Solid–fluid sequentially coupled simulation of internal erosion of soils due to seepage

In slopes where a mixture of coarse and fine particles is present, the infiltration of rainfall can cause the migration of fine particles. This migration alters the hydraulic properties of the soil and has implications for slope stability. In this study, the slope under investigation is a tailings dam composed of loosely consolidated soil with a wide particle size distribution. Due to rainfall infiltration, fine particles tend to migrate within the voids of the coarse particle framework, leading to changes in hydraulic properties and inducing slope instability. The classical internal erosion constitutive model, known as the Cividini and Gioda erosion criterion, is commonly used to predict the behavior and effects of fine particle erosion in geotechnical engineering. However, certain parameters in this erosion criterion equation, such as long-term density, are challenging to obtain through experiments. To investigate the coupled evolution of seepage and erosion within landfill slopes under the influence of rainfall infiltration and to understand the mechanisms of slope instability, this research assumes the erosion of fine particle suspension and adopts the Worman and Olafsdottir erosion criterion to establish a coupled model of unsaturated seepage and internal erosion. The developed model simulates the coupled response of seepage and erosion in unsaturated landfill slopes under three different rainfall intensities. It is then combined with the infinite slope model to quantitatively analyze the impact of fine particle migration on soil permeability and slope stability. The numerical simulations provide the following findings: The Worman and Olafsdottir erosion criterion, unlike the Cividini and Gioda erosion criterion, only requires the determination of the soil’s gradation curve to estimate the erosion rate. Internal erosion primarily occurs within the leading edge of moisture penetration, accelerating the advancement of the wetting front and reducing slope stability. When the rainfall intensity is lower than the saturated permeability coefficient, the influence of internal erosion can be disregarded. However, under rainfall intensities equal to or greater than the saturated permeability coefficient, considering internal erosion results in a difference in the depth of the wetting front of up to 34.2 cm after 6 h in the R2 scenario. The safety factor without considering internal erosion is 1.12, whereas considering internal erosion yields safety factors between 1.08 and 1.09. In the R3 scenario, the difference in the depth of the wetting front reaches 53.8 cm after 6 h, with a safety factor of 1.12 without considering internal erosion and safety factors between 1.06 and 1.07 when considering internal erosion.

Internal erosion is a complex phenomenon that is one of the main risk factors to soil destruction. Its occurrence is mainly due to water infiltration and can cause slope instability. “Karst soil” is a type of loess with special soil and water sensitivity that makes it prone to landsliding. The processes of internal erosion include transport erosion and chemical dissolution, which strongly effect loess structure and strength. To reveal the internal processes and effects of the loess due to water infiltration, field investigations and indoor tests, including infiltration tests, undrained triaxial tests, particle analysis, chemical analysis, and scanning electron microscopy (SEM), were conducted. The results show that (1) the fine particles (clay and silt) and chemicals can move within the matrix of the macro-pores under seepage flow. The physical internal erosion is mainly due to fine particle migration out of the water and clay and silt particles, and the sample column settlement was 3.3 cm with a settlement ratio of 16.5%, which results in changes to the soil skeleton, increasing the porosity and infiltration rate of loess. (2) Chemical dissolution is also an important internal erosion process in loess, especially cations of Na, Mg, Ca, and K and anions of Cl, SO4, and CO3, which are mainly lost due to dissolution and flow out of with water and clay particles, resulting in altered physical characteristics of the soil. (3) Soil particles’ mitigation and chemical dissolution change the loess structure, leading to skeletal destruction and decreased peak strength and residual strength of the infiltrated sample to 7.75% and 8.13%, respectively. During internal erosion, physical fine particle migration and chemical dissolution are important for loess stability and loess slope susceptible to failure during water infiltration.

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.

Internal erosion processes in soils play an important role on the instability analyses of hillslopes and embankment dams. Field observations support the assumption that the internal fine particles may migrate among the channels formed by coarser particles under the high hydraulic gradient condition, where the enrichment of fine particles has great potential on the increase of local pore-water pressure due to their low permeability. Although a number of traditional seepage experiments in laboratory have provided data showing the effect of soil properties on the macroscopic permeability, however, much remains unknown particularly for microscopic erosion processes. Therefore, in the current study, a series of one-dimensional soil seepage tests were firstly conducted by controlling the coarse to fine particle size ratio, and then the X-ray tomography tests were carried out at beamline BL13W1 at the Shanghai Synchrotron Radiation Facility (SSRF) to obtain the particle distributions and three-dimensional pore structures. By coupling discrete element method (DEM) with Darcy’s law, the internal particle erosion processes were back-analyzed. The results reveal that the preferential erosion can occur in the top and bottom regions of the soil specimen, and the migrated fine particles can be supplied when the pore size is large enough along the seepage path.

The internal erosion effect causes fine particles in the soil to move through seepage, and the loss of these fine particles leads to changes in porosity, which in turn affects the soil's hydraulic properties and mechanical performance, posing a threat to the safety of dam and levee engineering. To understand the formation and development of internal erosion under reverse seepage, a simulation test device for internal erosion was designed, and experiments were conducted on three granite residual soil samples with identical soil properties under different water flow speeds (25L/H, 50L/H, and 100L/H). By comparing and analyzing the wetting front, the amount of internal erosion, and the water content, the influence of water flow speed on reverse seepage internal erosion was studied. The results show that under reverse internal erosion, as the water flow speed increases, the internal erosion rate accelerates, as evidenced by the faster advancement of the wetting front and the increase in cumulative internal erosion. As internal erosion develops, the fine particle accumulation curve enters a stable phase. After the soil's water content reaches its peak, it slightly decreases and then remains relatively stable. Fluctuations in the soil water content occur due to the formation of preferential internal erosion channels or the redeposition of fine particles. The soil particle movement, fine particle loss, and redeposition caused by internal erosion create an internal erosion channel that narrows from the inlet to the outlet.

Land subsidence is presented in many factors in different areas with urbanization. Internal soil erosion, owing to pumping confined groundwater during the deep foundation pit construction, has contributed to land subsidence. Four governing equations are presented to describe the process of internal soil erosion based on the mathematical–physical model. The finite element computation results, based on practical deep foundation pit engineering consisted of 8 layers of soil of Shanghai area, demonstrate that internal soil erosion will cause the increment of land subsidence and deformation and is related to the hydraulic gradient and the characters of the soils.

Cyclic hydraulic loading frequently affects embankment dams during reservoir regulation, tidal fluctuations, and intense rainfall. It potentially worsens fine particle migration during internal erosion and increases dam failure risks. This study is the first to systematically explore the influence of cyclic hydraulic loading on the critical hydraulic gradient (icr) of gap-graded soils, providing new insights into suffusion behavior. Transparent soil experiments, which enable direct observation of soil structural evolution, are combined with coupled DEM–Darcy simulations that offer microscopic mechanical insights, marking the first integrated use of these two approaches to investigate suffusion behavior. To quantify fine particle migration, we propose a novel modified grayscale threshold segmentation (MGTS) method for analyzing cross-sectional images captured during transparent soil experiments. The results from both methods show consistency in fine particle migration, clogging formation, and failure, with differences in permeability and icr remaining within acceptable limits. Fine particle content significantly influences the post-cyclic icr of internally unstable soils. For soils with lower fine particle content (15%), icr increases after cyclic hydraulic loading and rises with the mean hydraulic gradient during cycling. Conversely, soils with higher fine particle content (20%) exhibit a decrease in post-cyclic icr. This behavior is explained by changes in the average contact force between fine particles (Fff) observed in DEM simulations.

Internal erosion of debris-flow deposits triggered by seepage

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.

Internal erosion caused by broken sewer pipes often leads to ground subsidence in urban area, which is a major risk to public safety and has caused substantial socioeconomic loss. In order to ensure the ground stabilization and the safety of buried pipelines, it is necessary to understand the process of internal erosion around a submerged defective pipe. In this paper, the Dynamic Fluid Mesh (DFM) is coupled with the three-dimensional discrete element method (DEM) to simulate internal erosion in gap-graded soils above a defective pipe. In this fluid-solid coupling scheme, the fluid mesh can be generated according to the soil skeleton formed by coarse particles and updated at regular intervals. Seepage forces are calculated and applied on solid particles in the DEM model. The approach accounts for permeability and porosity changes due to soil skeleton deformation and internal erosion. In this study, some gap-graded soils samples with different size ratio are established. A defective pipe is placed below the sample. After that, different hydraulic gradients are applied to the sample. Fine particles are washed away from the hole in the pipe. The results indicate that the erosion process can be divided into three stages according to changes in the erosion rate. In the initial stage, numerous fine particles are washed away, and the flow rate increases with the increase of eroded particles. Subsequently, the erosion rate decreases and the flow rate tends to reach a steady state. Finally, only a small proportion of particles fall down from the outlets and the erosion rate levels off to zero gradually. Parametric studies show that the increase of hydraulic gradient increase the eroded particle mass. The number of erosion particles from the bottom layer is much larger than those from the upper layers as more fine particles in the upper layers are locked.

The structure of natural hydrate-bearing sediments that exist offshore or onshore is a combination of coarse-grained sediments and fine-grained particles. During gas production from hydrate-bearing sediments, fine particles may migrate with the flowing fluids within pore space and cause clogging of the pore space of the porous media. Therefore, fine particles play a significant role during methane production from hydrate-bearing sediments as it impact the overall sediment formation performance and production efficiency. The migration of fine particles and its impact on clogging have been investigated in a single-phase flow, but it has not been clearly understood in a multi-phase flow. This research focuses on the study of fines migration and clogging behavior during single and multi-phase flow which can be implicated in gas production from hydrates bearing sediments. Microfluidic pore models that mimic porous media with different pore throat sizes were fabricated and utilized to study fines migration and clogging behavior in porous media. Artificial particles and natural fine particles were selected to represent fine particles. The impact of flow rate, pore-fluid types, particle concentration, and pore-throat to fine particle size ratio was investigated. Fine particles used in this research include polystyrene latex particles, silica, and kaolinite. Pore-fluids used in this study include deionized (DI) water, and sodium chloride (NaCl) brine (2M concentration). The particle concentrations covered from 0.1% to 10%. And the pore-throat widths were fabricated from 40 μm to 100 μm. Single-phase flow experiments were conducted to show that the concentration of fine particles required to form clogging in pores increased as flow rate decreased. The results obtained using polystyrene latex particles provide the insight at a relatively higher flow rate (50 μl/min) than literature studies that fine particles with 2% concentration can migrate in the pore throat without bridge or clogging at the various pore throat and fine particle size ratios (o/d = 2.6∼36.4). Furthermore, silica presents higher critical clogging concentration (0.5% in brine) compared with kaolinite (0.2% in brine) when the pore-throat width equal to 60 μm due to the larger pore throat and fine particle size ratio. On the other hand, the findings show that clogging easily occurred at a lower pore-throat to fine particle size ratio even with a few number of fine particles. In addition, pore-fluid type directly influences the tendency of fine's to form clusters which in turn impacts the clogging behavior. For instance, silica fines clogging easier occurs in brine solution compare within deionized water due to larger cluster size in brine, while kaolinite shows an opposite result which means the kaolinite has higher clogging possibility in deionized water compared within brine solution. On the other hand, findings of multi-phase flow experiments show that fine particles accumulate along the liquid-gas interface and migrate together, which in turn cause bridging or clogging to occur easily in pores. These observations imply that a multi-phase flow during gas production could easily form clogging in pores, in which the flow permeability of porous media decreases even though clogging has not occurred in the same conditions with a single-phase flow. Thus, the permeability of porous media in engineering applications should be estimated by considering relatively easy clogging in pores in a multiphase flow compared to a single-phase flow. Findings of this research show the vital impact of pore-fluids and fluid-fluid interphase on fine particles migration and clogging in porous media. It provides a better understanding of the fines migration and clogging mechanisms. In addition, the results indicate the need to understand the types of fines and fluids in reservoir before evaluating if there will be a clogging potential during gas production from hydrates bearing sediments.

Effects of pre-shearing stress ratio on the mechanical behaviours of gap-graded soils subjected to internal erosion

Formation damage in the vicinity of the wellbore is a major cause of low productivity1–6 and the difficulties attendant to restoration of the damage leave a significant number of wells that do not respond to remedial treatment.1 Operators have recognized the importance of damage control during drilling, workover and other operations and have used filtration processes to reduce the damage caused by fluids invading the formation. Until recently, however, little could be done to control permeability damage caused by the movement of fine particles native to the formation of interest. Prevention of the migration of fine particles (defined) as those having a diameter less than 44 micrometers and which will pass through the openings of a 325 U.S. mesh screen7) is a complex problem. Certain cationic organic polymers (COP's) have been developed to control the swelling of water-expandable clays such smectite in the presence of low salinity fluids.8–14 While damaging in itself, the major detrimental aspect of smectite swelling is the associated release of fine particles.8 This arises because, when present, the particles. This arises because, when present, the smectite clay frequently acts as the cementation medium holding sand grains and fine particles in place.7,8

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.

Keywords: Ephemeral gully erosion Erodibility Internal erosion Landslides Pipeflow Soil pipes. Abstract. Many field observations have led to speculation on the role of piping in embankment failures, landslides, and gully erosion. However, there has not been a consensus on the subsurface flow and erosion processes involved, and inconsistent use of terms have exacerbated the problem. One such piping process that has been the focus in numerous field observations, but with very limited mechanistic experimental work, is flow through a discrete macropore or soil pipe. Questions exist as to the conditions under which preferential flow through soil pipes results in internal erosion, stabilizes hillslopes by acting as drains, destabilizes hillslopes via pore-pressure buildups, and results in gully formation or reformation of filled-in ephemeral gullies. The objectives of this article are to review discrepancies in terminology in order to represent the piping processes better, to highlight past experimental work on the specific processes of soil pipeflow and internal erosion, and to assess the state-of-the-art modeling of pipeflow and internal erosion. The studies reviewed include those that examined the process of slope stability as affected by the clogging of soil pipes, the process of gullies forming due to mass failures caused by flow into discontinuous soil pipes, and the process of gully initiation by tunnel collapse due to pipes enlarging by internal erosion. In some of these studies, the soil pipes were simulated with perforated tubes placed in the soil, while in others the soil pipes were formed from the soil itself. Analytical solutions of the excess shear stress equation have been applied to experimental data of internal erosion of soil pipes to calculate critical shear stress and erodibility properties of soils. The most common numerical models for pipeflow have been based on Richards’ equation, with the soil pipe treated as a highly conductive porous medium instead of a void. Incorporating internal erosion into such models has proven complicated due to enlargement of the pipe with time, turbulent flow, and episodic clogging of soil pipes. These studies and modeling approaches are described, and gaps in our understanding of pipeflow and internal erosion processes and our ability to model these processes are identified, along with recommendations for future research.