Soil elements in situ are subjected to multidirectional shearing during earthquakes. Ignoring the effect of two horizontal shear components generally results in an underestimation of the liquefaction resistance of soils during earthquakes. The actual earthquake sequence generally consists of a mainshock and subsequent aftershocks. Soils may experience liquefaction during the mainshock and then reliquefy again during the subsequent aftershocks. Previous studies on multidirectional loading paths have mainly focused on single liquefaction events. This study employs 3D discrete element modeling to simulate reliquefaction behavior of sands with various multidirectional cyclic simple shear loading histories. The specimens are initially subjected to various strain histories under multidirectional loading paths and then reconsolidated to initial stress states. Subsequently, each soil specimen is subjected to unidirectional cyclic loading in two different directions in the reliquefaction tests. The influence of multidirectional cyclic loading histories on the post-liquefaction drainage compression and reliquefaction resistance are analyzed. Moreover, the evolution of soil fabrics and interaction between fabric orientation and loading direction in the reliquefaction test are investigated. The results highlight that reliquefaction behavior of soils depends on both the fabric and the interaction between the fabric orientation and the loading direction. This study aims to provide micromechanical insight for understanding the effects of multidirectional shearing histories on reliquefaction resistance of sands.

Particle segregation is a widespread phenomenon in nature. Vertical vibration systems have been a focal point in studying particle segregation, providing valuable insights into the mechanisms and patterns that influence this process. However, despite extensive research on the mechanisms and patterns of particle separation, the consequences, particularly the mechanical properties of samples resulting from particle segregation, remain less understood. This study aims to investigate the segregation process of a binary mixture under vertical vibration and examine the consequences through monotonic and cyclic triaxial drained tests. The results reveal that large and small particles segregate nearly simultaneously, with more thorough separation observed for large particles. The segregation index, Ds, effectively describes this evolution process, offering a quantitative metric for both mixing and segregation. Granular temperature analysis unveils three distinct states during segregation: solid-like, fluid-like, and solid-liquid transitional phase, corresponding to varying activity levels of particle segregation. Drained triaxial shear tests demonstrate the sensitivity of stress-strain relationships to the degree of segregation. Interestingly, ultimate strength is found to be essentially unrelated to the degree of segregation. When the segregation index approaches zero, signifying particles approaching a uniform distribution, the granular system reaches a harmonic state. This state exhibits optimal mechanical performance characterised by maximum peak stress, friction angle, and the highest elastic modulus. These findings underscore the potential impact of segregation on the mechanical response of granular mixtures and emphasise the necessity of a comprehensive understanding of particle segregation in soil mechanics.

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.

Seepage plays a crucial role in the mechanical behavior and damage modes of geotechnical materials. In this work, based on the unsteady seepage equation, a hydraulic coupling numerical simulation algorithm combining interpolation finite difference method (FDM) and discrete element method (DEM) is proposed to explore the intrinsic mechanism of the interaction between geotechnical materials and the seepage process. The method involves constructing an irregular fluid calculation grid around each particle and deriving the two-dimensional unsteady seepage governing equation and its stability conditions using interpolation and the FDM. The efficiency of the seepage calculation was investigated by numerically varying the parameters of the difference format. The method was applied to simulate the generation of gushing soil in a sinking area of a sunk shaft under hydraulic drive conditions. The results indicate that the improved FDM can effectively simulate the two-dimensional seepage of soil with high calculation efficiency. The hydraulic conductivity and time step positively correlate with the calculation efficiency of the difference format, whereas the spatial step has a negative correlation. The proposed method also accurately reflects the process of gushing soil damage. These results provide a solid theoretical basis to study the geotechnical seepage field and its associated damage mechanisms.

The direct simple shear (DSS) test serves as a vital method in geotechnics, allowing the measurement of peak and post-liquefaction shear strengths, along with the critical state friction angle of soils. Additionally, the simple shearing mode applied in a DSS test is the predominant failure mode in many geotechnical engineering problems. Although the DSS test is widely used to determine soil strength, a significant challenge with the DSS device is the non-uniformity of stress and strain distributions at the specimen boundaries. This non-uniformity depends on not only the specimen size but also the size of soil particles. The influence of specimen size on boundary effects is typically evaluated using the ratio of specimen diameter (D) to height (H). The median particle diameter (D50), as an indicator of a soil's particle size, could be another influential factor affecting the non-uniformities of stress and strain on specimen boundaries in a DSS test. Through three-dimensional discrete element method (DEM) simulations, this research explores these factors. Specimens were generated with a particle size distribution (PSD) scaled from a coarse sand sample. Laboratory monotonic DSS testing results on the coarse sand were employed to calibrate the DEM model and ascertain the modeling parameters. Boundary displacements were regulated to maintain a constant-volume condition which represents undrained shearing behavior. Various specimen diameters were simulated with identical void ratios to investigate the influence of D/H on stress path, peak and post-peak shear strengths, and critical state behavior. DEM simulations allowed the generation of several particle size distributions through different scaling factors applied to the sand gradation to determine the combined effect D50 and D/H. Limiting D/H and D50/D ratios are subsequently proposed to mitigate specimen boundary effects.

Preexisting cracks inside tight sandstones are one of the most important properties for controlling the mechanical and seepage behaviors. During the cyclic loading process, the rock generally exhibits obvious memorability and irreversible plastic deformation, even in the linear elastic stage. The assessment of the evolution of preexisting cracks under hydrostatic pressure loading and unloading processes is helpful in understanding the mechanism of plastic deformation. In this study, ultrasonic measurements were conducted on two tight sandstone specimens with different bedding orientations subjected to hydrostatic loading and unloading processes. The P-wave velocity was characterized by a similar response with the volumetric strain to the hydrostatic pressure and showed different strain sensitivities at different loading and unloading stages. A numerical model based on the discrete element method (DEM) was proposed to quantitatively clarify the evolution of the crack distribution under different hydrostatic pressures. The numerical model was verified by comparing the evolution of the measured P-wave velocities on two anisotropic specimens. The irreversible plastic deformation that occurred during the hydrostatic unloading stage was mainly due to the permanent closure of plastic-controlled cracks. The closure and reopening of cracks with a small aspect ratio account for the major microstructure evolution during the hydrostatic loading and unloading processes. Such evolution of microcracks is highly dependent on the stress path. The anisotropy of the crack distribution plays an important role in the magnitude and strain sensitivity of the P-wave velocity under stress conditions. The study can provide insight into the microstructure evolution during cyclic loading and unloading processes. (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 paper focuses on the use of rotary-percussive drilling for hard rocks. In order to improve efficiency and reduce costs, it is essential to understand how operational parameters, bit wear, and drilling performance are related. A model is presented therein that combines multibody dynamics and discrete element method (DEM) to investigate the influences of operational parameters and bit wear on the rate of penetration and wear characteristics. The model accurately captures the motion of the bit and recreates rock using the cutting sieving result. Field experimental results validate the rod dynamic behavior, rock recreating model, and coupling model in the simulation. The findings indicate that hammer pressure significantly influences the rate of penetration and wear depth of the bit, and there is an optimal range for economical hammer pressure. The wear coefficient has a major effect on the rate of penetration, when wear coefficient is between 1/3 and 2/3. Increasing the wear coefficient can reduce drill bit button pressure and wear depth at the same drill distance. Gauge button loss increases the rate of penetration due to higher pressure on the remaining buttons, which also accelerates destruction of the bit. Furthermore, a more evenly distributed button on the bit enhances the rate of penetration (ROP) when the same number of buttons is lost. (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/).

Discrete element simulation of triaxial tests is an important tool for exploring the deformation and failure mechanisms of geotechnical materials such as sands. A crucial aspect of this simulation is the accurate representation of lateral boundaries. Using coupled finite difference method (FDM)-discrete element method (DEM) approach, numerical simulations of consolidated-drained and consolidated-undrained triaxial tests were conducted under flexible lateral boundary conditions. These results were then compared with those of corresponding triaxial tests using rigid lateral boundaries. The results indicate that, compared to the rigid lateral boundary, the triaxial test using the FDM-DEM coupled flexible lateral boundary better captures both the macroscopic mechanical response and the microscopic particle kinematics of laboratory triaxial specimens. In the consolidated-drained triaxial tests, the strain softening and shear dilatancy of the specimen with the flexible lateral boundary are significantly weaker after reaching peak strength than those of the specimen with the rigid lateral boundary. In the consolidated-undrained triaxial tests, when the axial strain is large, the specimen with the flexible lateral boundary exhibits both a lower deviator stress and a smaller absolute value of negative excess pore pressure. Furthermore, in the consolidated-undrained triaxial tests, as the axial strain increases, the flexible lateral boundary provides weaker lateral constraint and support to the specimen compared to the rigid lateral boundary. Consequently, the stability of the force chains in the specimen with the flexible lateral boundary is lower, leading to more buckling events of force chains within the shear band. As a result, both the anisotropy and the deviator stress are reduced.

The physical and mechanical properties of granular soils are strongly related to the overlying stresses to which they are subjected. In particular, during the engineering construction phase, which involves activities like foundation stacking and building construction, the applied loads on the soil increase continuously over time. Unfortunately, current stress-controlled compression geotechnical tests have not adequately considered this situation. Therefore, this study aims to examine the effects of various factors, including void ratio, confining stress, stress loading rate, and particle shape, on both macroscopic shear properties and microscopic characteristics of granular soils under conditions of increasing axial stress in biaxial compression numerical simulations. The results show that: (1) In stress-controlled tests on granular soils, samples exhibit three different shear behaviors as the void ratio varies; (2) the confining stress and particle shape will change the magnitude of the deviatoric stresses and axial strains in the peak state of the sample, but not their trends; (3) the stress loading rate does not affect the strength of the samples. Therefore, the loading rate can be increased appropriately to improve the computational efficiency of the numerical model. These findings will enhance understanding of the time-dependent behavior of granular soils and provide valuable insights for engineering applications, particularly in soil mechanics, foundation treatment, and slope stability.

This paper presents a novel and efficient method for generating three-dimensional (3D) mesoscale structures of concrete using the discrete element method (DEM). The proposed approach enables the flexible and precise simulation of various aggregate shapes and volume fractions, which is crucial for sensitivity analysis and computational studies. Unlike traditional digitalization methods, this technique focuses on creating adjustable synthetic models to investigate the impact of different parameters on the mechanical properties of concrete. This technique demonstrates significant advantages in generating complex concave aggregates and high aggregate volume fraction models while allowing for flexible control of particle spacing, thus enhancing computational efficiency and model accuracy. Numerical simulations using the proposed method show excellent agreement with laboratory experimental results, validating its reliability. This method not only facilitates deeper sensitivity analysis but also aids in optimizing concrete designs and applications by providing insights into the effects of various parameters on concrete performance.