initial dynamic elastic modulus of the specimens with two geogrid layers increased by an average of 15.6%, and the maximum damping ratio increased by an average of 12.9%. While both geogrid reinforcement and higher wet density effectively increased the dynamic elastic modulus, only an increase in wet density notably improved the damping ratio. Finally, predictive models for the enhanced dynamic elastic modulus and damping ratio, which incorporate wet density and the number of reinforcement layers, were established. These models indirectly reflect the dynamic deviator stress–strain relationship of GRFLS. This study provides a theoretical basis for engineering construction. Full article

Rock pillars in deep underground mines are subjected to complex stress environments. The combined effects of in situ stress and cyclic disturbances from mining activities lead to a redistribution of the surrounding rock mass stress field, which readily triggers instability and failure, posing severe threats to mining engineering safety. To investigate the damage mechanism of cyclic loading on rock and its weakening effect on the bearing capacity of mine pillars, this study takes limestone as the research object. A series of uniaxial compression tests were conducted on limestone specimens subjected to triaxial cyclic pre-damage, complemented by numerical simulations to further characterize the energy and deformation evolution of the damaged limestone under cyclic loading conditions. The findings are as follows: (i) Triaxial cyclic tests on limestone show that both the input energy and dissipated energy follow similar trends, decreasing rapidly in the initial stage before stabilizing. The elastic strain energy remains largely constant, with most of the input energy being stored as elastic strain energy. Under constant stress levels and cycle numbers, increases in confining pressure and frequency reduce the rock’s input energy, elastic strain energy, and dissipated energy. (ii) The peak stress of damaged limestone exhibits a positive correlation with the pre-damage confining pressure and cyclic frequency, while it decreases with an increasing number of cycles. Higher confining pressure and frequency raise the input energy, elastic potential energy, and dissipated energy at the peak stress point. (iii) Deformation and failure in damaged limestone originate from the development and propagation of localized deformation zones. Increased lateral displacement within these zones promotes the formation of macroscopic fractures. Due to significant structural heterogeneity inside the localized areas, the evolution of deformation energy is influenced by regional characteristics. (iv) Simulation results indicate that the uniaxial compressive failure of limestone involves the accumulation and propagation of micro-scale tensile cracks, which ultimately coalesce into macro-scale shear fracture surfaces. During uniaxial loading of pre-damaged limestone, newly generated cracks predominantly initiate around pre-existing cracks, with only a limited number distributed randomly. Their peak intensity shows a positive correlation with the pre-damage confining pressure. Full article

Fractured coal in the residual-strength stage is a primary medium for gas migration and drainage in deep mining areas. To investigate the hydro–mechanical seepage response of post-peak fractured coal under constant-pressure-difference conditions, triaxial CO₂ seepage tests were conducted on coal specimens collected from the Xinyuan Coal Mine. A Weibull-based damage constitutive model was established to characterize the confining-pressure-induced hysteresis in the damage-evolution path. The flow-rate evolution and Reynolds number analysis indicated that gas flow remained within the linear Darcy regime. A controlled-variable analysis was used to examine the competing effects governing permeability evolution. Mechanical compaction induced an exponential decrease in permeability, whereas the decrease in permeability with increasing pore pressure was interpreted, within the proposed model framework, as the combined effect of possible adsorption-induced matrix swelling and weakened gas slippage. To address the limitations of conventional constant-slip-factor models, a pressure-dependent slip modulation coefficient was introduced into a composite permeability equation incorporating effective stress, adsorption-related deformation, and dynamic gas slippage. Global nonlinear fitting yielded R² = 0.97 and an RMSE of 0.1909, with the residuals generally distributed around zero, supporting the fitting reliability of the model within the investigated stress–pressure range. Response-surface analysis identified mechanical compaction as the dominant controlling mechanism, while adsorption-related deformation and gas slippage acted as secondary correction mechanisms. The proposed framework provides a quantitative basis for distinguishing the mechanical and fluid-related effects governing permeability evolution in post-peak fractured coal. Full article

Hydraulic fracture height growth in thin sandstone–mudstone interbeds is often limited by bedding interface failure and multi-cluster stress interference. In this study, a coupled fracture–matrix interface finite element model was developed for the He-8 sandstone–mudstone interbeds in the Sulige Gas Field and validated against previously published true triaxial hydraulic fracturing experiments. The simulations indicate that vertical–horizontal stress difference (VSD; the difference between overburden stress and minimum horizontal stress within a layer) promotes fracture-height growth, whereas interlayer stress difference (ISD; the minimum horizontal stress contrast between adjacent layers) acts as a stress barrier that promotes bedding interface shear failure and arrests vertical growth. For the investigated reservoir configuration, each 4 MPa increase in VSD increased fracture height by approximately 1.5 m in the three-cluster case and 1.8 m in the four-cluster case, whereas each 2 MPa increase in ISD reduced the average fracture height by approximately 4.0 m in the three-cluster case and 3.5 m in the four-cluster case. Under moderate ISD, increasing the fluid viscosity was more effective than increasing the injection rate alone, although the benefit depended on cluster number and interface failure state. These results clarify how stress contrast, interface strength, and multi-cluster stress shadows jointly control cross-layer fracture propagation in thin interbedded reservoirs. Full article

Cyclic triaxial tests are often used to evaluate the behavior of soils under seismic loads. The stress conditions imposed on a soil specimen during a cyclic triaxial test, however, are very different than those acting on an element of soil during an earthquake. One major difference is that the element in the field is subjected to a change in total confining stress, whereas in a conventional cyclic triaxial test the total confining stress (as applied through the cell pressure) is held constant. This use of constant cell pressure is usually justified by the assumption that in a saturated specimen the change in total stress is offset by a change in pore pressure, thus resulting in no change in the effective confining stress or liquefaction susceptibility. A laboratory study using cyclic triaxial tests was conducted on several soils to assess the validity of this assumption. For each soil, two series of stress-controlled cyclic triaxial tests were run: one set with a constant cell pressure, and thus a constant total confining stress, and a second set with a variable total stress/cell pressure. These tests were then compared in terms of both the resulting cyclic resistance curves and the amount of energy dissipated to trigger liquefaction. It was found that the two conditions of confining stress yielded results that were not statistically different. Therefore, the assumption that the change in pore pressure caused by the variation in total stress is offset by the change in pore pressure and thus results in no change in effective stress or liquefaction susceptibility appears valid. Based on these findings, cyclic triaxial tests performed with constant cell pressure, and thus a constant total confining stress, provide valid results for liquefaction analyses. Full article

Hot forging of Ti-6Al-4V is extensively utilized in the manufacture of orthopedic implants; however, the coupled influence of strain rate and temperature on ductile damage evolution during the forging of femoral stems remains insufficiently quantified. In this study, a finite element framework is developed to analyze and optimize the hot forging process, incorporating strain rate- and temperature-dependent plasticity, as well as the Johnson–Cook damage criterion. Mesh convergence is established, and the assumption of quasi-adiabatic conditions is substantiated via Péclet number analysis. A full factorial design is implemented by varying the ram velocity (0.1–0.5 m/s) and initial billet temperature (850–950 °C) to evaluate the forging load, stress triaxiality, equivalent plastic strain, and damage accumulation. Results indicate that process kinetics govern the mechanical response: increasing the ram velocity enhances strain-rate hardening, resulting in higher peak loads, while explicitly reducing stress triaxiality and suppressing ductile damage evolution. Conversely, temperature exhibits a secondary influence within the investigated domain. Validation of the damage criterion confirms safe operating windows, identifying low-velocity forging as a high-risk condition for localized defect formation. These findings provide practical guidelines for the strain-rate-based optimization of thermomechanical processing parameters for Ti-6Al-4V femoral stems. Full article