Internal Erosion Research Articles

ASSESSING LOCAL PORE WATER VELOCITY ALONG PREFERENTIAL FLOW IN EARTHEN DAM USING SALT TRACER

The local pore water velocity along the preferential flow path signifies the hydraulic parameter responsible for erosion within an earthen dam. This study introduces an empirical approach to ascertain the local pore water velocity within the earth dam's leakage zones by monitoring the travel time of the salt tracer through the corresponding electric potential anomalies in the ground. The alignment of electric potential anomalies with the movement of the salt tracer plume over time was confirmed through experiments on a physical model coupled with numerical simulations. The pore water velocity, calculated based on the location of the maximum electric potential anomaly, demonstrated excellent agreement with the experimental value, with an error of under 6%. For illustrative purposes, a field-scale salt tracer test was conducted at a leaking earthen dam in Vietnam. The tracer breakthrough curve originating from the leakage point revealed that the seepage water's travel time is approximately 40 days. The results of electric potential anomalies over time indicate that the pathway of seepage flow from upstream to the leakage point forms a horizontal V-shape, with the local pore water velocity ranging from 1.7 to 9.9x10-5 m/s. These local pore water velocities are subsequently compared with the critical seepage velocity to assess in-situ information regarding the internal erosion status of the target dam.

Monitoring of Älvkarleby test embankment dam using 3D electrical resistivity tomography for detection of internal defects

Electrical resistivity tomography (ERT) is a potential-based method for detecting internal erosion in the core of embankment dams using the electrodes installed outside. This study aims at evaluating the practical capability of ERT monitoring for detecting internal defects in embankment dams. A test embankment dam with in-built well-defined defects was built in Älvkarleby, Sweden, to assess different monitoring systems including ERT and the defect locations were unknown to the monitoring teams. Between 7500 and 14,000 ERT data points were acquired daily, which were used to create the distribution of electrical resistivity models of the dam using 3D time-lapse inversion. The inversion models revealed a layered resistivity structure in the core that might be related to variations in water content or unintentional variations in material properties. Several anomalous zones that were not associated with the defects were detected, which might be caused by unintentional variations in material properties, temperature, water content, or other installations. The results located two out of five defects in the core, horizontal and vertical crushed rock zones, with a slight location shift for the horizontal zone. The concrete block defect in the core was indicated, although not as distinctly and with a lateral shift. The two remaining defects in the core, a crushed rock zone at the abutment and a wooden block and a crushed rock zone in the filter, were not discovered. The results cannot be used to fully evaluate the capability of ERT in detecting internal erosion under typical Swedish conditions due to limited seepage associated with the defects. Furthermore, scale effects need to be considered for larger dams.

Mitigating Bend Pipe Erosion with the Incorporation of Twisted Tape Inserts

Mitigating Bend Pipe Erosion with the Incorporation of Twisted Tape Inserts

Experimental analysis and model prediction of elbow pipe's erosion in water-cooled radiator

The radiator with heat transfer capability is able to guarantee the stable operation of hydro generator set, while the long-term and continuous scouring on radiator pipes by cooling medium will lead to thinning or even perforation of pipe wall, which triggers wall failure. This paper analyzes and predicts the failure mechanism of radiator’s pipe wall, and investigates the effects of water flow velocity, sand content and sand particle size on erosion damage of radiator pipe by establishing a test bench for pipe erosion. The results show that the increase of above parameters will lead to the increasing erosion rate, especially when the sand content is 1%, the velocity is 8 m/s and the sand particle size is 0.85 mm, the erosion damage will be particularly serious. Based on experimental data, BP and LSSVM models are employed to predict the pipe wall failure, and PSO algorithm is used to optimize the two models. The optimized PSO-BP has the highest accuracy with the mean absolute error (MAE) of 0.2070 and the mean absolute percentage error (MAPE) of 4.702%. The findings provide a reference for wall failure analysis of radiator, which is of great significance for unit's safe operation.

Erosion Analysis and Optimal Design of Sand Resistant Pipe Fittings

Erosion of solid particles in a pipe elbow containing a 90° angle is investigated by simulation methods. In the process of shale gas exploitation, the impact of solid particles carried by fluid on the inner surface wall of pipes, as well as the turbulent flow, cause the erosion of pipes, which brings about heavy economic losses for the oil and gas industry. In the impact erosion of the inner surface wall of the pipe, the worst erosion occurs at the elbow. In this study, the erosion of a pipe elbow which has been widely used in actual production is analyzed, and the influence of the fluid velocity, the solid particle size, and the wall roughness on the erosion is investigated. Additionally, the simulation results of the erosion with the rebound and freeze boundary conditions are compared, indicating that setting the freeze boundary condition could significantly improve the computational efficiency by 74% with the acceptable accuracy. In order to reduce the impact erosion in the pipe elbow containing a 90° angle, an optimal design is proposed that can reduce the maximum erosion rate by 52.4%. These results complement the research of elbow erosion and provide ideas for the optimization problem of a pipe elbow containing a 90° angle.

Quantitative Evaluation of the Onset and Evolution for the Non‐Darcy Behavior of the Partially Filled Rough Fracture

AbstractThe non‐Darcy flow behavior in unfilled and fully filled rough fractures has been investigated thoroughly for decades. Natural fractures usually be partially filled with porous media due to long‐term internal and external dynamic disturbances and water flow erosion. However, how to evaluate the nonlinear seepage characteristics of the partially filled rough fracture (PFRF) is never scrutinized. In the present study, 2D direct numerical simulations are conducted to investigate the non‐Darcy behavior for flow through 6 PFRF models under different filling conditions (i.e., filling degree, roughness, and hydraulic conductivity capability of the filling medium), which included coupling between the free and seepage flow in the unfilled and filled region, respectively. The results show that the critical non‐Darcy factor E = 0.02 can be used to judge the onset of the non‐Darcy regime in PFRF, which is also the critical point of eddy region (ER) volume mutation. The higher the filling degree and the weaker the hydraulic conductivity of the filled medium, the more significant the non‐Darcy behavior. Furthermore, the variation of ER volume can be divided into three stages with the increasing inertia effect, that is, stable stage (E < 0.02), slow nonlinear growth stage (0.02 ≤ E ≤ 0.1), and rapid linear growth stage (E > 0.1). In addition, a new model for quantitatively evaluating the non‐Darcy behavior of PFRF is developed via gene expression programming based on 735 simulation data set, which is further employed to establish the relationship between the friction factor.

Loess susceptibility to erosion: Interaction of cohesion sources, air slaking and confinement

AbstractErosion in loess is a widespread phenomenon, as loess covers about 10% of the Earth's land surface. While erosion of loess soil has been intensively studied, the mechanisms controlling erosion in gullies and pipes in loess are poorly understood. Cohesion plays an important role in erosion in loess. Interactions between erosional processes and stabilizing mechanisms in loess are poorly understood. This study focuses on the interaction between air slaking and stabilization by confinement to improve leaching techniques for identifying cohesion sources in undisturbed loess from four sites in Czechia. The decrease in tensile strength of samples after leaching in distilled water, dithionate, HCl, and hydrogen peroxide was used to selectively remove cohesion sources such as Fe‐Al oxides and hydroxides, carbonates, and organic matter. Experiments showed that confinement and overburden stress are important but neglected stabilizing mechanisms in loess. Leaching of unconfined loess samples gave misleading results because of ubiquitous air slaking. To reliably identify cohesion sources, samples confined in compacted sand were used. Leaching of confined samples showed that Fe‐Al oxides and hydroxides are major sources of cohesion (60–90%), while carbonates and organic matter are of minor importance (0–30%). To avoid misleading results, examination of loess structure after leaching is critical to identify samples with damaged structure due to enhanced air slaking caused by bubbles generated during leaching. Air slaking is a powerful and rapid damage mechanism, but it occurs only in dry or semi‐dry loess near the soil surface. In contrast, chemical weathering is able to remove cohesion sources in deeper parts of the loess profile.

Insights into the cause of the Oroville dam spillway failure, 2017, California.

Earth internal seepage erosion in weathered bedrock under infrequently used hydraulic structures is often overlooked, which causes some solid particles to break away from the solid skeleton, degrading the earth's strength, and even causing unanticipated hydraulic engineering failures. The flood on the Oroville dam spillway in California in 2017 was caused by disturbed water flow due to a crack in the spillway chute caused by internal erosion in poorly weathered bedrock. The abnormal water flow of the spillway in the early stage and subsequent investigation revealed that the main reason for the accident was the insufficient weathered bedrock under the spillway chute. In this study, we formulated a coupled hydro-mechanical mechanism for internal erosion in weathered bedrock during the early stages. Using this model, we conducted an internal erosion numerical simulation at early stage, and the results showed that the physical characteristics of the weathered bedrock were degraded. Our results show the coupling analysis of quantitative computation during the early stage of internal erosion in weathered bedrock, which can provide an early warning method for the occurrence of internal erosion to avoid hydraulic disasters.

Wall and pillar drains in puddle clay embankment dams

Puddle clay core embankment dams were built prior to the use of rolled clay core dams and comprise a large proportion of UK dams, with the dams relying for their watertightness on the addition of water to (puddle) the clay until it has an undrained shear strength of around 10 kPa, and thus has increased ability to deform without cracking. Such dams sometimes had early forms of unfiltered drains, comprising coarse cobbles, in the downstream shoulder, either to assist construction and/or to lower the phreatic surface and improve the slope stability once the reservoir is in operation. These drains are separate from foundation drains dealing with springs encountered in the foundation excavation and drains to deal with surface runoff. This paper gives examples of such drains in dams constructed between 1863 and 1940. These drains were installed prior to the development of classic filter design rules and tools to analyse seepage. This is important as the drains have no filter, and are thus vulnerable to internal erosion which, if not detected, could lead to failure of the dam. These drains may also be the true root cause of historic incidents at these dams, due to embankment fill materials washing into the drain and creating sinkholes.

Internal erosion of a gap-graded soil and influences on the critical state

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.

Implications of bioturbation induced by Procambarus clarkii on seepage processes in channel levees

River levees are subject to bioturbation by various animals which can actively excavate into earthen structures producing an internal erosion that, during the passage of a flood, can grow in time making the levee unstable. This phenomenon can lead to river levee breaching and, as a consequence, collapse, even for relatively minor flood events. A well-known animal burrower is represented by the North American crayfish Procambarus clarkii (P. clarkii), an invasive species in Europe, mainly introduced for commercial purposes, causing a decline in biodiversity and profound habitat changes. The physical damages caused by P. clarkii on levees and banks, such as in rice fields, irrigation ditches, and small channels, have not been fully studied and behavioral components underlying this impact are mostly occasional. To understand the impact of burrowing activity on the seepage process, a field survey was done in a drainage channel in Tuscany, Italy, to evaluate the density and geometry of the internal burrows that were excavated by the crayfish. Based on these observations and some previous laboratory experiments, three dimensional (3D) numerical simulations of the seepage processes were done inside burrowed levees. Numerical results allowed the increase in the hydraulic vulnerability of levees to the process of internal seepage to be disclosed. In particular, for a given river water level, the reduction of the time scale for the phreatic line to reach the levee field side appears to be a function of a quantity here defined as the burrow hydraulic gradient. This quantity is here defined as the ratio between the hydraulic head inside the burrow and the horizontal distance from its end to the field side of the levee. Moreover, a comparison between the 3D with the analogous more common two dimensional (2D) numerical simulations illustrated the schematization which is better suited for describing the seepage processes when animal burrows, not only by crayfish, are present.

The effects of internal erosion on granular soils used in transport embankments

The flooding of embankments used for rail and other infrastructure has the potential to cause lasting weakening of slopes via the movement of fine particles induced by seepage. In laboratory experiments, internal erosion was induced in granular soil samples, with properties consistent with those used to construct transportation embankments, to assess how particle migration through, and out of, samples caused shear wave velocity, strength, stiffness and permeability changes. Shear wave velocity changes, measured using horizontal bender elements, of up to 19 % were observed following fine particle removal of up to 1 % of initial sample mass. Shear wave velocity change was found to be an indicator for identifying the development of permeability change during seepage-induced particle migration. Median measured permeability changes were +5 % and −34 % for samples containing 15 % and 30 % fines, respectively. The largest directly observed permeability and shear wave velocity changes occurred during the initial stages of seepage. Negative correlation was observed between mass of material removed from samples and peak friction angle. Following seepage, soils displayed a dual stiffness behaviour. Stiffness and strength changes were attributed to redistribution of fine particles and opening of pore spaces. Our results have implications for the monitoring of earthworks affected by flooding and seepage as the associated redistribution of fine particles may lead to large changes in slope properties.

Sinkhole stability chart for geotechnical investigation

Sinkholes are a common geohazard in karst areas where soluble carbonate bedrock is present. Active groundwater flow causes internal soil erosion that creates cavities in the overburden soil layer. The subsurface cavity propagates upward due to the continuous erosion process, and ultimately, they reach to the ground surface, resulting in a sudden collapse. This cover-collapse type is generally a sudden event, particularly in the area where a thick cohesive soil layer is embedded. In this study, a sinkhole stability chart was developed to quantitatively evaluate the safety against the collapse-type sinkhole. Finite-element (FE) modeling was used to simulate the collapse of sinkhole. Field testing data collected from multiple sinkhole sites were used as input to the FE model. To simulate the progress of sinkhole activity, a number of different scenarios with varied size of cavity, strength of overburden soil were simulated, and the corresponding safety factors were determined. Even though each site has varied soil profile, a general trend of the safety factor was observed. The envelopes of safety factors (or equal-safety lines) were statistically determined by using a multivariate regression method. The chart that quantitatively evaluate the stability of sinkhole is used as a decision-making tool for further engineering investigation or mitigation.

A 3D time-dependent backward erosion piping model

Backward erosion piping (BEP) is a failure mechanism of hydraulic structures like dams and levees on cohesionless foundations subjected to seepage flows. This article models the time-dependent development of BEP using numerical simulation of the erosion process. A 3-dimensional finite element equilibrium BEP model is extended with a formulation for the sediment transport rate. The model is compared to and calibrated with small- and large-scale experiments. Finally, a large set of simulations is analyzed to study the effects of factors such as grain size, scale (seepage length) and overloading on the rate of pipe progression. The results show that the development of BEP in the small-scale experiments is predicted well. Challenges remain for the prediction at larger scales, as calibration and validation is hard due to limited large-scale experiments with sufficiently accurate measurements. The results show that the progression rate increases with grain size and degree of overloading and decreases with seepage length, which is consistent with experimental observations. The model results provide a better physical basis for incorporating time-dependent development in the risk assessment and design of levees.

Prediction of Internal Erosion Parameters of Clay Soils Using Initial Physical Properties

This paper aims to predict the internal erosion rate index and critical shear of soils based on the initial physical properties of soils. Regression statistical analyses were employed on sixteen types of clayey soils prepared at different initial dry densities and water contents. The Hole Erosion test was conducted to determine the internal erosion parameters: the erosion rate index and the critical shear. Another set of specimens with the same initial dry unit weight and water content was remolded in the direct shear box and tested using the direct shear test to determine the shear strength parameters (i.e., the cohesion and the angle of internal friction). The various physical properties of soil (initial dry unit weight, initial water content, plastic index, liquid limit, optimum water content, maximum dry density, cohesion, and angle of internal friction) were used to develop models that predict both the erosion rate index and the critical shear. The findings show that the initial physical properties can be used to predict the erosion rate index and the critical shear. The coefficient of determination (R2) was found to be between 0.83 and 0.92 to predict the erosion rate index and between 0.85 and 0.9 to predict the critical shear. The high R2 implies that the models can be used to rate the soil erodibility in advance based on simple laboratory testing instead of time-consuming tests. Additionally, the findings give varied options for prediction depending on the availability of the soil initial physical properties.

Experimental study of seepage in levee with transverse structure and water barrier

A quasi-real-scale experiment was conducted to understand seepage in levees with internal transverse structures. The experiment involved fabricating levees with transverse structures in the same location and using the same method as that in sand levee experiments previously conducted to examine the feasibility of the optical-fiber-distributed temperature system. Levees with internal structures are susceptible to internal erosion and piping risks, which necessitates the installation of water barriers to mitigate these issues. However, conventional concrete water barriers require curing time, leading to potential secondary damage from flooding during construction. As a countermeasure, a compounded polypropylene (PP) water barrier was developed, which is easy to construct and cost-effective. The performances of both concrete and compounded PP water barriers were verified and compared. Water pressure changes inside the levees were tracked, and to monitor temperature changes inside the levees and gain a deeper understanding of seepage, distributed optical fiber cables were employed. The experimental results revealed that levees with transverse structures had a higher risk of internal erosion and piping due to seepage compared to those without such structures, and such levees require continuous monitoring. Furthermore, internal temperature measurements demonstrated the effectiveness of water barriers in mitigating seepage and backward erosion piping.

Occurrence of sand boils landside of a river dike during flooding: A geophysical perspective

The study of internal erosion of earth dams, dikes, and levees is an essential ingredient to address their safety. Indeed, internal erosion is known to be a major cause of failure for dams and levees. Several sections of the Agly river dike (south-west France) are affected by the appearance of sand boils on the side of the protected zone. These phenomena generally occur without any significant erosion of the dike itself. In order to better understand the reasons for these phenomena, we took advantage of a drought (dry riverbed) to image, using induced polarization tomography, the subsurface from the riverbed to the plains extending inland behind the areas prone to these pathologies. Induced polarization imaging is a geophysical technique that extends the classical electrical resistivity tomography to include low-frequency polarization mechanisms. This approach shows that the river dike is partly built on a sand-filled paleochannel, allowing water to flow under the dike and its clay core during river floods. During floods, underground flow under the dike is responsible for sand boil phenomena on the side of the protected zone. Numerical hydraulic simulations show that the discharge zone has specific concentration points, particularly at the toe of the dike even when the permeability of the sand filling the paleochannel is homogeneous. These concentrated discharges are controlled by the water level in the river, the geometry of the dike and the shape of the clay-sand interface of the paleochannel. This study highlights the role of geophysical techniques, especially induced polarization, in providing key information to better understand erosion and fluidization phenomena affecting river dikes.

Impact of specimen preparation on erosion and post-erosion response of gap-graded soils

Internal erosion is a major threat to the safety and operation of water management structures. Recent failures of embankment dams reported in Australia, Canada, and the US are fresh reminders of the importance of this ongoing issue. Suffusion, the focus of this paper, is one of the main modes of internal erosion occurring in internally unstable soils. It initiates internally, continues slowly with no visible signs, and leads to complete failure if not detected early. Despite recent advances in understanding the influence of suffusion on the mechanical behaviour of internally unstable soils through laboratory investigations, the impact of sample preparation techniques on the soil response during erosion and afterwards is yet to be fully undrstood. When it comes to laboratory investigation of remoulded soils, it has been understood that soil specimens prepared by different techniques show different characteristics and soil fabrics. This research studied the influence of sample preparation methods on the erosion of non-plastic fine particles in internally unstable soils and their mechanical consequences. Soil samples with two gradations, on the borderline of stable and unstable and one fully unstable, were prepared using moist-tamping and wet pluviation techniques and were subjected to a two-hour downward seepage. The erosion progress and pre- and post-erosion drained mechanical behaviour of samples were investigated. Tests result showed a difference in the erodibility of fine particles for the sample on the borderline of internal stability prepared by different techniques, but similar mechanical behaviours for both gradations.

Levee monitoring: DFOS Applications for understanding Levee Seepage

Enhancing the understanding of the geotechnical features of existing levees through on-site examination and monitoring is essential to estimate their security and that of the adjacent environment. However, these tasks are difficult due to the extensive length of the levees and the significant variability of soil texture within their structure and foundation, particularly in mountainous areas or/and when paleo rivers are present. Traditional methods for levee monitoring involve visual inspections, borehole drilling, in-situ tests, and the deployment of devices like piezometers and tensiometers. Regrettably, these methods often grapple with limitations imposed by their constrained spatial resolution. The investigation here presented tries to overcome the limits of traditional techniques, specifically probing the application of distributed fiber optical sensors (DFOS) for gauging temperature variations. Under normal conditions, temperature changes in the soil are mainly influenced by seasonal heat transfer from the air. However, during flooding events, notable temperature fluctuations both in space and time can revealing preferred flow paths, due to the presence of more permeable soils or large voids related to undergoing internal erosion. Two case studies are here presented, both aimed to the monitoring of a segment of Adige River’s embankment in the province of Bolzano (Italy). The pilot tests differ for the DFOS installation configurations. Since the river encountered a significant flood event in Autumn 2023, the gathered data serve a dual purpose: advancing the comprehension of the hydraulic dynamics and the safety conditions of this segment of levee, but also appraising the reliability and potential of these cutting-edge monitoring methodologies.

Shake table tests on levees deteriorated by seepage-induced internal erosion in geotechnical centrifuge

Shake table tests on levees deteriorated by seepage-induced internal erosion in geotechnical centrifuge