Discrete element modeling (DEM) is a useful tool for linking global responses of granular materials to underlying particle-level interactions. A DEM model capable of capturing realistic soil behavior must be calibrated to a reference dataset, typically consisting of laboratory experiments. Calibration of a DEM model often requires numerous simulations as contact parameters need to be iterated upon until the simulation results satisfactorily replicate the experimentally observed behaviors. This paper presents a sensitivity investigation that examines the effects of the contact parameters on the drained triaxial compression response of a poorly-graded sand. It then introduces a calibration procedure capable of providing contact parameters that satisfactorily reproduce the results of laboratory triaxial results in a few simulations. Results show that friction and rolling resistance coefficients jointly influence the mobilized peak and critical state friction angles, secant shear modulus, maximum dilation rate, total volumetric strain, and strain softening magnitude. These parameters also influence the mode of failure at contacts and the evolution of fabric anisotropy. The influence of mu r or mu on the triaxial response and particle-level interactions is coupled, becoming more profound as the other parameter is increased. Contact stiffness is shown to influence the shear modulus and volumetric change behavior independently of mu and mu r. An algorithm that estimates values for mu and mu r needed to reproduce experimental results is developed using triaxial response parameters from experimental datasets. The performance of the proposed calibration method is demonstrated for three natural sands showing that it provides appropriate calibrated parameters for poorly graded sands with different relative densities and confined with varying effective stress magnitudes.

Understanding the response of sand to complex loading conditions is vital for practical geotechnical engineering. Circular rotational shear is a special stress path where the magnitudes of three principal stresses vary following a circular stress trajectory in the it-plane with their directions fixed. Although experimental studies under such stress paths are limited, the discrete element method appears to be an appealing approach to examine the response of granular materials to varying complex loading paths in numerical virtual tests. This study presents comprehensive numerical simulations of granular samples subjected to a circular stress path under varying conditions, including samples prepared with different bedding-plane angles and densities and subjected to different stress ratios. Both macroscopic and microscopic behaviors are presented and interpreted. A contactnormal-based fabric tensor is adopted in a detailed analysis to measure the internal structure of the granular assembly. The fabric, strain, and strain increment tensors are decomposed with respect to the stress tensor, and the evolutions of these components are presented along with the key influential factors. The results obtained in this study provide useful physical insight for the development of constitutive models for granular soils under general loading conditions.

Acoustic emission (AE) offers the potential to monitor and interpret soil-pipe interaction behavior by sensing particle-scale interactions. However, application of AE is limited by gaps in understanding related to how particle-scale interactions influence AE activity. Discrete element method (DEM) simulations of buried pipe uplift with energy tracking were performed and compared with experimental mechanical, displacement, and AE measurements, to ensure realistic behavior was captured by the modeling approach. A parametric investigation was then performed to evaluate the influence of pipe displacement direction and pipe diameter on plastic energy dissipation, and hence AE. Trends of dissipated plastic energy and measured AE with stress level (via burial depth) and pipe velocity were analogous. Relationships were quantified (R2 ranging from 0.74 to 0.98) between AE, dissipated plastic energy, and pipe velocity. Measured AE and dissipated plastic energy were linked with a general expression, comprising increments of friction (sliding and rolling), damping, and damage energies. Sliding friction energy accounted for >80% of the total dissipated energy on average during buried pipe deformation. Exemplar relationships were established between dissipated energy, pipe movement direction, embedment ratio, and mobilized soil volume (R2 values ranging from 0.92 to 0.97). A conceptual framework for interpreting buried pipe behavior using AE monitoring was presented.

The evaluation of triggering potential and induced deformations is crucial for liquefaction assessment of geosystems constructed on, or comprised of, coarse-grained soils. A series of cyclic constant volume direct simple shear (DSS) tests were simulated using the discrete element method (DEM) to investigate the influence of gradation on the pre- and post-liquefaction behavior of coarse-grained soils, including the resistance to initial liquefaction, accumulation of shear strains, and evolution of fabric. The DEM simulations were conducted on isotropically consolidated cubical specimens composed of non-spherical particles with a coefficient of uniformity (CU) C U ) range between 1.9 and 6.4. At similar relative densities, specimens with broader gradations yield lower liquefaction triggering resistance than poorly-graded specimens. However, at the same initial state parameter, the more broadly-graded specimens have higher liquefaction triggering resistance. Regardless of the parameter of state used for comparison, the specimens with broader gradation accumulate post-triggering shear strains at smaller rates than the poorly-graded specimens. The presence of coarser particles in the broadly-graded specimens promotes stable packing and enhanced interlocking by forming a substantially large number of contacts, which limits the particle movement and produces lower post-liquefaction strain accumulation. In contrast, the finer particles contribute minimally to the overall specimen stability. Additionally, wider gradations increase the anisotropy in the contact forces, with the strong forces aligning in the major principal stress direction and the weak forces providing resistance against buckling. These insights contribute to a better understanding of the pre- and post-liquefaction triggering behavior in granular soils and the implication for the design of resilient infrastructure built on, or comprised of, coarse-grained broadly-graded soils.

This study presents an investigation into the mechanical behavior of geogrid-reinforced ballast subjected to cyclic loading focusing on the macro- and micromechanical features of the geogrid-ballast interaction mechanism. Key areas of interest include the effects of geogrid placement depth, aperture size, and stiffness on the motion of ballast particles, formation of contact force chains, and energy dissipation. A three-dimensional discrete element model, calibrated with experimental data, simulates ballast box tests performed on 300-mm-thick ballast layers reinforced by geogrids placed at depths ranging from 50 to 250 mm below the tie. The findings reveal that geogrids located within the upper 150 mm of the ballast layer significantly reduce tie settlement by minimizing particle movement, creating well-connected soil structures, and decreasing energy dissipation. Upon identifying 150 mm as the optimal geogrid placement depth, a parametric study evaluates the impact of the geogrid aperture size (A) and stiffness on the behavior of geogrid-reinforced ballast. The geogrid aperture size (A) is varied to give aperture size to ballast diameter (D) ratios ranging from 1.09 to 2.91, while the geogrid's stiffness ranges from 9.54 to 18.00 kN/m. Results indicate that A/D ratios greater than or equal to 1.45 are required for geogrids to perform satisfactorily, while stiffness appears to wield a negligible influence on the response of geogrid-reinforced ballast.

The current study adopts a micromechanical approach to explore the nature of stress transmission in wet granular materials. First, we derive the discrete form of the capillary stress tensor obtained from homogenization to show the virial nature of capillarity through the application of point -wise capillary forces in Discrete Element Modeling (DEM). Furthermore, the non -spherical character of the capillary stress tensor is highlighted through a series of DEM triaxial simulations. Contrary to common thinking, the capillary stress tensor has indeed both mean and deviatoric components due to the underlying micromechanical aspects. Relevant key dimensionless parameters are identified to evaluate the relative magnitude of the capillary stress to the externally applied and contact (intergranular) stresses, thus determining the specific conditions under which the contribution of the deviatoric part becomes considerable. In addition, a DEM simulation of a simple shear test is performed to confirm the anisotropy (non -sphericity) of capillary stress tensor. Finally, the effective nature of the contact stress in the sense of Terzaghi for the constitutive behavior of wet granular materials is investigated via a DEM stress probing analysis. Results suggest that a single contact stress variable - germane to an effective stress - cannot relate to strain for the constitutive law in triphasic condition.

Reinforcing calcareous sands with geogrids is a potentially effective method for large-scale geotechnical constructions in coastal lands. The breakable nature of polygonal calcareous sands determines the complex particlegeogrid interactions. A three-dimensional numerical model of geogrid reinforced calcareous sand (GRCS) was established to investigate the potential mechanical laws based on the discrete element method (DEM), and the reasonableness of the numerical model was verified by comparing with the indoor triaxial test. It follows that the macro-microscopic mechanical behavior of GRCS under the influence of aperture size and tensile resistance of geogrids was further investigated via effective DEM simulations. The presented results show that the decreased aperture size and increased tensile resistance are beneficial to enhance the macro-mechanical properties of GRCS, including strength, internal friction angle and pseudo cohesion. Particle crushing is mainly affected by shear strain and confining pressure. The bulging deformation of GRCS is partially suppressed due to the confining effect of geogrids. Besides, the source of strength enhancement of GRCS is revealed based on the microscopic particlegeogrid interactions, and the calculation method of horizontal and vertical additional stresses in the reinforced soil element considering the effects of tensile resistance and aperture size is further established.

Design of geosystems built on coarse-grained soils with broader gradations are typically based on methodologies developed for clean sands without explicit consideration of the effects of gradation, potentially leading to uncertainty in performance predictions. This study investigates the effect of changes in gradation on the shear strength, stress-dilatancy behavior, critical state parameters, and fabric evolution of coarse-grained soils using three-dimensional (3D) discrete element method (DEM) simulations. The simulations of monotonic isotropically-consolidated drained and undrained triaxial tests were conducted on specimens with coefficients of uniformity (CU) between 1.9 and 6.4 composed of nonspherical particles following the calibration of parameters against experimental triaxial data. Results are used to evaluate the peak and critical state shear strengths, dilatancy responses, critical state lines, shear-induced pore pressures, and fabric evolution. Notably, an increase in CU leads to increases in peak shear strength, total dilation, rate of dilation, negative pore pressure magnitude, and rate of pore pressure generation. The results show that the state parameter better captures the effect of gradation than the relative density because the former accounts for the difference between the initial and critical states. The trends in triaxial parameters are compared with established frameworks to highlight the differences in response resulting from variations in CU. The particle-level measurements indicate that gradation affects the packing characteristics and contact force transmission, where broader gradations result in greater interlocking between coarser particles, and the presence of coarser particles increases the anisotropy of the strong force networks. The finer particles provide resistance to buckling within these strong force networks. Additionally, particles smaller than D10 are inactive in stress transmission, and the percentage of particles inactive in stress transmission decreases with an increasing CU. The combination of macro- and microresults contributes to understanding the mobilization of stress and its dependency on dilatancy in soils of varying gradation.