Internal erosion induces alterations in the initial microstructure of soils, simultaneously affecting physical, hydraulic, and mechanical properties. The initial soil composition plays a crucial role in governing the initiation and progression of seepage-induced suffusion. This study employs the controlled variable method to develop granular soil models with varying particle size ratios, initial fine particle contents, and coarse particle shapes. Seepage suffusion simulations coupled with microstructural analyses are conducted using the CFD-DEM approach. Results demonstrate that particle size ratio, fine particle content, and coarse particle shape exert distinct influences on cumulative erosion mass, fine particle distribution, contact fabric, and mechanical redundancy at both macroscopic and microscopic scales. This numerical investigation advances the fundamental understanding of internal erosion mechanisms and informs the development of micro-mechanical constitutive models. Furthermore, for binary granular media composed of coarse and fine particles, careful control of the particle size ratio and fine content is recommended when utilizing gap-graded soils in embankment and dam construction to improve structural resilience and resistance to internal erosion.

Hydraulic structures such as embankments and dams are essential for water storages, flood control, and transportation, but are vulnerable to suffusion under complex loading conditions. This study investigates the effect of suffusion on the cyclic shear behavior of gap-graded soils using the coupled computational fluid dynamics and discrete element method (CFD-DEM). A series of seepage infiltration and drained cyclic shear tests are conducted on specimens with varying mean stresses and initial stress anisotropy to systematically evaluate the mechanical consequences of suffusion. The findings reveal that the higher mean stress and initial stress anisotropy significantly exacerbate fines loss and deformation, particularly along principal seepage directions during suffusion. Furthermore, the eroded specimens exhibit substantial stiffness degradation and microstructural changes, including the deteriorated interparticle contacts and more pronounced fabric anisotropy. Notably, fines loss intensifies the load-bearing reliance on coarse particles during cyclic loading. These results provide new micromechanical insights into suffusion-induced degradation, offering valuable implications for developing advanced constitutive model of gap-graded soils accounting for suffusion-induced fines loss and cyclic loading conditions.

This study proposed a novel hybrid resolved framework coupling computational fluid dynamics (CFD) with discrete element method (DEM) to investigate internal erosion in gap-graded soils. In this framework, a fictitious domain (FD) method for clump was developed to solve the fluid flow around realistic-shaped coarse particles, while a semi-resolved method based on a Gaussian-weighted function was adopted to describe the interactions between fine particles and fluid. Firstly, the accuracy of the proposed CFD-DEM was rigorously validated through simulations of flow past a fixed sphere and single ellipsoid particle settling, compared with experimental results. Subsequently, the samples of gap-graded soil considering realistic shape of coarse particles were established, using spherical harmonic (SH) analysis and clump method. Finally, the hybrid resolved CFD-DEM model was applied to simulate internal erosion in gap-graded soils. Detailed numerical analyses concentrated on macro- -micro mechanics during internal erosion, including the critical hydraulic gradient, structure deformation, as well as particle migration, pore flow, and fabric evolution. The findings from this study provide novel insights into the multi-scale mechanisms underlying the internal erosion in gap-graded soils.

Gap-graded soils, extensively utilized in geotechnical and hydraulic engineering, exhibit diverse strength characteristics governed by their distinctive particle size distribution (PSD). To investigate the influence of PSD on the shear strength of gap-graded soils, this study utilizes the Discrete Element Method (DEM) to reproduce drained conventional triaxial tests of gap-graded soils across a wide range of fine particle content (FC = 1-40%) and particle size ratio (SR = 2.5-6.0). The simulation results reveal that the peak shear strength follows a characteristic unimodal curve versus FC, attaining its maximum value at about FC = 25%. SR governs peak strength through critical FC thresholds: negligible impact at FC < 10%, whereas significant enhancement occurs at FC = 25%. Micromechanical analysis reveals that branch anisotropy evolution controls strength behaviour. Shear strength inversely correlates with peak branch anisotropy as reduced branch anisotropy promotes homogenized contact force distribution. FC and SR collectively regulate macroscopic strength through coupled control of branch anisotropy evolution, where their synergistic interaction governs force chain reorganization and stress distribution homogeneity. Based on these insights, a novel predictive formula for peak strength incorporating both SR and FC were proposed, providing guidance for optimized deployment of gap-graded soils in engineering practice.

This research investigates the particle-scale stress transmission characteristics at the end of isotropic consolidation stage for sand-rubber mixtures, focusing on the effects of particle size disparity, density, and stress levels. The discrete element method was adopted with total 450 simulations being conducted for sand-rubber mixtures with increasing size disparities to quantify the particle-scale stress distribution between sand and rubber materials. This study reveals that the variation of coordination number and void ratio for sand-rubber mixtures align with those observed in conventional gap-graded soils, while the inclusion of deformable rubber clumps significantly increases coordination number values. A complex interplay between packing density and stress level was evident, illustrating the nuanced role of rubber in stress transmission. As packing density and stress levels decrease, the efficacy of deformable rubber clumps in stress transfer increases. An inverse relationship between the efficiency of stress transmission and particle size disparity was observed for all these sand-rubber mixtures. The findings indicate that, despite variations in size disparity, the proportion of stress transferred by rubber remains consistently lower than their volumetric contribution. This study underscores the complexities of using sand-rubber mixtures and highlights that the effect of particle property disparity outweighs the that of particle property disparity.

Internal erosion can induce significant changes in the mechanical properties of soils, posing various hazards to dam and dike structures. Despite its importance, our current understanding of this phenomenon remains incomplete. The influence of pre-shearing stress conditions on the mechanical behaviours of soils during the internal erosion process is particularly challenging, as existing experiments have not been able to maintain the constant pre-shearing stress ratios. To bridge this gap in knowledge, this paper presents a series of discrete element method (DEM) simulations focused on gap-graded cohesionless soil. The primary objective of these simulations is to investigate two specific cases of internal erosion: suffusion and suffosion processes. Soil specimens are subjected to different pre-shearing stress ratios in the standard triaxial tests before being submitted to different levels of erosion to study their constitutive responses. The results show that erosion-induced deformation (i.e. suffosion) only starts after a specific amount of mass loss. This mass loss and the pre-shearing stress ratio form a well-defined criterion for triggering suffosion, which is named suffosion surface. The volumetric strain is shown to be a better indicator to describe the suffosion process than the commonly used void ratio. The pre-shearing stress ratio significantly influences the suffosion response of the soil sample, with a higher preshearing stress ratio facilitating soil failure. Furthermore, soil specimens undergo both deviatoric and volumetric responses during the suffosion process. To this end, new DEM-based statistical equations were proposed to describe the observed mechanisms, which are helpful for the future development of constitutive models to describe internal soil erosion.

The discrete element method (DEM) coupled with the pore-scale finite volume (PFV) method was used to simulate the suffusion and post-suffusion behavior of gap-graded soil with different initial fines content (fc). A series of drained triaxial tests were performed on the non-eroded, eroded and reconstituted samples. The results indicate that the erosion ratio (Er) increases as the fc of the sample increases. The mechanical response of the sample with an initial fc of 15 % is almost unaffected by suffusion. The dilatancy and peak deviatoric stress ratios of sample with initial fc of 25 % and 35 % are significantly lower with increasing erosion ratio. When the Er is greater than 8.2, eroded samples with an initial fc of 35 % collapse during shearing, and the eroded and reconstituted samples behave as dilation and contraction, respectively. As the initial fc increases, the mechanical response of the reconstituted samples differs more and more from that of the corresponding eroded samples. The force chain analysis indicates more force chains in the eroded sample than in the reconstituted sample, resulting in higher dilatancy of the eroded sample with fc of 25 % and 35 % during shearing.