Granular materials usually copossess inherent and stress-induced anisotropy that significantly influences their mechanical behaviors. This paper presents a series of true-triaxial tests on aeolian sands to consider the inherent and stress-induced anisotropy in terms of soil deposition angles and intermediate principal stress coefficients, respectively. These results show that the deposition angle primarily affected the elastic-plastic stage under axisymmetric conditions. Otherwise, the deposition angle affects all deformation processes after the elastic stage when the intermediate principal stress coefficient changes. Moreover, the critical state is not unique but depends on the combined effect of the deposition angle and the intermediate principal stress coefficient, which indicates that the strength, stress-strain response, and dilatancy behavior of sands are affected by both inherent and stress-induced anisotropy.

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.

A series of true triaxial unloading tests are conducted on sandstone specimens with a single structural plane to investigate their mechanical behaviors and failure characteristics under different in situ stress states. The experimental results indicate that the dip angle of structural plane (B) and the intermediate principal stress (o2) have an important influence on the peak strength, cracking mode, and rockburst severity. The peak strength exhibits a first increase and then decrease as a function of o2 for a constant B. However, when o2 is constant, the maximum peak strength is obtained at B of 90 degrees, and the minimum peak strength is obtained at B of 30 degrees or 45 degrees. For the case of an inclined structural plane, the crack type at the tips of structural plane transforms from a mix of wing and anti-wing cracks to wing cracks with an increase in o2, while the crack type around the tips of structural plane is always anti-wing cracks for the vertical structural plane, accompanied by a series of tensile cracks besides. The specimens with structural plane do not undergo slabbing failure regardless of B, and always exhibit composite tensile-shear failure whatever the o2 value is. With an increase in o2 and B, the intensity of the rockburst is consistent with the tendency of the peak strength. By analyzing the relationship between the cohesion (c), internal friction angle (4), and B in sandstone specimens, we incorporate B into the true triaxial unloading strength criterion, and propose a modified linear Mogi-Coulomb criterion. Moreover, the crack propagation mechanism at the tips of structural plane, and closure degree of the structural plane under true triaxial unloading conditions are also discussed and summarized. This study provides theoretical guidance for stability assessment of surrounding rocks containing geological structures in deep complex stress environments. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

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.

Under the effect of wave loads, continuous and cyclic principal stress rotation (PSR) occurs, with constant principal stress values in foundation soil units. The stability of coastal engineering structures in permafrost regions is inevitably subjected to the persistent impact of wave loads, which poses a significant challenge to their durability. Consequently, a series of experimental studies were carried out using a frozen hollow cylinder apparatus (FHCA) to investigate the influence of crucial three-dimensional stress state parameters, including the coefficient of intermediate principal stress (b), mean principal stress (p), and principal stress rotation radius (R), on the deformation characteristics and dynamic property evolution of frozen soils. The results indicated that under continuous principal stress rotation, the mean principal stress p has a limited impact on the deformation behavior and mechanical property evolution of the frozen soil. In contrast, b and R significantly influence the mechanical properties of frozen soil. When b and R at low values, the continuous rotation of principal stress causes axial strain to develop positively, decreases the mechanical property parameter damping ratio, increases the elastic modulus, and densified the sample. However, with the increase in b and R beyond a threshold, the repeated principal stress rotation causes the axial strain to develop negatively, increases the damping ratio continuously, decreases elastic modulus, and leads to significant softening of the frozen soil with an increase in rotation cycles.

Existing literature on true triaxial and torsional shear tests indicate that the mechanical response of a granular assembly is significantly influenced by the magnitude of the intermediate principal stress ratio. The present study aims to explore the mechanism behind such effects in reference to the particle-level interaction using 3D DEM simulations. In this regard, true triaxial numerical simulations have been carried out with constant minor principal stress and varying 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 employing rolling resistance-type contact model to mimic particle shape. The numerical simulations have been validated against the true triaxial experiments reported in the literature for dense Santa Monica beach sand. The macro-level shearing response of the granular assembly has been examined in terms of the evolution of stress ratio and volumetric strain for different rolling resistance coefficients. Further, such macro-level response has been assessed in reference to the micro-scale attributes, e.g. average contact force, number of interparticle contacts, mechanical coordination number, contact normal orientation, and fabric tensor as well as meso-scale attribute like strong contact force network. Lade's failure surface has been adopted to represent the stress and fabric at peak state in the octahedral plane, and mathematical expressions have been proposed relating the failure surface parameters to the rolling resistance coefficient.

Fracture (fault) reactivation can lead to dynamic geological hazards including earthquakes, rock collapses, landslides, and rock bursts. True triaxial compression tests were conducted to analyze the fracture reactivation process under two different orientations of Q2, i.e. Q2 parallel to the fracture plane (Scheme 2) and Q2 cutting through the fracture plane (Scheme 3), under varying Q3 from 10 MPa to 40 MPa. The peak or fracture reactivation strength, deformation, failure mode, and post-peak mechanical behavior of intact (Scheme 1) and pre-fractured (Schemes 2 and 3) specimens were also compared. Results show that for intact specimens, the stress remains nearly constant in the residual sliding stage with no stick-slip, and the newly formed fracture surface only propagates along the Q2 direction when Q3 ranges from 10 MPa to 30 MPa, while it extends along both Q2 and Q3 directions when Q3 increases to 40 MPa; for the pre- fractured specimens, the fractures are usually reactivated under all the Q3 levels in Scheme 2, but fracture reactivation only occurs when Q3 is greater than 25 MPa in Scheme 3, below which new faulting traversing the original macro fracture occurs. In all the test schemes, both epsilon 2 and epsilon 3 experience an accumulative process of elongation, after which an abrupt change occurs at the point of the final failure; the degree of this change is dependent on the orientation of the new faulting or the slip direction of the original fracture, and it is generally more than 10 times larger in the slip direction of the original fracture than in the non-slip direction. Besides, the differential stress (peak stress) required for reactivation and the post-peak stress drop increase with increasing Q3. Post-peak stress drop and residual strength in Scheme 3 are generally greater than those in Scheme 2 at the same Q3 value. Our study clearly shows that intermediate principal stress orientation not only affects the fracture reactivation strength but also influences the slip deformation and failure modes. These new findings facilitate the mitigation of dynamic geological hazards associated with fracture and fault slip. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Dispersive clay is widely distributed in the Songnen Plain of northeast China, causing serious embankment damage to hydraulic engineering, and the research on the relevant failure mechanism is still incomplete. In this study, based on the real stress path of dispersive clay failure, a mechanical experimental method under plane strain conditions was adopted to investigate the strength properties of dispersive clay. The results showed that under plane strain conditions, the stress-strain curve of dispersive clay exhibited the strain hardening type, distincted from the conventional strain softening type under triaxial vertical conditions, and the strength difference was approximately twice at a consolidation stress of 50 kPa. The stress-strain relationship of the principal stress also showed the strain hardening type, and the relationship between the stress-strain was approximately linear. Under low consolidation stress, the coefficient of the intermediate principal stress reached 0.44, indicated a significant influence of the intermediate principal stress on the strength of the clay. Under low consolidation stress, the failure mode of dispersive clay was characterized by swelling with no obvious spatial shear band, while under high consolidation stress, the failure mode exhibited a shear band located diagonally. Additionally, the strength properties of dispersive clay were weakened by the leaching of chemical ions in the clay, showed different compaction and strength under different consolidation stresses.

Currently, the mechanical properties of calcareous sand are mainly studied through triaxial tests, as traditional uniaxial compression tests fail to capture real loading conditions and soil strength anisotropy. To address this, true triaxial tests were conducted to examine the effect of the intermediate principal stress parameter (b) on the three-dimensional strength and deformation behavior of calcareous sand. In the constant b and sigma 3 tests, as the b value increased, both the strength and peak friction angle (phi ps) of calcareous sand were increased, while the tangent slope of the dilatancy curve showed a gradual rise.. The phi ps of calcareous sand was found to be higher compared to silica sand and coarse-grained soils. In the constant mean effective stress (p) and b test, the strength was increased with higher values of both b and p. The Matsuoka-Nakai 3D strength criterion proved more effective in fitting the 3D strength of calcareous sand in pi plane. As the b value increased, the critical stress ratio (Mc) was decreased. A quadratic function can better represent the Mc of calcareous sand in the pi plane under varying confining pressures. Furthermore, the Mc of calcareous sand was higher than that of silica sand and completely decomposed granite soil. This study provides a valuable experimental basis for understanding the 3D strength and deformation characteristics of calcareous sand in oceanic engineering infrastructure.

This study investigates, for the first time ever, the suffusion on gap-graded granular soils under torsional shear conditions from a microscopic perspective. A numerical model of the hollow cylinder torsional shear test (HCTST) using the discrete element method (DEM) is first developed, where an algorithm for simulating the real inner and outer rubber membranes of the hollow cylinder apparatus (HCA) is introduced. After the validation, the computational fluid dynamics (CFD) approach is introduced for the coupling between the particle and fluid phases. Then, a series of the coupled CFD-DEM suffusion simulations considering the rotation of the major principal stress axis (alpha) and intermediate principal stress ratio (b) are conducted. It is found that more fine particles are eroded in cases having smaller alpha and b, and the clogging phenomenon in the middle zones becomes more significant as both alpha and b increase. From the microscopic perspective, the specimens whose contact anisotropy principal direction is close to the fluid direction will lose more fines, and the anisotropy magnitude also plays an important role. In addition, the differences in structure and vertical connectivity of the pores in HCTST samples under various complex loading conditions cause fine particles to have different migration paths, further resulting in different fines mass loss.