True triaxial tests were conducted on artificially frozen sand. The effects of the intermediate principal stress coefficient, temperature and confining pressure on the strength of frozen sand were studied. The stress-strain curves under different initial conditions indicated a strain hardening. In response to increases of either the intermediate principal stress coefficient or the confining pressure or to a decrease of temperature, the strength typically increased. Furthermore, a new strength criterion was proposed to describe the strength of artificially frozen sand under a constant b-value stress path, combining the strength function in the p-q and pi planes. Considering the low confining pressure, the strength criterion in the p-q plane fitted the linear relationship in the parabolic strength criterion well. The strength criterion in the pi plane was combined with stress invariants, and a new strength criterion was established. This criterion considers unequal tension and compression strength, and integrates temperature. Test results indicated its validity. All parameters of the strength criterion could be easily determined from the triaxial compression and triaxial tensile tests.

Rockfill, a coarse granular material commonly used in dam construction, exhibits complex mechanical behavior under generalized stress conditions. This paper investigates the mechanical properties of rockfill through a series of stress-path tests conducted on a self-developed, large-scale true triaxial apparatus with cubical specimens of 60 x 30 x 30cm. Three test series are carried out by varying the mean effective stress, the deviator stress and the Lode's angle, respectively. An elastoplastic constitutive model is presented to describe the behavior of rockfill. An improved dilatancy equation is introduced by considering the phase transformation stress ratio instead of the critical stress ratio.

This study examines the behavior of anisotropically consolidated granular assemblies under undrained cyclic true triaxial loading paths. To achieve this, the Discrete Element Method (DEM) is conjugated with the Coupled Fluid Method (CFM) to account for fluid-solid interaction in undrained conditions. The examined loading paths include two phases: anisotropic consolidation and undrained cyclic true triaxial loading. During consolidation, samples are sheared at various Lode angles to reach a spectrum of initial static shear stress levels. In the second stage, undrained cyclic loading is applied with constant shear stress amplitudes at various Lode angle values. The results indicated that the monotonic and cyclic Lode angle, initial static shear stress, and amplitude of deviatoric stress have pronounced effects on the secant shear modulus degradation and the rate of excess pore water pressure generation of granular assemblies. In tandem with macro-scale observations, the evolution of the microstructure within assemblies is analyzed using the coordination number, redundancy index, inter-particle contact fabric tensor, and particle orientation fabric tensor. The micro-scale findings confirm that the anisotropy induced by changes in the loading direction significantly impacts the shear strength of the assemblies. Additionally, the fabric of assemblies aligns along the preferential direction corresponding to the major principal stress, influencing the dilative response.

The variability in particle morphology significantly impacts the mechanical properties of rockfill materials. To enhance the understanding of this influence, this study collected basalt rockfill particles from 6 different site sources, with their morphology captured by 3D scanning technology, and then the morphological characteristics categorized through cluster analysis. True triaxial tests for these 6 particle groups were simulated using discrete element method (DEM), and the effects of elongation, flatness, convexity, and intermediate principal stress coefficient on the stress-strain relationship and peak strength were qualitatively assessed through principal component analysis (PCA). Further, by controlling the elongation, flatness, and convexity, 3D reconstructed particle models were created by spherical harmonics (SH) analysis, and the true triaxial tests on these models were simulated to quantitatively clarify the influence of morphological parameters on the macroscopic stress- strain relationship, peak strength, microscopic contact, anisotropic evolution, and other characteristics. Considering the size effect in rockfill materials, multi-scale models incorporating particle morphology were further evaluated across four sample scales. The results indicate that, on the macro scale, the three morphological parameters and the middle principal stress coefficient each have substantial effects on peak strength independently, while the interaction among these parameters does not have a notable influence on the strength. With increasing convexity, the peak strength of samples gradually decreases, while an increase in elongation and flatness leads to a trend of initially increasing and then decreasing strength. On the micro scale, the increase in both elongation and flatness results in a more uniform fabric in the main and lateral directions, while the coordination number shows a trend of initially increasing and then decreasing before stabilizing gradually. The influence of elongation on the main direction fabric is slightly smaller than that of flatness, while convexity has minimal effect on these microscopic features. Additionally, the morphological parameters not only impact the deformation capacity of samples but also demonstrate heightened sensitivity to the strength-size relationship of the sample due to interlocking and boundary constraints between particles. This underscores the pivotal role of morphological parameters in governing the mechanical motion of particles during the sample size scaling process, consequently influencing the strength of the material.

True triaxial and hollow cylinder tests are among the best alternatives to explore the effects of stress paths oriented along different Lode angles on soil behavior. However, those experiments are not easy to conduct in the laboratory, especially for cyclic loading. This study investigates the undrained cyclic behavior of granular soils under true triaxial loading conditions using the discrete element method (DEM) coupled with fluid method (CFM). Numerical specimens with elongated particles oriented along three different bedding planes and in an isotropic condition were prepared and subjected to constant volume cyclic loading. Loading direction effects on the liquefaction potential were considered, applying the deviatoric stress amplitude along different Lode angles. The impact of initial fabric orientation and stress anisotropy on the micro- and macro-scale response of particulate assemblies was intensively studied. The results show the significant effect of the Lode angle on the liquefaction susceptibility and inclination of the phase transformation line of granular assemblies. It can be concluded that particulate assemblies become more prone to the onset of liquefaction by alternating the Lode angle. The inherent anisotropy and Lode angle influence the number of cycles to reach liquefaction, the slope of the phase transformation line, and the failure line.

Consolidated-drained true triaxial tests with constant b\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$b$$\end{document} values were performed on normally consolidated cross-anisotropic kaolin clay. Isotropic stress probes were incorporated into these true triaxial tests to study the orientations of plastic strain increment vectors and positioning of the plastic potential surface at different levels of shearing. An isotropic compression test was also performed to characterize the cross-anisotropic response of the clay. Pronounced cross-anisotropy was observed in the K0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$K_{0}$$\end{document} consolidated kaolin clay during shear, particularly when the major and minor principal stresses were perpendicular and parallel to the axis of material symmetry, respectively. A simple rotational kinematic hardening mechanism incorporated into the single hardening constitutive model for soil has been found to fairly accurately simulate the evolution of anisotropy in the form of expansion and rotation of the yield and plastic potential surfaces during true triaxial shearing.

Particle size significantly influences the macroscopic and microscopic responses of granular materials. The main purpose of previous works was to investigate the macroscopic response, but the influence of particle size on the evolution of microstructures is often ignored. The particle size effect becomes more complex under true triaxial stress conditions. Using the discrete-element method, a series of true triaxial numerical tests were carried out in this study to investigate the particle size effect. The mechanism of the particle size effect was elucidated from the perspective of similarity theory first. Then, the evolution of the stress and fabric for the whole, strong, and weak contact network was investigated. Meanwhile, the role played by strong and weak contacts in the particle size effect was discussed. The numerical results demonstrate that the peak stress ratio of the granular materials is enhanced as the particle size increases, which is caused by strong contacts. The peak stress ratio shows a linear relationship with particle size. The particle size effect on the strength is greater under the triaxial compression condition than under the triaxial extension condition. The proportion of sliding contacts within weak contacts gradually increases as the particle size increases. At nonaxisymmetric stress conditions, stress and fabric display noncoaxial behavior on the pi-plane, and an increase in particle size enhances the noncoaxiality, which mainly originates from the weak contacts.

Fissured loess is one of the important factors that cause geological disasters in the Loess Plateau. This paper analyzes the mechanical characteristics of fissured loess under different confining pressures and different intermediate effective principal stress coefficients b-values by conducting true triaxial tests on fissured loess with different fracture angles. The results show that there are Concave Points in the peak stress curves of the fracture loess samples with different angles, and the Concave Points are affected by both intermediate effective principal stress coefficients b-values and confining pressure. When the confining pressure is 100 kPa, the major effective principal stress growth rate K-value increases the most under different intermediate effective principal stress coefficients b-values, and the change trend is basically the same, and the major effective principal stress growth rate K-value growth rate decreases with the further increase of confining pressure. The internal friction Angle of the fusible loess sample is not affected by the change of joint and joint dip Angle; With the same intermediate effective principal stress coefficients b-values, the cohesion increases first, then decreases and then increases with the increase of crack Angle. When the fracture Angle is the same, the cohesion approximately increases with the increase of intermediate effective principal stress coefficients b-values. The 0 degrees crack has the greatest influence on cohesion, the 60 degrees crack has the second, and the 30 degrees and 90 degrees crack has the least influence on cohesion.

This study reveals the mechanical behavior of silt in the Yellow River floodplain under 3D stress. A true triaxial apparatus was used to conduct consolidated drained shear tests under different intermediate principal stress coefficients (b) and consolidation confining pressures to investigate the influence of the intermediate principal stress on the deformation and shear strength of silt. The stress-strain curves exhibited strong strain-hardening characteristics during shearing. Due to enhanced particle interlocking and microstructural reorganization, the silt demonstrated complex b-dependent deformation and strength characteristics. The cohesion rose with increasing b, whereas the internal friction angle followed a non-monotonic pattern, increasing and decreasing slightly as b approached 1. The strength envelope of the silt fell between that predicted by the Lade-Duncan and the extended von Mises strength criteria., which is best predicted by the generalized nonlinear strength criterion when the soil parameter alpha was 0.533. The findings reveal the stress-path-dependent mechanisms of Yellow River floodplain silt and provide essential parameters for optimizing the design of underground engineering projects in this region.

The contact network of granular materials is often divided into strong and weak subnetworks, which play different roles in micromechanics. Within the strong contact network, there exists the largest connected component, that is, the largest cluster, which may connect system boundaries and could be the most important structure in force transmission of the whole system. This paper concerns the particular features of the largest cluster in the strong contact network of granular materials, by considering the combining effects of loading path and particle shape. A series of true triaxial tests with various intermediate principal stress ratios are conducted for granular assemblies of different shaped particles using the discrete element method (DEM). Both the macroscopic stress-strain responses and the microscopic topological changes of the contact network are investigated. It is found that both particle shape and loading path will influence the shear strength and the topological features of the strong network. The threshold zeta$\zeta $ (the ratio to the average force) is used to distinguish the strong and weak networks, and a critical threshold can be identified by comparing the network-based metrics. The largest cluster within the strong network approaching the critical threshold can span the boundaries in each direction with minimum contacts, which occupies a small portion of particles and contacts but transmits a considerable portion of the applied stress. In addition, the similar contribution weight of the largest cluster to the deviatoric stress is identified for granular materials with different particle shapes.