CFD–DEM Analysis of Internal Soil Erosion Induced by Infiltration into Defective Buried Pipes

Frictional instabilities in fluid-saturated granular materials underlie natural hazards, including submarine landslides and earthquake initiation. Experimental evidence shows distinct failure behaviors under subaerial and subaqueous conditions due to the coupled influences of mechanical deformation, interparticle friction, and particle–fluid interactions. We use three-dimensional coupled computational fluid dynamics–discrete element method (CFD–DEM) simulations to investigate the collapse and runout of dense and loose granular assemblies in both environments. Parametric analyses demonstrate that pore-pressure evolution controls the failure mode in saturated settings (fast vs slow sliding), consistent with prior laboratory experiments and lattice-Boltzmann–discrete element method simulations: dense assemblies stabilize via dilation, whereas loose assemblies compact rapidly and transiently fluidize. At the mesoscale, we coarse-grain particle-contact statistics and Eulerian fluid fields to define apparent friction and normalized pore pressure, and organize inertial and viscous responses using log10(In/Iv). Spatiotemporal analyses of these coarse-grained fields reveal strain-rate-dependent behavior governed by evolving porosity and effective stress. In both environments, friction in the failure shear zone is rate-strengthening with respect to the inertial number (In, for dry medium) and viscous number (Iv, for fluid-saturated medium). We further utilize the mesoscale stress framework to compare the evolution of pore pressure in the CFD–DEM simulations of subaqueous slope collapse with an analytical solution for the development of the failure front, using inputs derived from numerical triaxial DEM tests on the same assemblies. The analytical model reproduces steady-state excess pore pressures and captures fluid–particle coupling; however, a mismatch near failure onset suggests a role for transient frictional behavior in grain–fluid interactions. These results support the development of physics-based models of natural hazards and advance our mechanistic understanding of saturated granular failure.

Lattice-Boltzmann method (LBM) simulations of a gas-fluidised bed have been performed. In contrast to the current state-of-the-art coupled computational fluid dynamics–discrete element method (CFD–DEM) simulations, the LBM does not require a closure relationship for the particle–fluid interaction force. Instead, the particle–fluid interaction can be calculated directly from the detailed flow profile around the particles. Here a comparison is performed between CFD–DEM and LBM simulations of a small fluidised bed. Simulations are performed for two different values of the superficial gas velocity and it is found that the LBM predicts a larger bed expansion for both flowrates. Furthermore the particle–fluid interaction force obtained for LBM simulations is compared to the force which would be predicted by a CFD–DEM model under the same conditions. On average the force predicted by the CFD–DEM closure relationship is found to be significantly smaller than the force obtained from the LBM.

Forces acting on a single introduced particle in a solid–liquid fluidised bed

Seepage effect on failure mechanisms of the underwater tunnel face via CFD–DEM coupling

Accurately assessing the erodibility of geomaterials is of great significance for the design of earthen structures and the prevention of the associated failure induced by seepage force. Recently, the un-resolved Computational Fluid Dynamics–Discrete Element Method (CFD–DEM) has been widely used to investigate internal erosion. However, due to the use of wall boundary and the fact that the fixed CFD domain cannot be changed with the soil sample’s volume contraction during the erosion test, a larger porosity at the boundary of the CFD domain is commonly formed, resulting in sidewall preferential flow (i.e., relatively more fine particles migrate along the boundary of the DEM domain) and thereby overestimating the soil erodibility. In this study, a new method based on particle boundary is developed to tackle this problem. The newly proposed particle boundary can prevent its particles from erosion via inter-particle bonding and transfer stress from servo walls to the simulated sample. An optimal particle boundary thickness is determined by considering sample contraction and computational efficiency. The performance of the new method was compared with the conventional method and also verified using experimental results. The results show that the newly proposed method has significantly improved the uniformity of fluid velocity distribution. Furthermore, the cumulative eroded mass of fine particles in the new model is approximately 15% lower than in the conventional model. It is convincingly demonstrated that the new method can simulate internal erosion better and give a more accurate assessment of geomaterial erodibility.

Seepage water may move soil particles and cause internal erosion of soils, leading to sinkholes and the collapse of embankments and slopes. To account for the effects of confinement and constricted seepage exit, a test apparatus was developed to study the internal erosion of granular soils under various confining pressures, particle sizes, and sizes of the seepage exit opening. As indicated in the literature, the behavior of internal erosion has been largely studied by laboratory experiments and field investigations, and mechanical models that help describe the failure mechanism of internal erosion are less prevalent. A hydro-mechanical model that incorporates the fluid drag force and the shear strength of soil was therefore developed for quantifying the internal erosion experiments conducted in this study. The experimental results showed that the greater the confining pressure or the particle size, the greater the critical velocity; the greater the seepage exit opening, the smaller the critical velocity. The critical velocity predicted by the proposed hydro-mechanic model compares reasonably well with the experimental data. In addition to the confining pressure, particle size, and size of the seepage exit opening, the proposed model also showed that the friction angle and porosity of the soil are factors influencing the critical velocity, which is consistent with the experimental findings of this study.

In internal erosion studies, the effect of seepage flow direction in soil and soil layers has received little attention. However, in geotechnical structures, such as earth dams or even natural deposits, the flow may not be perpendicular to the layers or in line with the gravity. Hence, in this article, a special physical model is introduced with both the possibility of changing the flow angles relative to the direction of gravity and the possibility of studying heterogeneous specimens in internal erosion. In addition, the effect of flow direction, inclination of layers, and plasticity index on the internal erosion (suffusion) of gap-graded soil with two types of fines, bentonite and rock flour, has been tested and analyzed. Additionally, a new method for specimen preparation of the gap-grading soils called the “ideal moisture content method” has been presented. Based on the results, the greater the angle of the flow vector is in relation to the direction of gravity; the occurrence and continuation of erosion takes place in higher gradients. Furthermore, in general, in the case of layers perpendicular to the flow, the rate of flow discharge and amount of the eroded particles in the same conditions are less than those of layers parallel with the flow. Moreover, layer direction affects the shape of pipes created because of the erosion.

SummaryIt has been reported that sand production, which is a simultaneous production of soil particles along with gas and water into a production well, forced to terminate the operation during the world's first offshore methane production test from hydrate‐bearing sediments in the Eastern Nankai Tough. The sand production is induced by internal erosion, which is the detachment and migration of soil particles from soil skeleton due to seepage flow. The inflow of the eroded soil particles into the production well leads to damage of the production devices. In the present study, a numerical model to predict the chemo‐thermo‐mechanically coupled behavior including internal erosion during hydrate dissociation has been formulated based on the multiphase mixture theory. In the proposed model, the internal erosion is expressed as mass transition of soil particles from soil skeleton to the fluidized soil particles. Since the internal erosion is considered to depend on the soil particle size, mass of soil particles are divided into several groups that have different representative particle diameters, and the constitutive equations for the onset condition and the mass transition rate of the internal erosion are formulated for each group. Also, transportation of soil particles in the liquid phase is formulated for each particle size group in the proposed model. Finally, a simulation of the methane gas production from the hydrate‐bearing sediment by depressurization method is presented, and the internal erosion and the dissociation behavior are discussed.

Urban groundwater infiltration resulting from tropical storm rainfall can lead to internal soil erosion and ground subsidence. The long-term effects of these phenomena may result in uneven settlement of the subway system, posing risks to its safety and operational integrity. Suffusion poses a significant hazard to long-term subway operation. It is a type of soil internal erosion characterized by the migration of fine particles from the pore channels between coarse particles. This study focused on the performance degradation of different gap-graded sands caused by internal erosion, observed through triaxial compression tests. Using the cap plasticity model and considering mechanical parameters before and after soil erosion, a refined two-dimensional finite element model of a soil subway station structure based on ABAQUS was developed to analyze the deformation and damage pattern of the subway structure resulting from soil internal erosion and earthquake. The results of this study demonstrated that localized internal soil erosion significantly contributes to uneven settlement in subway stations, potentially compromising their seismic performance during oblique seismic excitations.

Fluid–soil interaction plays a pivotal role in various geotechnical engineering applications, as it significantly influences processes such as erosion, sediment transport, and soil stability. Modeling fluid–soil particle interactions in these contexts presents substantial challenges due to the inherent complexity of the interactions occurring across multiple characteristic scales. The primary challenge lies in the vast disparities in magnitude between these scales, which demand sophisticated modeling techniques to accurately capture the intricate dynamics involved. Coupled fluid–soil particle models have emerged as essential tools for understanding the mechanisms underlying fluid–soil interactions. Among these, the CFD-DEM (computational fluid dynamics–discrete element method) approach has gained significant attention. This method provides an effective compromise between high-resolution sub-particle fluid modeling and coarser mesh-based techniques for fluids and particles. By doing so, CFD-DEM facilitates large-scale simulations while maintaining computational efficiency, making it a promising solution for studying fluid–soil interactions in complex geotechnical scenarios. This review highlights the application of CFD-DEM models in geotechnical engineering, with a specific focus on soil erosion processes and the critical role of turbulent flow. It explores various fluid–soil particle interaction computational mechanisms and their implications for erosion dynamics, emphasizing several key aspects, including the following: laminar vs. turbulent flow models: understanding the distinctions between flow regimes is critical for accurately predicting fluid-induced soil particle movement. Shear stress effects: the influence of flow-induced shear stress on the detachment of soil particles is analyzed, particularly in erosion-prone environments. Sediment transport mechanisms: factors such as particle size, density, and water velocity are examined for their roles in governing sediment transport. Knowledge gaps and future directions: these involve identifying unresolved issues in current fluid–soil interaction models, with an emphasis on improving the accuracy and scalability of CFD-DEM simulations. By delving into these aspects, the review aims to advance the understanding of fluid–soil interactions and provide insights into optimizing modeling techniques for geotechnical engineering applications. It also outlines future research directions to bridge existing knowledge gaps, emphasizing the importance of integrating advanced turbulence modeling and computational strategies to enhance the predictive capabilities of fluid–soil interaction frameworks.

This paper discusses the thermal and exergy efficiency analysis of the hydrothermal liquefaction (HTL) process, which converts sewage sludge into biocrude oil in a continuous plug–flow reactor using a linear Fresnel solar collector. The investigation focuses on the influence of key operational parameters, including slurry flow rate, temperature, pressure, residence time, and the external heat transfer coefficient, on the overall efficiency of biocrude oil production. A detailed thermodynamic evaluation was conducted using process simulation principles and a kinetic model to assess mass and energy balances within the HTL reaction, considering heat and mass momentum exchange in a multiphase system using UDF. The reactor’s receiver, a copper absorber tube, has a total length of 20 m and is designed in a coiled configuration from the base to enhance heat absorption efficiency. To optimize the thermal performance of biomass conversion in the HTL process, a Computational Fluid Dynamics–Discrete Element Method (CFD-DEM) coupling numerical method approach was employed to investigate improved thermal performance by obtaining a heat source solely through solar energy. This numerical modeling approach allows for an in-depth assessment of heat transfer mechanisms and fluid-particle interactions, ensuring efficient energy utilization and sustainable process development. The findings contribute to advancing solar-driven HTL technologies by maximizing thermal efficiency and minimizing external energy requirements.

A new modeling framework for internal erosion in heterogeneous stratified soils is developed by combining existing methods used to study sediment transport in canals, filtration phenomena, and stochastic processes. The framework is used to study the erosion process in sediments caused by a flow of water through a covering filter layer that deviates from geometrical filter rules. By the use of spectral analysis and Laplace transforms, mean value solutions to the transport rate and the variance about the mean are derived for a transport constraint at the upstream boundary and a constant initial transport. Increasing the heterogeneity of a defective filter layer in a canal retards the propagation of the expected bed degradation. An implication is that the heterogeneity of the riprap in this respect prolongs the expected degradation process and the expected lifespan of the structure. The increased variance and decreased correlation length of the riprap reduces the confidence of the mean value solution. The st...

To investigate the mechanism of road collapse induced by structural defects in underground drainage/sewerage pipelines in water-rich sands, laboratory physical model tests were conducted to reproduce the macroscopic development of surface subsidence. A computational fluid dynamics-discrete element method (CFD-DEM) model was then established and validated against the tests to assess its reliability. Using the validated model, we examined the effects of defect size and groundwater level on the progression of groundwater-ingress-driven internal erosion and tracked the evolution of vertical stress and intergranular contacts around the pipe. Results show that internal erosion proceeds through three stages—initial erosion, slow settlement, and collapse—culminating in an inverted-cone collapse pit. After leakage onset, the vertical stress in the surrounding soil exhibits a short-lived surge followed by a decline on both sides above the pipe. The number of intergranular contacts decreases markedly; erosion propagates preferentially in the horizontal direction, where the reduction in contacts is most pronounced. Within the explored range, higher groundwater levels and larger defects accelerate surface settlement and yield deeper and wider collapse pits. Meanwhile, soil anisotropy strengthens with increasing groundwater level but peaks and then slightly relaxes as defect size grows. These qualitative findings improve understanding of the leakage-induced failure mechanism of buried pipelines and offer references for discussions on monitoring, early warning, and risk awareness of road collapses.

Using an extended CFD–DEM for the two-dimensional simulation of shock-induced layered coal-dust combustion in a narrow channel

Deep-sea pneumatic lifting systems face critical efficiency challenges due to complex gas–liquid–solid interactions within vertical pipes. This study establishes a computational fluid dynamics–discrete element method (CFD–DEM) coupled framework, based on the Eulerian multiphase model, to simulate these interactions. It systematically simulates particle transport for different particle sizes (8–20 mm) in various pipe diameters (80–220 mm). Key findings reveal: smaller particles (8 mm) exhibit superior flow stability due to significant flow field forces, while larger pipe diameters (160–180 mm) optimize efficiency by balancing turbulent suspension and energy consumption. Narrow pipes (100 mm) generate strong inlet acceleration but intensify flow instability for particles >16 mm, whereas wider pipes (160 mm) enhance output at reduced velocities. The optimal mechanism (pipe diameter: 160–180 mm; particle size: 8–14 mm), determined by suppressing inertial dominance and maximizing interfacial momentum transfer, achieves a transport efficiency of 20%. This study investigates factors influencing transport efficiency in vertical pneumatic lift systems, reveals the gas–liquid–solid flow mechanisms in deep-sea mining lifts, provides reliable references for engineering applications, and holds significant engineering implications for improving transport efficiency.