Analysis of the pipe depth development in small-scale backward erosion piping experiments

Errors in finite element analysis of backward erosion piping

Backward erosion piping (BEP) is an internal erosion mechanism by which erosion channels progress upstream, typically through cohesionless or highly erodible foundation materials of dams and levees. As one of the primary causes of embankment failures, usually during high pool events, the probability of BEP-induced failure is commonly evaluated by the U.S. Army Corps of Engineers for existing dams and levees. In current practice, BEP failure probability is quantitatively assessed assuming steady state conditions with qualitative adjustments for temporal aspects of the process. In cases with short-term hydraulic loads, the progression rate of the erosion pipe may control the failure probability such that more quantitative treatment of the temporal development of erosion is necessary to arrive at meaningful probabilities of failure. This report builds upon the current state of the practice by investigating BEP progression rates through a series of laboratory experiments. BEP progression rates were measured for nine uniform sands in a series of 55 small-scale flume tests. Results indicate that the pipe progression rates are proportional to the seepage velocity and can be predicted using equations recently proposed in the literature.

With the utilization of underground space, backward erosion piping (BEP) has been observed in many underground structures (e.g., shield tunnels) founded on sandy aquifers. However, due to invisibility, the geometry of the eroded pipe and its spatial evolution with time during the piping process was still not clear. In this study, we developed a Hele-Shaw cell to visualize the dynamic progression of BEP. With imaging process technology, we obtained a typical process of BEP (the erosion process can be divided into a piping progression phase and a piping stabilization phase), quantitatively characterized the formation of erosion pipes, and compared the patterns of erosion (e.g., the erosion area A and the maximum erosion radius Rmax) that spontaneously develop under different fluxes of water. The most interesting finding is that the sand grains in a thicker Hele-Shaw model are easier to dislodge, which is possibly due to the granular system in a thicker model having more degrees of freedom, reducing the stability of the sand grains.

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.

Backward Erosion Piping (BEP) is a form of internal erosion widely recognized as a major contributor to dam and levee failures. This process occurs when seepage forces erode soil particles at an unprotected (unfiltered) outlet, initiating the formation of a pipe. If conditions allow, the pipe gradually progresses backward toward the seepage source, ultimately undermining the structural integrity of the overlying dam or levee. Over time, this backward progression undermines the stability of the structure and, if left unmitigated, can ultimately lead to failure. Extensive research has explored various aspects of BEP initiation and progression under steady-state conditions. However, since loading on levees and many dams is inherently transient, these structures are expected to experience multiple loading events throughout their lifespan. Such scenarios include river flooding events, tidal fluctuations, and storm surges, where water pressures and flow regimes are dynamic, affecting the progression of internal erosion. Understanding the effects of multiple loading events on BEP progression is important for assessing the long-term risk associated with BEP failure mechanisms. To investigate this phenomenon, specialized testing equipment was developed in this study to replicate various scenarios of converging flow at the base of a vertical defect in an overlying low-permeability blanket. The apparatus includes a cylindrical soil chamber measuring 0.91 m (3 ft) in diameter and 0.62 m (2 ft) in height, allowing seepage water to flow radially toward a riser in the centre of the chamber. This setup induces BEP into a central riser, where testing is conducted for different riser diameters. An array of precision pore-pressure sensors measures pore water pressures and captures changes in the flow regime caused by the progressing internal erosion. The results from the test are analysed using an inverse-analysis technique based on three-dimensional finite element analyses. This approach enables interpretation of the seepage regime that aligns with both the observed development of the pipe and the measured pore pressure data. This paper provides an overview of the experimental procedure and analysis of the results, offering insights into the complex mechanisms of BEP.

<p>Backward erosion piping (BEP) has been proven to be one of the main failure mechanisms of water-retaining structures worldwide. Dikes, which are often built on sandy aquifers, are particularly vulnerable to this special type of internal erosion. In this research, we propose a numerical solution that combines a 2D Darcy groundwater solution with Exner’s 1D sediment transport mass conservation equation. The inclusion of criteria for incipient particle motion, as well as the linkage of the bedload transport rate to the pipe progression, enables us to build a stable time-dependent piping model. 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 a real-scale dike failure 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. The main finding 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. (1963, 1979) to simulate backward erosion piping in dikes.</p>

Piping is a major failure mechanism in both dams and dikes. In this work, we develop a transparent specimen tank that allows a camera to clearly record the piping formation process. In addition, this apparatus is able to maintain a constant hydraulic head and to apply K0 stress states. The eroded soil and the discharged water are measured using a collection system. We conduct a series of laboratory tests to investigate the evolution process and mechanisms of piping in gap-graded soils under a K0 stress state. Our results show that the piping process can be divided into three stages: seepage, pipe formation, and pipe wall erosion. No visible soil erosion and deformation is observed in the seepage stage. In the pipe formation stage, the flow channel is formed gradually from the position where water is injected into the exits. In the pipe wall erosion stage, grains are eroded mainly from the pipe wall. Moreover, erosion is more profound on the convex side than on the concave side of the pipe. From the sensitivity analysis, a large percentage of coarse grains can hinder the formation of the flow channel is observed. Finally, the transition mechanism from suffusion to piping based on different testing conditions is revealed.

Backward erosion piping is a form of internal erosion that endangers the structural stability of levees and dams. Understanding the factors that influence this form of erosion can result in improved risk assessment and more appropriate modifications to new and existing structures. Historically, it has been assumed that the presence of silt size particles would reduce the gradient required for erosion. This study investigated the influence of fines content on backward erosion piping through a series of laboratory experiments on silty sands. Laboratory results show that as the fines content increased in the samples, so too did the gradient required to produce and progress piping to failure. The results indicate that a new factor is needed to properly account for silt content in backward erosion piping (BEP) risk assessment of silty sands.

Up to now, the phenomenon of backward erosion piping in uniform sands covered by a cohesive top layer has been predicted using a 2D approach. The Sellmeijer model (Sellmeijer 1988) includes the 2D-flow towards the pipe, Poiseuille flow in the infinitely wide pipe and equilibrium of grains on the pipe bottom to predict the critical head.Arecent series of experiments indicates that the original Sellmeijer model fails to predict the critical head correctly for coarser sand types, and an empirical correction has been proposed to correct for this (Sellmeijer et al. 2011).A theoretical solution is not presently forthcoming. In this study the pipes formed in small- and medium-scale experiments have been analysed for different sand types. It was found that the width of the pipe increases with increasing grain size. This indicates that a 3D-approach is required to correctly model the piping process and to solve the influence of grain size on critical head. 3D numerical simulations of the piping experiments provide insight in the pipe hydraulics and difficulties encountered upon modeling of piping

Backward erosion piping (BEP) has been recognised as a major cause of failures in water-retaining structures. However, the fundamental mechanisms controlling the phenomenon are not well understood. This research applies the theory of rate processes to develop a constitutive relationship between energy density of the seepage flow and the erosion rate of soils during the evolution of BEP. The resulting equation is used to analyse four datasets of previously reported experimental observations. The mechanical parameters estimated through the proposed model fall into the ranges of values that were reported in the literature. To validate the proposed approach, the constitutive model was incorporated into a multiphase numerical framework to simulate evolution of BEP in embankment soil and compared with reported experimental observations. The numerical framework with the proposed constitutive model is shown to be capable of reproducing both the observed evolution of local hydraulic gradients and pipe progression in the structure.

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.

The coarse sand barrier (CSB) is a novel remediation measure to reinforce embankments against the internal erosion mechanism of backward erosion piping (BEP). The feasibility of the CSB is investigated in a research programme consisting of experiments at different scales, numerical analysis and application at a pilot site in the Netherlands. Laboratory experiments showed that the CSB increased the critical head drop for BEP by one order of magnitude. In this paper, the strength of the CSB is quantified using numerical modelling of the experiments. A local, scale-independent, strength criterion for the CSB is derived based on medium-scale experiments. The application of this local criterion is illustrated for a field-scale model, in which the critical head drop that can be retained increases by a factor two to three with the CSB.

Parametric study on thermal performance of horizontal earth pipe cooling system in summer

This paper presents the findings of a preliminary research program that aimed to investigate the mechanisms governing the progression of piping erosion in organic soils and attempted to use the findings to reduce the severity of piping erosion of sand. The hypothesis was that the presence of organics within mineral soils results in a reduction in piping erosion progression. Erosion behaviors of the soils were quantified using a simple erosion test with a preformed hole to simulate an initial piping channel. The research was split into five test phases. In Phase 1, the influence of grain-size distribution was eliminated to develop a preliminary understanding of the role that organic matter plays in the erosion process. The results indicated that organic matter likely contributed to a reduction in piping erosion. The second phase excluded both grain-size distribution and individual particle shape as variables in the erosion process. The results further confirmed the connection between organic matter content and erosion resistance. The third phase revealed the positive correlation between the reduced piping erosion and increased organic matter content. The fourth phase investigated the quantitative relationship between certain biologically derived substances (polysaccharides and glomalin) and piping erosion reduction. The positive correlations that were preliminarily derived from this phase indicated that these substances likely play a major role in reducing piping erosion. The final phase comprised of introducing organic matter into mineral soil and quantifying the subsequent changes in geomechanic properties. The results suggested that the introduction of organic materials into mineral soil decreases strength, increases consolidation settlement, and reduces permeability.

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.