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.

Stress release of the surrounding soil is the fundamental reason for many accidents in tunnel engineering. There have been a great number of numerical simulations and analytical solutions that study the tunneling-induced ground stress. This paper conducts a series of physical model tests to measure the stress state evolution of the surrounding soil during the tunnel advancing process. The ground compactness, as the most critical factor that determines the mechanical properties of sand, is the control variable in different groups of tests. The measurement results show that at the tunnel crown, the minor principal stress sigma 3, which is along the vertical direction, decreases to 0 kPa when the relative density (Dr) of the ground is 35% or 55%. Therefore, we can deduce that the sand above the crown collapses. When Dr = 80%, sigma 3 does not reach 0 kPa but its variation gradient is very fast. At the shoulder, the direction angles of three principal stresses are calculated to confirm the existence of the principal stress rotation during tunnel excavation. As the ground becomes denser, the degree of the principal stress rotation gradually decreases. According to the limited variation of the normal stress components and short stress paths at the springline, the loosened region is found to be concentrated near the excavation section, especially in dense ground. As a result, different measures should be taken to deal with the tunnel excavation problem in the ground with different compactness.

The stress state is the fundamental for evaluating the soil strength and stability, playing a crucial role. However, during the stress testing, local damage and other uncertain factors may lead to partial sensor data missing, causing the existing three-dimensional stress calculation method to fail. To accurately restore the soil stress state during data missing, a three-dimensional stress calculation method was developed based on three-dimensional stress testing principles, incorporating axisymmetric and one-dimensional compression characteristics. The three-dimensional stress, principal stress , the first invariant of stress I-1, the second in variant of stress J(2) and stress Lode angle of a sandy soil foundation under one-dimensional compression conditions with different data missing were calculated and compared to results with complete data. The results show that the method is highly accurate; as the load increases, the relative error decreases and converges. The principal stresses, the first invariant of stress I-1, the second invariant of stress J(2) and the stress Lode angle align with one-dimensional compression response, suggesting that this calculation method supports advanced data mining. This study offers a novel approach and a practical method for fully utilizing the test data.

Shield tunneling inevitably disturbs the surrounding soil, primarily resulting in changes in stress state, stress path, and strain. Modifications to certain parameters, such as shield thrust, shield friction, and soil loss, are made based on the elastic mechanics Mindlin solution and the mirror method, and a calculation expression for additional soil stresses induced by tunneling was derived. Additional soil stresses are calculated using the parameters of the Hangzhou Metro Kanji section. 3D principal stress paths and deviations of the principal stress axes near the tunnel crown, waist, and invert during shield tunneling were obtained by applying a transition matrix orthogonal transformation. These results are compared with experimental data to validate the theoretical solution's accuracy. The stress distribution along the tunneling direction and the 3D principal stress paths and deviations of the principal stress axes in the surrounding soil are determined. The results are as follows: The additional soil stresses along the tunneling direction follow a normal distribution and an S-shape. Under the combined influence of three construction mechanics factors, the shear stress component is approximately 1/3 to 1/2 of the normal stress and should not be neglected. During shield tunneling, the deviation angle of the principal stress axis at the tunnel crown changes from 90 degrees to 180 degrees, with little change in the magnitude of the principal stress. At the invert, the magnitude of the principal stress rapidly increases from 0.25 kPa to 8 kPa, with minimal deviation in the principal stress axis. At the shoulder, the principal stress variation and axis deviation are small. At the foot of the arch, the deviation angle of the major and minor principal stress axes is larger, while the magnitude of the principal stress slightly changes. At the waist, the deviation angle of the major principal stress is larger, and the magnitude of the minor principal stress significantly changes. A strategy for addressing changes in soil stress paths during shield tunnel construction is also proposed.

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/).

To investigate the effects of the maximum principal stress direction (theta) and cross- shape on the failure characteristics of sandstone, true-triaxial compression experiments were conducted using cubic samples with rectangular, circular, and D-shaped holes. As theta increases from 0 degrees to 60 degrees in the rectangular hole, the left failure location shifts from the left corner to the left sidewall, the left corner, and then the floor, while the right failure location shifts from the right corner to the right sidewall, right roof corner, and then the roof. Furthermore, the initial failure vertical stress first decreases and then increases. In comparison, the failure severity in the rectangular hole decreases for various theta values as 30 degrees > 45 degrees > 60 degrees > 0 degrees. With increasing theta, the fractal dimension (D) of rock slices first increases and then decreases. For the rectangular and D-shaped holes, when theta = 0 degrees, 30 degrees, and 90 degrees, D for the rectangular hole is less than that of the D-shaped hole. When theta = 45 degrees and 60 degrees, D for the rectangular hole is greater than that of the D-shaped hole. Theoretical analysis indicates that the stress concentration at the rectangular and D-shaped corners is greater than the other areas. The failure location rotates with the rotation of theta, and the failure occurs on the side with a high concentration of compressive stress, while the side with the tensile and compressive stresses remains relatively stable. Therefore, the fundamental reason for the rotation of failure location is the rotation of stress concentration, and the external influencing factor is the rotation of theta. (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/).

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.

Improvement of granular soils mechanical properties can be achieved by the addition of bonding agents. In this research, low amount of Portland cement was added to a sand and its beneficial shear strengthening effects were evaluated under a range of multiaxial stress paths. The influence of the orientation of the principal axes of stress and strain on the stress-strain response and failure of cemented sand has only been scarcely investigated. Therefore, this experimental investigation reports the results of a series of consolidated drained hollow cylinder torsional tests with constant principal stress path direction, alpha sigma, varying from 0 degrees to 90 degrees Results were compared with the shear behaviour of the uncemented sand tested under similar loading conditions. Results show that the addition of cement to the sand matrix increases the soil strength for all multiaxial stress path directions. The suitability of two multiaxial strength criteria for reproducing the shape of the failure envelope as a function of the orientation of principal stress axis alpha sigma has also been analysed.