In geotechnical engineering, the development of efficient and accurate constitutive models for granular soils is crucial. The micromechanical models have gained much attention for their capacity to account for particle-scale interactions and fabric anisotropy, while requiring far less computational resources compared to discrete element method. Various micromechanical models have been proposed in the literature, but none of them have been conclusively shown to agree with the critical state theory given theoretical proof, despite the authors described that their models approximately reach the critical state. This paper modifies the previous CHY micromechanical model that is compatible with the critical state theory based on the assumption that the microscopic force-dilatancy relationship should align with the macroscopic stress-dilatancy relationship. Moreover, under the framework of the CHY model, the fabric anisotropy can be easily considered and the anisotropic critical state can be achieved with the introduction of the fabric evolution law. The model is calibrated using drained and undrained triaxial experiments and the results show that the model reliably replicates the mechanical behaviors of granular materials under both drained and undrained conditions. The compatibility of the model with the critical state theory is verified at both macroscopic and microscopic scales.

This study presents a novel micromorphic continuum model for sand-gravel mixtures with low gravel contents, which explicitly accounts for the influences of the particle size distribution, gravel content, and fabric anisotropy. This model is rigorously formulated based on the principle of macro-microscopic energy conservation and Hamilton's variational principle, incorporating a systematic analysis of the kinematics of coarse and fine particles as well as macro-microscopic deformation differentials. Dispersion equations for plane waves are derived to elucidate wave propagation mechanisms. The results demonstrate that the model effectively captures normal dispersion characteristics and size-dependent effects on wave propagation in these mixtures. In long-wavelength regimes, wave velocities are governed by macroscopic properties, whereas decreasing wavelengths induce interparticle scattering and multiple reflections, attenuating velocities or inhibiting waves, especially when wavelengths approach interparticle spacing. The particle size, porosity, and stiffness ratio primarily influence the macroscopic average stiffness, exhibiting consistent effects on dispersion characteristics across all wavelength domains. In contrast, the particle size ratio and gravel content simultaneously influence both macroscopic mechanical properties and microstructural organization, leading to opposing trends across different wavelength ranges. Model validation against experiments confirms its exceptional predictive ability regarding wave propagation characteristics, including relationships between lowpass threshold frequency, porosity, wave velocity, and coarse particle content. This study provides a theoretical foundation for understanding wave propagation in sand-gravel mixtures and their engineering applications.

Further investigation into the progression of soil arching under the impact of noncentered tunnel is warranted. This study addresses this need by examining trapdoor models with varying vertical and horizontal spacings between the tunnel and the trapdoor through the discrete element method. The numerical model underwent calibration utilizing data from previous experiments. The results indicated that the soil arching ratio under the impact of noncentered tunnel exhibits four distinct stages: initial soil arching, maximum soil arching, load recovery, and ultimate stage, aligning with observations unaffected by tunnel presence. The minimal disparity in stress ratio within the stationary region was observed when the vertical spacing between the tunnel and the trapdoor ranges between 150 and 200 mm. Moreover, the disturbed area on the left part of the trapdoor extended significantly beyond the trapdoor width, with notably higher disturbance height compared to the right side. When the tunnel deviated from the centerline of the trapdoor, the stress enhancement on the right side was considerably greater compared to the left. Additionally, the displacement of the trapdoor resulted in a reduction of contact force anisotropy in the soil on the side more distant from the tunnel, while increasing it on the side closer to the tunnel.

Microbially induced calcite precipitation (MICP) is a promising technology for soil improvement, where the treated soil can be regarded as the structural one. In this study, a micromechanics-based model is proposed to investigate the mechanical behaviors of inherently anisotropic MICP-cemented sand, which consists of a hexagonal close-packed (HCP) particle assembly (2D) composed of bonded elliptical particles with same size. A size-dependent bond failure criterion is adopted to define the microscopic mechanical reactions between the particles to model the nonlinear characteristics of the soil. Based on the homogenization theory and lattice model, the stress-strain relationship, strength criteria, and corresponding macroscopic mechanical parameters with respect to microscopic parameters for MICP-cemented sand are derived and verified by DEM simulation based on the regularly arranged particle assembly. The effects of key parameters, including cement content, initial void ratio, inherent anisotropy, and confining pressure, on the mechanical behaviors of MICP-cemented sand is investigated in detail, and the good agreement between the theoretical solution and laboratory test results validates the applicability of the theoretical solution for analyzing MICP-cemented sand.

The discrete element method (DEM) is adopted to investigate the influence of the particle shape on the smallstrain stiffness and stiffness degradation of granular materials during triaxial compression tests. Clumped particles are used to simulate irregular granular particles. The simulation results show that a more irregular particle shape causes an increase in the initial stiffness at very small strains and more delayed stiffness degradation. The micromechanism is explored on the basis of the analytical stress-force-fabric relationship, which reveals that increased particle irregularity leads to higher relative contribution of the tangential force anisotropy to the deviatoric stress. The achievable slip ratio and the mechanical coordination number also increase with increasing particle irregularity, resulting in larger resistance to deformation. An equivalent spherical particle analysis method is proposed, which reveals that the irregularity of particle shapes significantly increases both the sliding resistance and the rotational resistance between two particles, resulting in greater stability in the contact network and thus contributing to higher macroscopic stiffness and slower stiffness degradation.

This study investigated contact distribution and force anisotropy associated with elliptical particles in granular soils within the pendular state of unsaturated soils, employing the discrete element method. The high cost of determining the micromechanical factors through laboratory tests justifies the use of this method. The macromechanical behavior of unsaturated granular soils depends on interparticle contact characteristics and liquid bridge behavior. The findings indicated that as the degree of saturation increased, both the shear strength and the anisotropy of the normal and shear forces initially rose before subsequently declining. Notably, the contact normal anisotropy exhibited minimal variation with changes in saturation. Furthermore, it was observed that as confining pressure increased at a specific eccentricity and degree of saturation, the associated anisotropies exhibited a continuous increase. In this context, as the eccentricity of the particles increased, the peak shear strength and its corresponding anisotropies initially increased and then decreased. Conversely, residual soil strength showed a consistent increase in shear strength and anisotropy with rising eccentricity.

Jet grouting is a geotechnical consolidation technique commonly used to improve soil mechanicals. Despite its successful applications, understanding micro-level interactions between the jet and soil is incomplete. This paper utilizes the Smoothed Particle Hydrodynamics (SPH) and Arbitrary Lagrangian-Eulerian (ALE) methods to simulate fluid-soil interactions in both non-submerged and submerged environments. Analysis covers the flow fields and soil erosion. Findings show erosion velocity remains steady in non-submerged conditions, with the jet compacting and flushing soil. In submerged conditions, the simulated jet flow field under soil constraint is similar to that in the free submerged conditions. However, influenced by soil deformation, damage, and the backflow of the slurry, the jet flow field under soil constraint displays distinct features. For instance, velocity distributions in certain cross-sections cannot be accurately described by normal distribution, and axial velocity distribution curves exhibit different partitions compared to free submerged jet theory. Comparative simulations vary jet pressures, grout water-cement ratios, and soil compactness to analyze the erosion process. It is found that jet pressure significantly affects the depth of the erosion pit. The limit erosion distance in ALE simulations were compared with theoretical values derived from an established theory, and a model experiment was also conducted to analyze the jet-grouted diameter at different left speeds and rotational speeds of rod. The results show that ALE method can offer high accuracy in predicting the jet-grouted diameter and proves to be a feasible approach for fluid-soil interaction simulations in jet grouting.

Coral soil in large quantities of islands has been used for the construction of islands with the development of global marine construction projects. At present, the research on the macro and micromechanical behavior of coral soil during loading is insufficient, which is related to the development of marine engineering. Using the self-developed high-pressure geotechnical CT-triaxial apparatus, the consolidated drained triaxial tests were conducted on coral gravel under confining pressures ranging from 200 to 800 kPa, all the while employing realtime CT scanning to monitor the sample's deformation. The deformation, particle breakage, and porosity of coral gravel could be directly observed by CT images and its post-processing. The results show that the stress-strain relationship of the samples is strain hardening. Notably, particle breakage during consolidation predominantly manifests as corner breakoff, whereas shearing processes primarily induce splitting. The relative breakage Br is not only approximately linear with the average coordination number C-N of particles, but also with the logarithm of average particle size d, porosity n, and local strain s. Observing the evolution of the sample during loading, the increase of confining pressures leads to the decrease of the sample porosity, resulting in a diminishment in pore dimensions, a densification of particle packing, and the increase of contacts between particles. Consequently, this induces particle breakage and continuous volumetric contraction, thus the stress-strain relationship is hardening. The reciprocal influence between macroscopic and microscopic mechanics manifests in coral gravel. The experimental findings could provide valuable insights for marine engineering construction.

Previous studies on the hollow cylinder torsional shear test (HCTST) have mainly focused on the macroscopic behavior, while the micromechanical responses in soil specimens with shaped particles have rarely been investigated. This paper develops a numerical model of the HCTST using the discrete element method (DEM). The method of bonded spheres in a hexagonal arrangement is proposed to generate flexible boundaries that can achieve real-time adjustment of the internal and external cell pressures and capture the inhomogeneous deformation in the radial direction during shearing. Representative angular particles are selected from Toyoura sand and reproduced in this model to approximate real sand particles. The model is then validated by comparing numerical and experimental results of HCTSTs on Toyoura sand with different major principal stress directions. Next, a series of HCTSTs with different combinations of major principal stress direction (a) and intermediate principal stress ratio (b) is simulated to quantitatively characterize the sand behavior under different shear conditions. The results show that the shaped particles are horizontally distributed before shearing, and the initial anisotropic packing structure further results in different stress-strain curves in cases with different a and b values. The distribution of force chains is affected by both a and b during the shear process, together with the formation of the shear bands in different patterns. The contact normal anisotropy and contact force anisotropy show different evolution patterns when either a orb varies, resulting in the differences in the non-coaxiality and other macroscopic responses. This study improves the understanding of the macroscopic response of sand from a microscopic perspective and provides valuable insights for the constitutive modeling of sand. (c) 2024 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/).

A large amount of waste clay is generated in shield tunnel engineering. Although the physical properties such as the unconfined compressive strength of the clay have been improved through lime modification, some still fail to meet the specifications for road embankment materials. In this study, a indoor carbonation device was used to carbonize lime-modified clay samples with different lime contents. The changes in water content (omega), plasticity index (IP), undrained compressive strength (qu), and softening coefficient (Kr) of the samples were analyzed. Microscopic mechanisms were investigated through tests such as Scanning electron microscope (SEM) and Diffraction of x-rays (XRD). The results showed that with increasing carbonation period, the omega and IP of soil samples with different lime contents gradually decreased and stabilized. After 2 hours of carbonation, the qu, elastic modulus (E50), and Kr of the soil samples significantly increased. After 1 hour of carbonation and 7 days of curing, the modified clay with lime content greater than 7% met the subgrade strength standards for secondary and lower-grade highways (IP = 500 kPa). The carbonation reaction resulted in the formation of CaCO3 crystals on the surface of soil particles and Ca(OH)2. With carbonation further developed, a CaCO3-Ca(OH)2-C-S-H hybrid material was generated, which filled the pore space in clay reinforcing the structure and improving the strength of lime-modified clay.