Internal Erosion Processes Research Articles

The influence of fines content on ground collapse due to internal erosion of sand-fines mixtures around defective pipes

Ground collapse caused by defective pipelines has become a global issue with the continuous advancement of urbanization. This study reproduced the internal erosion process of sand-fines mixtures around defective pipes through a series of model tests considering a relatively wide range of fines content and three hydraulic conditions. The erosion characteristics and the evolution of the cavity were analyzed to determine the triggering mechanisms of ground collapse. The results indicate that the formation and evolution of the cavity predominantly depend on fines content under hydraulic loading, which is replaced by ground settlement at lower fines content. Moreover, the scouring of groundwater amplifies the impact of internal erosion, which means the marked ground settlement at lower fines content or the expansion of the cavity even to ground collapse at higher fines content. To prevent sudden ground collapse, it is not recommended to use sand-fines mixtures with a fines content of more than 10% as embedment materials.

Internal erosion process and its influence factors in widely graded loose soils due to rainfall infiltration

Internal erosion process and its influence factors in widely graded loose soils due to rainfall infiltration

Internal erosion in granular soils with different microstructures under cyclically increased hydraulic gradients

Internal erosion is one of the leading causes of failures and accidents of embankment dams, dikes, and slopes. The hydraulic loading acts as the driving force to detach the soil particles, while the initial soil microstructure determines the susceptibility of particle loss. In engineering practices, the soil may be subjected to cyclic hydraulic loadings due to water level fluctuations by extreme weather of intensive rainfall and drought. Under such conditions, the soil internal erosion process will be different from that under the steady unidirectional seepage, which has not been fully studied and needs urgent investigation. Thus, in this study, the internal erosion process and hydraulic characteristics of soils with different microstructures were investigated by both laboratory seepage tests and the discrete element method (DEM) simulation. The results indicate that the soil with a higher fine content was more structurally stable, and required a larger hydraulic gradient for erosion initiation. Once the internal erosion occurred, the particle loss and the soil hydraulic conductivity increased with increasing fine content. Additionally, when the applied hydraulic gradient was essentially the same, the soil experienced a server erosion under the cyclically than monotonically increased hydraulic gradients, and the amount of eroded soils increased with the increasing gradient amplitude. The results of this study will expand our understanding of the physical mechanism and hydraulic behaviors of soils subjected to cyclically increased hydraulic gradients and with different microstrures.

Investigation on the failure mechanism of the internal erosion in inverse grading sand based on DDA and Darcy’s law

Inverse grading structures are commonly found in landslide accumulations of high-speed and long-runout landslides. However, there is limited research on the internal erosion of inverse grading deposits. This study developed a program called discontinuous deformation analysis (DDA) and Darcy’s law coupling program (DDCP) using C language to investigate the failure mechanism associated with internal erosion in inverse grading sand. The results indicate that as the hydraulic gradient increases, the internal erosion phenomenon becomes more pronounced. The failure of the inverse grading sand due to internal erosion can be attributed to three processes: loss of fine particles in the lower layer, accumulation of the particles from the upper layer in the lower layer, and flow of fine particles from the upper layer through the lower layer and exit from the bottom. During the internal erosion process, both the hydraulic conductivity and porosity initially decrease and then increase. Similarly, the average velocity of fine particles exceeds that of coarse particles in each layer. The stability of the inverse grading sand during the internal erosion increases with a higher content of fine particles in the lower layer. These findings are significant in understanding the mechanism of internal erosion within inverse grading deposits.

A Sand Boil Database for Piping Risk Management in the Po River, Italy

Sand boil formation at the toe of river embankments is the typical manifestation of the initiation and progression of an internal erosion process known as backward erosion piping, which is recognized to be a major concern in many river systems worldwide. In Italy, more than 130 sand boils have been detected along the Po River, many of them experiencing recurrent reactivations during high-water events. In recent years, as part of the activities of the European project LIFE SandBoil, the Italian authority responsible for flood protection and flood damage reduction along the Po River has implemented a GIS-based web application to catalogue the sand boils observed in its operating area. The resulting database allows keeping records of a comprehensive and varied set of information, in terms of predisposing factors, initiation conditions and surface effects. Taking as a reference a well-documented cross-section of the Po River prone to piping, this paper describes the main features of this versatile and flexible tool, whose long-term aim is to support vulnerability studies and the development of risk maps against piping. The database, which might also accommodate data from different river basins, is thus meant to help in flood risk management, by suggesting priorities for the implementation of mitigation measures and allowing the monitoring of intervention effectiveness.

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

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 pre-shearing 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.

On Internal Erosion of the Pervious Foundation of Flood Protection Dikes

This work focuses on the mechanisms that trigger internal erosion of the pervious foundation of flood protection dikes. The origin of these permeable layers is generally attributed to the presence of a paleo-valley and paleo-channels filled with gravelly-sandy sediments beneath the river bed and dikes. These layers may extend into the protected area. Visual observations of leaks, sand boils and sinkholes in the protected area testify to internal erosion processes in the underground soil. Local geological conditions are part of the information to be sought to explain these processes: presence of permeable soils and position of interfaces. Results obtained on Agly dikes (France), using two classical geophysical methods (EMI and ERT), were analyzed using cored soils and showed that it is not enough to simply conclude to the presence of backward erosion piping. The possibility of internal erosion, such as suffusion or contact erosion, must also be considered as the cause of leaks, sand boils and sinkholes. As the results obtained are explained by the presence of a paleo-valley and paleo-channels beneath the river bed and dikes—commonly encountered in this context—the methodology presented and the results obtained are likely to be relevant for many dikes.

Gully internal erosion triggered by a prolonged heavy rainfall event in the tableland region of China's Loess Plateau

Gully erosion is a severe form of soil erosion, but gully internal erosion processes are poorly understood, especially at the event scale. To investigate gully internal erosion intensity and understand the related gully development mechanism in an agricultural environment with gully head stabilization and vegetation restoration efforts, two successive field investigations were carried out just before and after a prolonged rainfall event in 2021 in the tableland region of China's Loess Plateau. Thirteen gullies were investigated and all experienced gully internal erosion, while most gully boundaries were stable during the heavy rainfall event based on the comparison of the UAV digital orthograph maps (DOMs acquired with Unmanned Aerial Vehicle) before and after the rainfall event. The proportion of gully internal erosion area to gully internal area of the 13 investigated gullies ranged from 3 to 55%, with average areal erosion proportion of the gully sidewall and gully bed of 21% and 36%, respectively. The erosion area of subdrainage units (SDUs) on the gully sidewall was positively correlated to the SDU area, average SDU slope gradient and vegetation type, while the erosion area on the gully bed was positively correlated to the gully area, gully depth and gully bed slope gradient. Gully internal erosion was not significantly correlated with gully drainage area because the connectivity between the upslope and gully areas was interrupted and the effective drainage area of the gully was obviously reduced by soil erosion conservation measures, including terraces on the upslope drainage area, shrub belts, and water barriers. Thus, gully internal erosion is still active under the heavy rainfall storm against the background of the ‘Grain for Green’ and ‘Gully Stabilization and Tableland Protection’ programs, and integrated measures for preventing both gully expansion and gully internal erosion must be further enhanced in the context of climate change.

Application of electrical resistivity tomography (ERT) in uranium mining earth dam

Abstract Dams are built to contain water or mining tailings and provide several benefits to society. The safety and stability of these structures are essential since eventual accidents can generate catastrophic damage. The main cause of ruptures is associated with abnormal seepage, which can progress to an internal erosion process. To assess the stability and safety of dams, periodic inspections are carried out, which typically include visual inspections and geotechnical instrumentation. Some geophysical methods have the potential to assist conventional geotechnical instrumentation using non-destructive and rapid acquisition investigations. The present study applied a finite element forward modeling to evaluate the percolation conditions and water stability in an earth dam of a decommissioned uranium mine in Brazil. The dam is the final structure of the contention system of mine water treatment. The electrical resistivity data were acquired using electrical resistivity tomography (ERT) and Schlumberger array. The results were presented from the inversion of data in 3D models. The data were processed by open-source software (modeling and inversion of geophysical data). The results indicate the presence of water in the vertical filter located in the crest area. The different levels of investigation indicated water infiltration in the rock mass of the dam foundation. These results are supported by structural surveys carried out in the area that indicate a fracturing pattern in the rock mass of the mine area. The results of the study were satisfactory for the desired purposes and demonstrate that geophysical methods constitute an important complementary tool for the geotechnical monitoring dams.

A grading method for analyzing internal erosion processes of nano-silica improved sand

Internal erosion (IE) often occurs in poor graded sand, one of the traditional treatments is reducing the permeability by grouting. Today, nano-silica becomes a choice of grouting materials as its low viscosity and good penetration capacity. According to present literature, the effect of decreasing loss mass during IE after the improvement of nano-silica was rarely studied. One of the important reasons is that, mass loss during IE was previously focused and was weighed after filtering the effluent by electronic balance, more accurate weighing method should be studied after adding nano-silica which cannot be precisely weighed by electronic balance. In this paper, a new grading method is conducted to monitoring the particle size distribution in the effluent and illustrate the process of IE. Erosion time and permeability are also recorded and analyzed as comparison. The experimental results show that the grading method can monitor precisely the mass loss and the composition of the effluent, grading range of 1-1000 can be adopted to monitor the coagulation of silica gel particles (1-50 , average diameter 11±5 ) and fine sand particles (50-100 , average diameter 65±7 ), grading range of 1-1000 can be adopted to monitor the smaller coagulation of silica gel particles (concentrated in the range of 1-250 nm). Through grading method, the IE of nano-silica improved sand can be divided into three stages: Removal and release of unbonded nano-silica particles and unbonded fine particles; Movement and discharge of bonded particles; Expansion of pores and instability of the whole sample skeleton.

Coupled CFD–DEM Investigation of Erosion Accompanied by Clogging Mechanism under Different Hydraulic Gradients

Soil internal erosion refers to any process that the finer grains are eroded from the matrix of coarser grains due to seepage. A coupled computational fluid dynamics and discrete element method (CFD–DEM) analysis of internal erosion is conducted on bimodal soils. The coupled influences of hydraulic gradient i and fine content Fc on different domination process of internal erosion is investigated. It is found that the erosion degree does not always increase as i increases for “underfilled” soil (e.g. Fc = 10 % and Fc = 20 %), there is a hydraulic gradient threshold ith above which local particle clogging plays a dominant role, resulting in a smaller erosion degree instead, the ith is larger for larger Fc, when Fc is equal to 10, the ith is 2 or less, while ith is 4 for the soil with a fine content of 20 %. For “transitional” soil (e.g. Fc = 30 %), no obvious clogging domination under larger i is observed. The mechanism is revealed through both macro- and microquantity perspectives. It is found that local clogging is formed by particle bridge formation under larger i values. These findings are highly significant for further understanding the influence of i on the erosion process and establishing the erosion law.

Geophysical monitoring of a laboratory‐scale internal erosion experiment

ABSTRACTEarth dams are structures used worldwide for water management. Their failure over time is notably due to water seepage generating internal erosion. There is a growing need to detect the processes at work as early as possible. This study presents a controlled laboratory experiment aimed at detecting and monitoring water seepage into a soil sample. The experiment was monitored with electrical resistivity tomography, velocimeters and video recording. The video recording of the downstream side of the soil sample shows successive episodes of mass movements associated with a progressive water flow increase. The electrical resistivity tomography, limited by a low temporal resolution, shows an evolution of the resistivity in agreement with the evolution of the soil sample (e.g., saturation and mass movements), but with strong limitations regarding the robustness of the results. The continuous seismic recording reveals extra rupture episodes that occur inside the volume of the soil sample, which were not recorded by the video. Their distribution in time and energy illustrates strongly nonlinear changes in the soil sample, with several phases of acceleration. A controlled source monitoring using external repetitive events allows probing the medium with an enhanced temporal resolution compared to electrical resistivity tomography. The apparent seismic velocity of the soil sample reveals a nonlinear decrease, high at the beginning of the experiment, and then stalled until the different mass movements enlarge the amount of water inside the sample along with the water flow. The different techniques used, especially seismic monitoring, describe a complex and strongly nonlinear process of internal erosion centred around the coupling between water flow and internal damage. Finally, these findings suggest that seismic methods could be able to distinguish the four different phases of internal erosion (namely, initiation, continuation, progression and failure) discussed in the geotechnical literature.

Influence of soil and hydraulic conditions on the processes of internal erosion, cavity formation and collapse behind coastal structures

Collapse of cavities formed by internal erosion has occurred at the back sides of seawalls and quaywalls all over the world. Here, the process of internal erosion was investigated by conducting prototype-scale experiments. A series of processes of cavity formation and collapse were reproduced considering different grain size distributions and water level conditions. Suction, that is, negative pore water pressure relative to atmospheric air pressure, affected the formation of cavities and the timing and scale of their collapse. The scale of the collapse was large under high suction. The types of hydraulic force also affected the cavity formation. A symmetrical cavity was formed under one-dimensional water level fluctuations, whereas the cavity expanded in size toward the wave-incident boundary under incident waves. Cavity collapse was found to be caused by a decrease in suction accompanied by a rise in groundwater level. The collapse occurred within the area where ground surface displacement was observed, and the maximum ground surface displacement (dmax) was correlated with the ratio of the maximum width of the cavity and its depth (B/L) regardless of the ground height and water level fluctuation range. Countermeasure against the internal erosion using a two-layer filter was systematically studied in a series of sand filtration tests and prototype-scale hydraulic experiments. The filter maintained a sufficient deterrent effect under various hydraulic and dynamic forcing when the uniformity coefficient of the filter (DF60/DF10) was higher than 3.0, and the median particle diameter ratio (DF50/DS50) was less than or equal to 20.

Internal Erosion of a 0–5 mm Crushed Sand in a Rigid Wall-Permeameter: Experimental Methods and Results

The article deals with internal erosion in 0–5 mm crushed sand, a material used in urban facilities. Several methods, designed for natural materials, predicted a risk of internal erosion for the crushed sand. The real risk was evaluated in clear-wall, watertight and airtight, rigid-wall permeameters with lateral piezometers. Each test started with a fully saturated specimen, verified by a mass-and-volume method. In their initial condition, the specimens were homogeneous, as directly confirmed by three methods. Each erosion test included successive seepage steps at a constant mean gradient. Migration of fine particles was observed through the clear wall. Dyed water colored three or four preferential seepage paths inside the specimen, not along the wall, confirming that the preparation method was correct. The predictive methods correctly forecast the size of mobile particles but overestimated the gradient that triggered the internal erosion process. The tests were the first ones to include nonreactive tracer tests, which gave the effective porosity value at different gradient steps. This value decreased with internal erosion, a novel result, which quantified the fact that more seepage concentrated in preferential seepage paths. Nonreactive tracer tests can thus be used to quantify the formation of preferential seepage paths.

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

Loose wide-grading soils are commonly found in the source areas of debris flows, and in landslides after an earthquake. During rainfall events, fine particles (fines) in the soils gradually migrate downward, and eventually the loss of fines results in an increase in the pore volume of the soil and a reduction in the stability of the soil skeleton, which can lead to subsequent slope failure. To gain more understanding of the fine migration process at the microscopic scale, a 3D discrete element-fluid flow sequentially coupled model is developed, based on Darcy’s Law, to simulate fluid flow through a porous medium and calculate the transportation of soil solids. The erosion model is verified using experimental data. Parametric studies are carried out to investigate the effects of coarse particle size. The results reveal that changes in pore structure caused by fine particle migration can change the local permeability of the material. For the case of the average pore throat diameter to fine particle ratio ( $$J$$ ) of 2.41, changes in local porosity with time from internal erosion in the sample can be divided into four stages: (1) a rapid increase with some variations in porosity, (2) a slow increase in porosity, (3) a rapid increase in porosity, and (4) a steady state with no change in porosity. Not all stages are present for all value of $$J$$ . Stages (1) (2) (4) are present for 2.48 ≤ $$J\le$$ 2.58 and stages (1) (4) are present for $$J$$ ≤ 2.24 and $$J\hspace{0.17em}$$ ≥ 2.74. A sharp increase in the fine’s erosion possibility occurs for a $$J$$ value lies between 2.58 and 2.74. The erosion possibility sensibility shows an exponential relationship with $$J$$ . The model provides an effective and efficient way to investigate the process of pore blockage and internal soil erosion.

Investigating the relationship between erosion-induced structural damage and lining displacement parameters in shield tunnelling

This study investigated the relationship between erosion-induced structural damage and lining displacement parameters for shield tunnelling using a finite element approach, which properly simulated the interaction between the eroded soil and tunnel structure as well as the details of the segmental liners. The interaction between the bolt rod and reserved bolt hole was simulated by a surface-surface contact. The lining concrete and bolt materials were simulated by plastic constitutive models. The internal erosion process was qualitatively modelled by a strength-and-stiffness reduction method with a progressive flow boundary on the tunnel surface. The structural damage mechanism and its relationship with the lining displacement parameters were then clarified by conducting a series of parametric studies. It was found that the damage to the lining concrete and the number of yielded circumferential bolts could be properly related to the maximum and average vertical convergence rates of the liners, respectively, while the maximum internal force of the longitudinal bolts could be identified by the axial deflection angle of adjacent liners.

A transient backward erosion piping model based on laminar flow transport equations

Backward erosion piping is an internal erosion process, which compromises the stability of water retaining structures such as dams and levees. In this paper, we propose a numerical solution that combines a 2D Darcy groundwater solution with Exner’s 1D sediment transport mass conservation equation. As an estimate of sediment transport, we tested four different empirical transport equations for laminar flow. The model performance was evaluated based on the results of the real-scale IJkdijk experiment. Through this, we were able to demonstrate the applicability of existing sediment transport equations to the description of particle motion during piping erosion. The proposed transient piping model not only predicts the pipe progression in time, it also allows for an identification of pore pressure transitions due to the erosion process. A major conclusion of the study is that from the four different modeling approaches for laminar flow, it is recommended to follow the approach of Yalin et al. regarding the simulation of backward erosion piping for dike configurations similar to those of the IJkdijk experiment.

Embankment Breach Research: Observed Internal Erosion Processes

HighlightsThis study provides data from internal erosion tests on four intermediate-scale homogeneous embankment dams.Soil properties influence the breach formation process and breach timing.Results showed that observed erosion rates of the internal flow path varied by several orders of magnitude.Quality control of embankment construction can greatly influence breach development.Abstract. Internal erosion and embankment overtopping are the two most common causes of embankment dam and levee failures and incidents. Internal erosion is the removal of soil material by the flow of water through a continuous defect, cavity, or crack within a compacted fill and/or its foundation. Internal erosion initiates from vulnerabilities within the embankment. The embankment soil material plays a key role in both the erosion process and rate of failure, but characterizing soil properties and how they relate to the rate of failure can be challenging. Soil properties such as texture, density, strength, moisture content, and erodibility can vary greatly; thus, it is important to study the effects of these properties on the breach formation process and breach timing. The USDA Agricultural Research Service performed internal erosion breach experiments on four intermediate-scale homogeneous earthen embankments constructed of soils ranging from a silty sand to a lean clay material. The embankments were constructed to a height of 1.3 m, a top width of 1.8 m, and upstream and downstream slopes of 3(H):1(V). The embankment materials were characterized by water content, density, texture, strength, and erodibility. Erodibility was measured using a jet erosion test (JET) apparatus. A 40 mm diameter, continuous steel pipe was placed through each embankment during construction and removed to form an open-ended void through the embankment connected to the upstream reservoir. The removal of the pipe initiated internal erosion. The objectives of the experiments were to observe the development of the internal erosion process over time and to examine the influence of soil properties on the erosion rate, breach timing, geometry of the breach opening, and breach outflow. The rate of erosion and failure observed in these tests varied by several orders of magnitude, with the silty sand embankment eroding most rapidly and the lean clay embankment with a mean moisture content of 18% dry basis at standard compaction eroding the slowest. These observations were indicative of the soil textures. Although the two lean clay embankments were constructed of similar soils, the difference in erosion rates speak to the importance of quality control (e.g., compaction moisture content) during construction. Soil properties including soil texture, erodibility, and compaction moisture content are key predictors of erosion rate and observed failure. Keywords: Breach, Dam failure, Dams, Embankments, Erodibility, Internal erosion, Levees, Overtopping.

DEM simulation of internal erosion around a submerged defective pipe

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 internal erosion process and effects of undisturbed loess due to water infiltration

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.