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.

Soil-rock mixtures (S-RM) are prevalent in both nature and practice, and stability of S-RM slopes is one of the focuses for engineers. In addition to soil strength, seepage erosion is one of the main factors affecting the stability of S-RM slopes. As water infiltration complicates the multi-field coupling effects and micro-scale mechanical behaviors of S-RM, it is essential to investigate seepage-induced S-RM landslides from both macro and micro perspectives. This study proposed a CFD-DEM fluid-solid coupling method, and the method was validated with Darcy experiments and lab slope stability experiments. The method was then applied to analyze seepage-induced slope instability, focusing on the impact of rock content and rock shape. The results indicate that slope failure under seepage showed the same characteristics as debris flow, with instability features such as sliding surfaces, damage range, and particle motions varying according to rock content and shape. As rock content increased, the accumulation of slope transitions through three distinct modes. Slope was prone to failure along the soil-rock interface, and low rock content further impaired the stability. The slope deformation was primarily driven by changes in particles contact. Once slope instability occurred, the system tended to adjust particle contacts to achieve new state of equilibrium.

Rainfall-induced instabilities in highly permeable earthen slopes typically originate at the slope toe; however, the triggering mechanism remains unclear. In this study, we captured the initial microscopic deformations and the overall macroscopic progressive damage of slope instability, extracted the stress paths and contact force chains of soil particles in different parts of the slope before and after rainfall, and revealed the triggering mechanism of soil slope instability induced by rainfall by conducting model tests and utilizing CFD-DEM (computational fluid dynamics-discrete element method) fluid-structure coupling numerical simulations. Our findings revealed that the slope toe exhibits stress concentration prior to rainfall and is a sensitive area of the entire slope before rainfall. After rainfall, rainwater infiltrates, and the seepage rate is the highest near the slope toe. The force-chain arch formed by the large particles at the slope toe, which play the role of the skeleton, is gradually weakened. The essence of rainfall-induced soil slope failure lies in the gradual erosion of the stable contact force chains between soil particles at the slope toe by seepage forces, leading to a progressive weakening, fracture, and disappearance from the outside inward in a collective movement. Once the failure of the slope toe is triggered, the damage area of the inter-granular contact force chains is significantly larger than the displacement plastic zone (or shear band), and the stress in the soil near the slope rapidly transitions from high to low. Subsequently, as the soil particles continue to slip and roll, the soil stress fluctuates and gradually increases, forming a stress-concentrated force chain arch at the rear edge of the slip surface highlighting the slope's certain self-stabilizing capability after failure. Throughout the process, the stress path at the foot of the slope is the longest.

This study investigates, for the first time ever, the suffusion on gap-graded granular soils under torsional shear conditions from a microscopic perspective. A numerical model of the hollow cylinder torsional shear test (HCTST) using the discrete element method (DEM) is first developed, where an algorithm for simulating the real inner and outer rubber membranes of the hollow cylinder apparatus (HCA) is introduced. After the validation, the computational fluid dynamics (CFD) approach is introduced for the coupling between the particle and fluid phases. Then, a series of the coupled CFD-DEM suffusion simulations considering the rotation of the major principal stress axis (alpha) and intermediate principal stress ratio (b) are conducted. It is found that more fine particles are eroded in cases having smaller alpha and b, and the clogging phenomenon in the middle zones becomes more significant as both alpha and b increase. From the microscopic perspective, the specimens whose contact anisotropy principal direction is close to the fluid direction will lose more fines, and the anisotropy magnitude also plays an important role. In addition, the differences in structure and vertical connectivity of the pores in HCTST samples under various complex loading conditions cause fine particles to have different migration paths, further resulting in different fines mass loss.

This study proposes a resolved framework coupling computational fluid dynamics (CFD) with discrete element method (DEM) to simulate the sedimentation of granular sand. Realistic sand particles were reconstructed by spherical harmonic representation combined with the multi-sphere clump method, and a fictitious domain method for irregular clumps was further developed to solve the fluid-particle interaction. This resolved CFD-DEM offers a direct and robust approach for computing real fluid forces on irregular-shaped granular sands, without relying on any empirical drag force models. Initially, the effectiveness and accuracy of the proposed CFD-DEM were validated through a series of single-particle free settling simulations for various ideal-shaped particles. Critical fluid-particle interacting behaviors in terms of drag force and wake structure were mainly investigated and corroborated with experimental data. The study subsequently progressed to simulate the sedimentation processes of various granular sand assemblies composed of realistic-shaped sand particles, utilizing the proposed CFD-DEM. Detailed numerical analyses concentrated on particle-scale mechanics during sedimentation, including settling trajectories and velocities of particles, as well as the coordination and anisotropy of inter-particle contacts. The results and findings gained from this study provide a novel insight into the micro-mechanisms underlying the sedimentation and accumulation process of granular soils in geological environments.

该文采用CFD-DEM方法对回转体水下运动兴波破冰问题进行了数值模拟研究，探究回转体运动速度、潜深对破冰效果的影响，对比分析了不同工况下冰盖的裂纹扩展规律和破坏程度，其中速度无因次化为弗劳德数Fr，潜深无因次化为回转体深度与回转体最大直径的比值，记作h。研究结果表明：在回转体运动兴波破冰过程中，回转体中后部冰盖的破坏情况较为严重；随着回转体的运动，冰盖裂纹向前扩展，出现新的纵向裂纹与横向裂纹，待裂纹完全发展后，冰盖呈现出以横向裂纹为主的破坏形式；存在临界速度Ccri，当回转体的运动速度处于临界速度附近时，冰盖的破坏效果较好，对应的速度区间为Fr=0.55～0.75；随着回转体潜深增大，冰盖破坏程度总体呈现出下降趋势，h>3.43后，冰盖不再发生明显破碎。

One of the critical steps in the root crop harvesting process is screening tubers from soil. However, low screening efficiency seriously hinders the rapid development of the root crop industry. Clarifying the tuber-soil mixture separation behaviour and establishing the connection between vibration, airflow parameters, and separation index (SI) is critical to increasing screening efficiency. Corydalis Yanhusuo is employed as the research object, and the three-dimensional scale distribution and mechanical properties of tubers and soil particles are first counted. Then, a vibration and airflow coupling separation model of the tuber-soil mixture was constructed using the computational fluid dynamics and discrete element method (CFD-DEM) coupling method, and the physical parameters in the model were calibrated. A new method for calculating the SI is proposed. The relationship between vibration amplitude, frequency, airflow velocity, SI, and separation velocity was analysed. Simultaneously, the porosity change in the particle group during the separation process was investigated, and the relationship between vibration, frequency, and airflow velocity on the separation dynamics of binary mixtures was revealed by utilising data visualisation and frequency domain analysis. The platform for the vibration and airflow separation physical test was built. The separation behaviour of mixed particles in various parameters was discussed, as was the feasibility and accuracy of the numerical simulation results. The results of this study can provide theoretical support for the efficient screening of tuber-soil mixtures and further promote the rapid development of the root industry.

Conventional CFD (Computational Fluid Dynamics)-DEM (Discrete Element Method) coupling methods encounter apparent difficulties in addressing the large deformation exhibited by soils with arbitrarily shaped fluid domains for undrained triaxial shear tests with flexible membranes. Herein, a novel CFD-DEM coupling method is proposed to address the main challenges of dynamically reproducing complex external boundaries and mapping for fluid fields. The workflow of surface mesh construction, mesh coarsening, and internal volume division is proposed to generate required meshes. The mapping of fluid information between updated and original meshes is implemented by a distance-weighted interpolation strategy. The coupling method is subsequently applied to investigate the effect of flexible membranes with and without clamped ends on undrained triaxial shear characteristics of soils after its comparison to the constant volume method for validation. The flexible membranes without clamped ends are proven to delay the shear dilation and weaken the inter-particle contact force. Moreover, they enable the free development of the shear band and induce significant octahedral shear strain at both ends of the band. The fluid pressure distributions of both boundary types are uniform and a vortex-shaped velocity field for the fluid is obtained due to the effect of the particle-fluid interaction.