Triaxial Loading Research Articles

Two-dimensional imaging of elastic properties of rock core samples from measurements of angle-dependent ultrasonic reflection coefficients

Laboratory and field measurements often fail to identify small-scale variations in rock elastic properties. Due to limited spatial resolution, conventional laboratory methods cannot properly describe rock formations exhibiting a high degree of heterogeneity, thereby masking differences between stiff and compliant layers. Continuous sample measurements can mitigate this problem but are not widely used by the industry. We build upon our previous work and apply laboratory angle-dependent ultrasonic reflection coefficient (ADURC) measurements to achieve detailed two-dimensional descriptions of the elastic properties of complex rock samples. This method successfully yields high-resolution information on P- and S-wave velocities, as well as bulk density, across the surface of rock samples. Elastic properties are estimated using a nonlinear inversion algorithm that matches laboratory measurements with numerical simulations. ADURC data acquired at various sample locations enable detailed rock descriptions, where the effective measurement area is determined by the size of the receiver, measurement frequency, and incidence angle. Consequently, the sampling area is smaller compared with triaxial loading and acoustic transmission tests, for which the resolution is controlled by sample size. Measurements conducted on samples exhibiting different levels of spatial complexity validate the capability of the ADURC method to identify small-scale heterogeneities. For the reported experiments, variations in angle-dependent reflectivity give rise to corresponding variations in the estimated P- and S-wave velocities and density, which can exceed 60%. These small-scale variations across heterogeneous rock samples often are overlooked by conventional laboratory methods.

Experimental and damage model study of layered shale under different moisture contents

To investigate the mechanical properties and damage evolution law of layered shale under varying moisture contents, we conducted triaxial compression experiments on rock samples with different bedding angles and moisture levels. This study analyzed the variations in mechanical properties of layered shale under different conditions, and established a predicted model for elastic modulus based on different bedding angles and moisture content. Additionally, the damage constitutive model of layered shale was improved. The study revealed that shale’s mechanical properties display anisotropy, which is influenced by the bedding angles and moisture contents. The elastic modulus of the rock increases with the rise of bedding angle, exhibiting a ‘U’-shaped change. Conversely, the mechanical properties of rocks deteriorate, and their brittleness weakens with the increase in moisture content. When the confining pressure increased, the overall mechanical properties of shale were enhanced, and the influence of bedding on shale was weakened, but the deteriorating effect of water on rocks was hardly affected. Based on the above experiments, a predicted model of equivalent elastic modulus of shale considering the coupling effect of bedding and different moisture contents was proposed, which could effectively predict the elastic modulus of layered shale with different moisture content under different confining pressures. Furthermore, based on the predicted model of elastic modulus, an improved damage constitutive model of layered shale under triaxial loading was established, and the damage accumulation trend of layered shale was obtained, which showed an “S”-shaped change with strain. Under the coupling effect of bedding and different moisture contents, the damage of shale was advanced, but the accumulation rate of damage slowed down. With the increase of confining pressure, the influence of bedding and moisture content on the damage characteristics of shale decreased, and the damage curves under different conditions gradually tended to isotropy. The developed damage constitutive model for layered shale under different moisture contents provides theoretical support for the study of reservoir fracturing and wellbore stability.

A Test System for Microwave-Assisted Dual-Mode Mechanical Cutting of Hard Rock Under True Triaxial Compression

This paper introduces a test system for microwave-assisted dual-mode mechanical cutting of hard rock under true triaxial compression. The test system was developed by Northeastern University, China, to simulate the cutting and breaking of hard rock by a tunnel boring machine (TBM). The test system is mainly composed of a dual-mode mechanical cutting subsystem, a true triaxial loading subsystem and a high-power microwave presplitting subsystem. The system performs dual-mode cutting under true triaxial stress, applies a single sensor to monitor the three-dimensional force of the cutter head, and integrates the reduction in the vibration and resistance to torsion during the cutting process. Mechanical cutting tests of basalt under different amounts of microwave irradiation and true triaxial stress conditions were conducted in Chifeng, Inner Mongolia, China. The results showed that microwave radiation presplitting effectively improved the efficiency of mechanical rock breaking. However, after 180 s of microwave irradiation, true triaxial high stress inhibited the mechanical breaking of basalt in terms of the overall excavation depth. During the tests, each system performed stably. The test results verified the reliability of the test system.

A damage constitutive model of layered slate under the action of triaxial compression and water environment erosion

Based on the macroscopic structure control theory, The slate with a significant bedding plane is a composite rock mass composed of rock blocks containing microscopic defects, joint surface closure elements, and shear deformation elements. Considering the coupling damage effect of water erosion and triaxial compressive load on bedding structure plane, the transversely isotropic damage constitutive model of slate under triaxial compressive load is derived with the dip angle of bedding and confining pressure as the variable. Firstly, based on the statistical theory of continuous damage mechanics and the maximum tensile strain criterion, the transversely isotropic deformation constitutive model of rock block with micro-defects is given; Secondly, based on the phenomenological theory of closed deformation and shear-slip deformation mechanism of layered structural plane under the coupling action of water erosion and triaxial compression load, the calculation formula of axial deformation of layered structural plane under the coupling action is given; Finally, to verify the accuracy of the established constitutive model, triaxial compression tests are carried out to study the influence of dip angle and confining pressure on the macroscopic mechanical properties and mechanism of slate. The results show that: the established triaxial compression damage constitutive model of bedding slate can accurately describe the stress–strain relationship of bedding slate after water environment erosion. With the increase of bedding dip angle, the strength and deformation capacity of the bedding slate first decreases and then increases, showing a U-shaped distribution as a whole. There are three main types of failure: tension shear composite failure, shear slip failure, and splitting tension failure.

Analysis of the Internal Structure of Brenna Sandstone Samples with Respect to the Differences in Measured Quasi-elastic Moduli

The Young’s modulus and Poisson’s ratio determined for the rocks under uniaxial or triaxial loading conditions represent necessary input parameters for solving many geotechnical tasks. However, these effective stiffness moduli, particularly the Poisson’s ratios, measured on different samples sometimes substantially differ, even for visually compact and macroscopically homogeneous rock. This has been observed for Brenna sandstone, especially under conditions of conventional triaxial extensions. The aim of this study was, thus, to reveal differences in rock structure that could cause such behavior. Several complementary methods were used to investigate the structure of this sandstone: X-ray computed tomography (CT), visual analysis using optical scanning and stereo-microscopy, and a new method combining water jet erosion with these visualization techniques. Analyses of this structure revealed mechanically more resistant compact thick and adjacent thin weak layers. In rock samples, these layers have different patterns and orientations. To demonstrate the influence of layer orientation on effective stiffness moduli measured under different loading conditions, FEM calculations were performed for idealized structural models. The outcomes of this numerical analysis are in qualitative agreement with the results of the loading experiments and the layered structure revealed in the samples. The ability of the proposed method using water erosion to analyze the rock structure in detail was verified. The method offers an interesting alternative to standard visual and X-ray CT techniques. The numerical calculations indicate the importance of latent layered defects in sedimentary rocks for fine measurements of stiffness moduli (especially the Poisson’s ratios) used as inputs for geotechnical applications.

Progressive internal erosion during triaxial shearing: an X-ray micro-computed tomography study

Fines removal during internal erosion results in the permeability increase and strength reduction of granular soils. This study examines internal erosion during shearing of granular soils under triaxial loading conditions using a specially designed apparatus and X-ray computed tomography scans. Two gap-graded soil samples with different fines content are tested and scanned multiple times to characterise sample- and pore-scale volume change and fines content using image-processing techniques. Results show that internal erosion increases gradually with axial strain and has a complex impact on permeability increase due to the spatial variation of fines content. Triaxial shearing induces both pore contraction and dilation, destabilises the clogging of fine particles and promotes internal erosion. Also, the erosion of force-supporting fine particles at a high fines content facilitates pore contraction. The progressive internal erosion due to mechanical disturbance leads to the formation of a flow channel from the upstream loading platen throughout the sample, resulting in a dramatic increase in permeability. Overall, this study provides novel insights regarding the interplay between the hydro-mechanical processes during shearing under constant seepage.

Acoustic emission and failure characteristics of coal and rock under single-free-face true triaxial loading

The airside roadway is frequently identified as the predominant site for dynamical disasters involving coal and rock. For the assurance of safety in mining zones, it becomes imperative to understand the mechanical response and energy characteristics of the coal and rock system within the airside roadway during mining processes. A triaxial static and dynamic load test system was used to conduct compression experiments on coal and rock samples under single-free-face true triaxial loading at different loading rates. The research findings are summarized as follows: (1) The damage and failure of the samples gradually propagate from the free face to the deeper sections of the samples and evident stage characteristics. Revealed that the damage mechanism was “microfracture localization – localized region development through – overall instability.” (2) A rising loading rate impedes the coal and rock's ability to make adaptive adjustments, thereby amplifying the sample's resistance against crack development, and playing a positive role in stimulating coal and rock failure, and the coal and rock system changed from progressive stable failure to sudden instability failure. The mesoscopic failure pattern gradually evolves into inadequacy and irregularity. (3) Throughout the entire failure episodes of coal and rock samples, the predominant mode of failure is tensile. However, influenced by the loading rate, the principal failure mechanism transitions from tensile to shear during the Ⅳ stage of failure. (4) There is noteworthy congruence between the macrofracture sites and the spatial positioning of high-energy fracture episodes. This furnishing pivotal insights for preempting and control deep impact disasters.

Study on the evolution of permeability properties of limestone under different stress paths

Abstract Stress path change has a great relationship with the effect of deformation and strength of rock. However, the underground rock body is in the engineering environment where the stress field, seepage field, and other fields are coupled to change, the law of fluid flow in the rock body is complex and variable. The change in the stress field has an important effect on the seepage characteristics of rock body; therefore, it is necessary to study the pattern of rock permeability evolution pattern by different stress paths. This study is based on the study of limestone, conducting triaxial unloading seepage mechanics experiments, the evolution of permeability properties of limestone specimens was analyzed based on the test results. The results show that in the conventional triaxial loading seepage test, the permeability of the limestone decreases before the rock stress reaches the peak intensity and increases after that. Increasing axial pressure unloading surrounding pressure compression section, permeability loss rate and effective stress are in line with the changing law of Gaussian distribution function. Under the action of constant axial pressure unloading surrounding pressure, with the increase in unloading amount, the permeability rate of change appears to increase slowly, and in the late stage of unloading section, the permeability rate of change appears to surge. Unloading section permeability change rate and cumulative unloading amount are in line with the law of change of the exponential function. Creep unloading pressure seepage test found that unloading pressure stage strain-time and permeability-time evolution characteristics are in line with the exponential rule of change. The experimental results of this study can provide an important experimental and theoretical basis for the permeability analysis of low-permeability rock body under complex stress conditions in underground engineering.

Mechanical Behavior and Energy Dissipation of Sandstone–Shotcrete under Monotonic and Cyclic Triaxial Loading

Mechanical Behavior and Energy Dissipation of Sandstone–Shotcrete under Monotonic and Cyclic Triaxial Loading

Dynamic multifractal characteristics of acoustic emission about composite samples with different stress loading and unloading conditions

Studying the failure characteristics of the common composite strata structure in western China is essential for evaluating stope stability and predicting coal mine dynamic disasters. To investigate the influence of different stress loading and unloading conditions on the instability characteristics of composite samples, three triaxial loading and unloading test schemes simulating different in-situ mining depths were designed. Complex triaxial tests were conducted on 12 sets of composite samples, and the bearing capacity, acoustic emission (AE) parameters and dynamic multifractal characteristics of the samples under different stress loading or unloading conditions were analyzed. The results indicate that samples tested by stress schemes simulating greater mining depths exhibit less damage, and the failure mode is a tensile-shear mixed failure, but the tensile failure is the main failure mode. The multifractal spectral parameters Δα\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta \\alpha$$\\end{document} of AE time series during the failure of composite samples tested with triaxial loading and unloading schemes simulating different mining depths show a decreasing trend in Δα\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta \\alpha$$\\end{document} values with increasing mining depth, while the change rules of Δfα\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta f\\left( \\alpha \\right)$$\\end{document} values are the opposite. The multifractal parameter changes degree in four-layer rock structure composite samples under different stress conditions are lower than those in three-layer rock structure composite samples, indicating that the microcrack propagation process in the three-layer composite sample is more complex, resulting in higher levels of damage. The dynamic change of multifractal parameters Δα\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta \\alpha$$\\end{document} and Δfα\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta f\\left( \\alpha \\right)$$\\end{document} during different stress loading and unloading stages reflects the influence of axial pressure or confining pressure changes on crack propagation in composite samples. Compared to the initial stress stage, the non-uniformity of AE signals increases in the residual stress stage, and the proportion of large signals becomes more prominent, signifying a complex micro-fracture process in the composite samples.

Dynamic imaging of force chains in 3D granular media

Granular media constitute the most abundant form of solid matter on Earth and beyond. When external forces are applied to a granular medium, the forces are transmitted through it via chains of contacts among grains-force chains. Understanding the spatial structure and temporal evolution of force chains constitutes a fundamental goal of granular mechanics. Here, we introduce an experimental technique, interference optical projection tomography, to study force chains in three-dimensional (3D) granular packs under triaxial shear loads and illustrate the technique with random assemblies of spheres and icosahedra. We find that, in response to an increasing vertical load, the pack of spheres forms intensifying vertical force chains, while the pack of icosahedra forms more interconnected force-chain networks. This provides microscopic insights into why particles with more angularity are more resistant to shear failure-the interconnected force-chain network is stronger (that is, more resilient to topological collapse) than the isolated force chains in round particles. The longer force chains with less branching in the pack of round particles are more likely to buckle, which leads to the macroscopic failure of the pack. This work paves the way for understanding the grain-scale underpinning of localized failure of 3D granular media, such as shear localization in landslides and stick-slip frictional motion in tectonic and induced earthquakes.

Experimental study on damage evolution process, seepage characteristics, and energy response of columnar jointed basalt under true triaxial cyclic loading

The physical response of Columnar Jointed Basalt (CJB) under a stress–seepage coupling field is of great significance for dam safety and oil and gas reservoir evaluation. A true triaxial experimental machine was used for investigating the deformation, damage evolution characteristics, seepage properties, and energy response of the CJB. Before the start of the test, a series of uniaxial compression tests were used to determine the ratio of similar materials, and the cyclic loading test results indicate that: The strength characteristics of CJB decreases slowly and then increases rapidly as the dip angle β varies from 0° to 90° under two loading conditions of the tiered cyclic loading (T–C) and the constant amplitude cyclic loading (CA–C), showing an obvious U–shaped trend, and four different loading stages can be distinguished by characteristic stress. In the yield stage, the stress–strain curve shows an obvious plastic hysteresis loop, there is a strong correlation between the failure modes of the CJB and β of the specimen, the joint surface gradually deteriorates owing to loading and unloading conditions, and structural control failure occurs widely in specimens. The damage evolution process is discussed in combination with the Load–Unloading Response Ratio, and the relative damage amount (Df) is defined by cycle index. The permeability–axial stress relationship of the specimen conforms to the power exponential relationship, and the permeability evolution characteristics are strongly correlated with the degree of damage. The energy response of the CJB were investigated by tress–strain curve, under two loading modes, T–C and CA–C, the cumulative curves of dissipation energy Ed can be fitted by an exponential function and a logarithmic function, respectively. The findings of this research hold immense importance for clarifying the mechanical and seepage properties of CJB under a stress–seepage coupling field.

Characterising mechanical properties and failure criteria of steel slag aggregate concrete under multiaxial stress states

To advance the application of steel slag aggregate concrete (SAC) in civil engineering, this study introduces a comprehensive experimental program to explore the failure mode, strength, deformation, and failure criteria of SAC under multiaxial stress conditions. In this investigation, SAC specimens containing 50% stabilised steel slag as a coarse aggregate were examined for mechanical behaviour (e.g. stress–strain relationships) under diverse loading scenarios, including uniaxial tensile and compressive loadings, biaxial compression, and true triaxial compression loadings. Subsequently, three distinct failure criteria (i.e. the Ottosen criterion [1], William–Warnke criterion [2], and Guo criterion [3]) for SAC were formulated and authenticated based on the experimental outcomes. The results showed varied failure modes in the SAC, such as prism type, splitting tensile, sheet-like tensile, and oblique shear, which depended on the stress state of the specimen during failure. A significant elevation in the ultimate strength and deformation of SAC specimens was observed, with an increment in the first-to-third principal stress ratio (σ1/σ3). Furthermore, the failure criterion introduced by Guo was validated as adept at reproducing all geometric characteristics of the concrete failure surface and precisely forecasting SAC’s ultimate strength of the SAC. Overall, this investigation proposed a reliable failure criterion based on true triaxial test data that can not only describe the multiaxial strength characteristics of SAC but also provide design advice for the applications of SAC in engineering structures.

Creep properties and model of fractured sandstone under freezing environment

Creep is an important mechanical property of fractured rock. To explore the creep mechanical properties and damage evolution law of surrounding rock mass under long-term external load during freezing construction, triaxial graded loading creep and CT tests are conducted under a freezing environment (−10 °C) on sandstone with different fracture dip angles. The test results reveal that the prefabricated fracture have a significant impact on the creep of sandstone under freezing environment. As the fracture dip angel increases, the creep duration, creep deformation, and long-term strength all decrease first and then increase, with lower values at 15° and 45°. At 0°、 15° and 45° dip angles, the rocks exhibit integrated shear through failure, whereas rocks with a dip angle of 75° and 90° exhibit the mode of tensile shear through failure. Notably, no microcracks or secondary cracks are observed in the rock samples. Finally, a nonlinear viscoelastic–plastic constitutive model of fractured sandstone is established via fractional calculus. Fitting the experimental curve with the theoretical model reveals that the proposed creep damage model could accurately describe the creep behavior of fractured sandstone under freezing, especially in the accelerated creep stage, which validates the reliability of the parameters.

Experimental Research on Anisotropy Characteristics of Shale under Triaxial Incremental Cyclic Loading and Unloading

Shale is a common rock type that is associated with underground engineering projects, and several important factors, such as bedding structure, confining pressure, and the loading and unloading path, significantly influence the anisotropy of shale. Triaxial monotonic loading tests and triaxial incremental cyclic loading and unloading tests of shale under three kinds of confining pressures and five types of bedding inclination angles (θ) were thus performed to investigate the anisotropy of shale in terms of mechanical behavior, acoustic emission (AE), and energy evolution, and reveal the mechanism by which shale anisotropy is weakened. The results show that (1) the compressive strength and elastic modulus of shale decrease and then increase as the θ increases, and that both σ3 and incremental cyclic loading and unloading reduce the anisotropy in terms of the compressive strength and elastic modulus of shale, with the ratio of plastic strain to total strain reaching its maximum at a θ of 60° during each loading and unloading cycle. (2) The failure modes of shale with θ of 0°, 30°, and 90° under triaxial monotonic loading are similar to the counterparts under triaxial incremental cyclic loading and unloading, while the failure modes of shale with θ of 45° and 60° differ significantly under the two loading conditions, and interestingly, the degree to which the bedding plane participates in shale crack evolution under incremental cyclic loading and unloading is considerably lower than that under triaxial monotonic loading. (3) The cumulative AE count and AE b-value of shale first decrease and then increase as the θ increases, while the Felicity ratio decreases as the number of cycles increases. (4) As the θ increases, the total energy density U0 and the parameter m, which reflects the accumulation rate of elastic energy, first decrease and then increase, with both reaching a minimum at a θ of 60°. (5) The mode by which cyclic loading and unloading leads to failure in shale with a θ of 60° is similar to that at a θ of 0° and is the main mechanism by which shale anisotropy weakening occurs as a result of cyclic loading and unloading. The results provide experimental support and a theoretical basis for safer and more efficient underground engineering projects that involve shale.

Experimental Study on the Pore Structure of Sandstone Damaged by Blasting and Changes in Energy Dissipation Under Triaxial Loading

Experimental Study on the Pore Structure of Sandstone Damaged by Blasting and Changes in Energy Dissipation Under Triaxial Loading

Experimental investigation on visualization and quantitative characterization of filling and plugging performances of fractures in coal

Underground coalbed methane extraction is an important way for preventing coal mine gas disasters and reducing greenhouse gas emission, but air leakage around extraction boreholes is a critical factor that limits efficient coalbed methane extraction. Dynamically filling and plugging fractures around extraction boreholes can prevent air leakage. Due to the invisibility and complexity of underground geological formations, observing the process and effectiveness of plugging is extremely challenging. To visualize and quantitatively characterize the filling and plugging performances of fractures in coal. First, experiments are conducted to fill the coal damaged by the uniaxial and the triaxial loading-induced using a novel fracture filling experimental system under the constraint conditions. Then the multidimensional (point, surface, volume) plugging performances of fractures in coal before and after filling is visualized by the method of the X-ray computed tomography imaging. Finally, the quantitative characterization of filling and plugging performances of fractures in coal is conducted by testing changes in weight, fracture volume, and permeability before and after filling process. The experimental results show that after filling, the width and quantity of fractures in the coal significantly decreases, and the damaged coal is repaired. The filling ratio of fractures in coal fractured by uniaxial loading is 72%, and the fracture filling ratio in coal under triaxial loading is 68.1%. Under the influence of seepage and diffusion of plugging materials, the fractures are continuously filled, leading to a continuous decrease in the permeability of the fractured coal. The significant reduction in permeability of the fractured coal body is evident. The filling and plugging performances of fractures in coal is effective. The findings in this paper provide meaningful references and guidance for determining the filling and plugging parameters in underground gas extraction engineering practices.

Changes in filtration and capacitance properties of highly porous reservoir in underground gas storage: CT-based and geomechanical modeling

The paper presents the results of comprehensive studies of filtration and capacitance properties of highly porous reservoir rocks of the aquifer of an underground gas storage facility. The geomechanical part of the research included studying the dependence of rock permeability on the stress-strain state in the vicinity of the wells, and physical modeling of the implementation of the method of increasing the permeability of the wellbore zone - the method of directional unloading of the reservoir. The digital part of the research included computed tomography (CT)-based computer analysis of the internal structure, pore space characteristics, and filtration properties before and after the tests. According to the results of physical modeling of deformation and filtration processes, it is found that the permeability of rocks before fracture depends on the stress-strain state insignificantly, and this influence is reversible. However, when downhole pressure reaches 7-8 MPa, macrocracks in the rock begin to grow, accompanied by irreversible permeability increase. Porosity, geodesic tortuosity and permeability values were obtained based on digital studies and numerical modeling. A weak degree of transversal anisotropy of the filtration properties of rocks was detected. Based on the analysis of pore size distribution, pressure field and flow velocities, high homogeneity and connectivity of the rock pore space is shown. The absence of pronounced changes in pore space characteristics and pore permeability after non-uniform triaxial loading rocks was shown. On the basis of geometrical analysis of pore space, the reasons for weak permeability anisotropy were identified. The filtration-capacitance properties obtained from the digital analysis showed very good agreement with the results of field and laboratory measurements. The physical modeling has confirmed the efficiency of application of the directional unloading method for the reservoir under study. The necessary parameters of its application were calculated: bottomhole geometry, stage of operation, stresses and pressure drawdown value.

Numerical study of effective parameters on true triaxial test results in rigid type apparatus

Operational parameters in true triaxial loading devices (TTA), such as the loading rod dimensions, friction between the loading platens and sample faces, loading gap, and one-side loading may affect the test results. Knowledge of the effect of each parameter is very important to correctly interpret rock behavior and to optimize these devices. Employing the Flac3D finite difference software, a rigid type true triaxial systemdesigned and builtat Amirkabir University of Iran (AUT-TTA) was simulated andinvestigated in this study. The results indicate that increasing the ratio of the rod cross-section to the loading platen area leads to an increase in the sample confinement, so, more uniform stress distribution occurs in the rock sample. The loading gap reduces strength of the rock specimen and leads to concentration of tensile stresses at the sample edges. The end effect leads to an increase in the stress level and a pseudo-hardening behavior in the samples. Also, the end effect is intensified under one-side loading condition compared to two-side loading. Comparison between results of AUT-TTA numerical simulation and analytical analysis (ideal conditions without considering operational parameters) indicates that the slope of the stress–strain diagram has low sensitivity to operational parameters. The most important operational factor affecting the stress level and dilatation stress is the end effect and other operational parameters have little effect. All parameters lead to an increase in the dilation strain. The clear edge and one-sided loading under low intermediate principal stresses and the end effect under high intermediate principal stresses have the greatest effect on the dilation strain.

Macro-microscopic mechanical behavior of geogrid reinforced calcareous sand subjected to triaxial loads: Effects of aperture size and tensile resistance

Reinforcing calcareous sands with geogrids is a potentially effective method for large-scale geotechnical constructions in coastal lands. The breakable nature of polygonal calcareous sands determines the complex particle-geogrid interactions. A three-dimensional numerical model of geogrid reinforced calcareous sand (GRCS) was established to investigate the potential mechanical laws based on the discrete element method (DEM), and the reasonableness of the numerical model was verified by comparing with the indoor triaxial test. It follows that the macro-microscopic mechanical behavior of GRCS under the influence of aperture size and tensile resistance of geogrids was further investigated via effective DEM simulations. The presented results show that the decreased aperture size and increased tensile resistance are beneficial to enhance the macro-mechanical properties of GRCS, including strength, internal friction angle and pseudo cohesion. Particle crushing is mainly affected by shear strain and confining pressure. The bulging deformation of GRCS is partially suppressed due to the confining effect of geogrids. Besides, the source of strength enhancement of GRCS is revealed based on the microscopic particle-geogrid interactions, and the calculation method of horizontal and vertical additional stresses in the reinforced soil element considering the effects of tensile resistance and aperture size is further established.