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 soil, characterized by the absence of certain particle sizes, is commonly used in infrastructure projects such as dams and roadbeds. A comprehensive understanding of both the macro- and micro-mechanical behaviors of discontinuously graded soils is essential for their effective use in engineering applications. In this study, drainage triaxial compression tests were conducted on four gap-graded soil samples with different fine-grain contents mainly using the DEM method, whereas the flexible boundary part was performed using the FDM-DEM method. The contacts were classified based on the magnitude of contact forces between coarse and fine particles, considering the coordination number of the particles involved and the normal angular distribution of these contacts. This classification enabled a detailed analysis of how fine particles contribute to stress transmission and structural evolution during shearing. The fabric tensor for these contact types provided further insights into the anisotropy of samples during shearing. On the microscopic scale, the evolution of contact numbers was found to closely align with the observed stress-strain behaviors. Increasing fine particle content significantly altered the role of fine particles in the stress transmission process. With low content of finer particles, initially, fine particles were situated within the voids formed by coarse particles, and the fine particles are gradually embedded into the coarse particles during the loading process. With the increase of fine particle content, fine particles constantly aggregate to block coarse particles and become the main medium of stress transmission.

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.

The influence of mechanical loading paths on the characteristics of gap-graded granular assemblies was investigated using the discrete element method (DEM). Dense and loose gap-graded assemblies with finer fraction content, f(c), ranging from 0-100% were prepared and subjected to drained triaxial compression and extension loading paths. After examining key macroscale quantities, micromechanical analyses were conducted to elicit the particle-scale characteristics including the evolution of the fabric of the assemblies under the different loading paths. The results of the DEM analysis confirm the validity of the Mohr-Coulomb failure criteria at the critical state. While the mobilised friction angle at the peak is higher under extension than in compression, no significant difference was obtained in the critical state friction angle for both loading paths. Despite the higher mean stress transmitted by the gap-graded assemblies under compression in comparison with extension, the contribution of the finer particles to the total mean stress is not significantly influenced by the loading paths. Our data show that the variation in the fabric of granular assemblies under different loading paths does not always stem from an initial inherent anisotropy. Fabric anisotropy is marginally higher under extension than in compression despite having an initial isotropic fabric.

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.

Hydraulically filled coral sand foundations are susceptible to various challenges within intricate marine environment. The friability of coral sand results in the production of large amounts of sub-graded fine particles under external stress. Meanwhile, the continual influence of oceanic forces leads to a gradual erosion of these fine particles from soil. The interaction between these two long-term effects plays a crucial role in particle breakage and soil mechanics of coral sand. To address this issue, consolidated drained triaxial tests and sieving analysis were conducted on the gap-graded coral sand with various fine contents. Three unique test methodologies are devised to alter the fine content, including hydraulic scouring, particle removal and particle replacement. The experimental results revealed that a specific amount of fine particle loss can significantly deteriorate the mechanical properties of dense coral sand. By replacing coarse particles with fine particles, larger strength parameters and less dilation were observed, yet there existed a critical threshold of 60% fine content, beyond which further substitution did not yield additional improvement in soil strength. Particle crushing was primarily concentrated in the middle layer of the specimen, influenced by the development of the shear band. Furthermore, the amount of newly generated finer particles exhibited a positive correlation with the increase in fine content in the initial gap-graded soil. These findings could enhance the understanding of the role that fines plays in determining the mechanical characteristics and particle breakage behavior of coral sand, and thus aid in more accurate assessments and designs of engineering applications involving coral sand.

Internal erosion involves the transport of soil particles from within or beneath a geotechnical structure due to seepage flow, influencing the subsequent mechanical and hydraulic behaviour of the soil. However, predicting changes in small-strain modulus ( G max ) with eroded fines and varying principal stress directions can be challenging due to various factors related to soil fabric. The present study investigates the impact of seepage flow on G max , as well as the effect of principal stress rotation (PSR), of gap-graded soil with a fines content of 20%, using a novel erosion hollow cylindrical torsion shear apparatus. The erosion test results indicate that, regardless of density, the G max generally increases with seepage time. The trend of G max measured in the vertical and torsional directions varies significantly, as seepage is applied always downward, resulting in a different impact on the vertical and horizontal bedding planes. After a cycle of PSR, the induced torsional shear strain is found larger for the eroded specimens, while vertical strain decreases due to fine removal accompanied by seepage flow. In the PSR tests, the specimens subjected to erosion exhibit a greater reduction in G max compared to non- eroded specimens, with increasing the angles of principal stress direction. This reduction may be due to the inefficacy of the reinforced soil skeleton established by erosion against shearing. The distribution of fine particles and anisotropy induced by seepage flow contribute to non-trivial mechanical behaviour during principal stress rotation, particularly regarding small-strain shear modulus. (c) 2024 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society. This is an open access article under the CC BY- NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Water retaining structures are critical elements of civil infrastructure. Internal erosion of soils forming the containment structures may occur progressively and lead to expensive maintenance costs or failures. The strength, stress-strain behavior and critical state of soils which have eroded, as well as the characteristics of the erosion, may be affected by hydraulic gradient, confining stress and relative density of the soil at the start of the erosion. Here, erosion and triaxial tests have been conducted on gap-graded soil samples. The tests and results are novel as the samples were prepared to be homogenous post-erosion and prior to triaxial testing by adopting a new sample formation procedure. The post-erosion homogeneity was evaluated in terms of particle size distribution and void ratio along a sample's length. The erosion-induced mechanical property changes can then be linked to a measure of initial state, more reliably than when erosion causes samples to be heterogeneous. The results show that erosion causes the critical state line in the compression plane to move upwards. The movement is lesser than the increase in void ratio caused by erosion. The state parameter is therefore reduced, consistent with the soil's reduced peak strength and its less dilative response. Regarding the erosion characteristics, the flow rate decreases with the increase in initial relative density or effective stress, but increases with the increase in the hydraulic gradient being applied. The cumulative eroded soil mass increases with the increase in hydraulic gradient and decreases with the increase in initial density and effective confining stress.