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Backward erosion piping (BEP) is a type of internal erosion that typically involves the erosion of foundation materials beneath an embankment. BEP has been shown, historically, to be the cause of approximately one third of all internal erosion related failures. As such, the probability of BEP is commonly evaluated as part of routine risk assessment...
Contexts in source publication
Context 1
... process of BEP is illustrated in Figure 1. For BEP to occur, it is necessary to have an unfiltered seepage exit through which soil can begin eroding. ...
Context 2
... is evident by the numerous sand boils that occur along the U.S. levee systems during each flood [5]. A sand boil is a small cone of deposited soil that occurs concentrically around a concentrated seepage exit, as shown in Figure 1. The presence of sand boils indicates that the process of BEP has initiated at a particular site. ...
Citations
... BEP is recognized as one of the major causes of severe damage or even failure of flood defenses worldwide (e.g., Richards and Reddy 2007;Robbins and Sharp 2016). In Italy, a total of 130 sand boils of remarkable size (Fig. 2) have been catalogued in recent years along the major Italian watercourse, the Po River (Aielli et al. 2019). ...
The article presents a three-dimensional (3D) finite element (FE) model of the groundwater flow beneath a river embankment, aimed at developing a simple and reliable numerical strategy for the identification of hydraulic conditions that cause the reactivation of sand boils in flood defense systems prone to recurrent backward erosion piping. The seepage model is calibrated on a cross section of the Po River, where a large natural sand boil has been periodically observed during past high-water events. Monitored river water levels, piezometric measurements, and geotechnical testing data have been used for the calibration study. The numerical analysis proposes a suitable way to simulate a preexisting eroded zone, identifies the key parameters to be collected in the field, and discusses the criteria for the assessment of piping reactivation. The sensitivity analysis proposed herein enables one to identify the set of model parameters capable of capturing field evidence.
Backward erosion piping (BEP) is a type of internal erosion that has caused the failure of many dams and levees and continues to threaten the safety of existing infrastructure. To manage this threat, failure risks are regularly evaluated to prioritize risk reduction measures. Unfortunately, current practice for assessing BEP is limited to simple calculation rules that have large uncertainty and error. While numerical models have been developed for simulating BEP, ambiguity regarding the erosion constitutive model, inconsistencies in the assumed physics, and lack of laboratory tests for measuring model parameters have made it difficult to validate tools for use in practice. No validated, widely accepted model for BEP exists today. This thesis develops and validates an approach for finite element modeling of BEP by introducing the concept of the critical secant gradient function (CSGF). The CSGF provides a spatial function of the hydraulic gradient upstream of the pipe tip. An analytical expression for the CSGF and a laboratory test for measuring the CSGF are developed. A steady-state finite element model for simulating BEP progression is then developed for both two- and three-dimensional domains. The model and CSGF concept are validated through hindcasting of BEP experiments. Remarkable agreement was obtained between the finite element predictions and the experiments despite the experiments having different scale, configurations, and boundary conditions. These results indicate that the CSGF may provide the needed link between theory, lab testing, and numerical models to reliably predict BEP progression in practice. Additionally, the results indicate that the steady state finite element algorithm proposed is capable of adequately describing the BEP process, and more complex models may not be necessary. After developing and validating an approach for finite element modeling of BEP progression, the remainder of the thesis demonstrates techniques that can be used to simulate BEP in practice. The use of adaptive meshing is demonstrated in two-dimensions as a means of efficiently simulating BEP progression for field scale problems. Additionally, the random finite element method is applied to demonstrate how to incorporate spatial variability in soil properties into BEP predictions. The results of this study demonstrate how both techniques, in conjunction with the CSGF, offer the potential for transformative improvements in the engineering practice of risk assessment of dams and levees susceptible to BEP progression.
Sand boils are the surface manifestation of an erosion process, known as backward erosion piping, which may take place beneath river embankments during high-water events. The risk of embankment failure greatly increases in locations affected by sand boils. Numerous studies have been carried out, mainly at the laboratory scale, providing significant advancements in this field. Nonetheless, there is still a gap between research and practice that needs to be filled.
This study presents a set of field measurements carried out on a large sand boil reactivated near the toe of an embankment along the river Po (Italy). Hydraulic heads, velocity and discharge, concentration and pipe geometry were measured as a function of the water level in the river during the November 2018 flood. The collected data are compared to predictions of a theoretical model which provides the head loss in the vertical pipe. Furthermore, the local exit gradients, as deduced from measurements, are discussed, together with the operational critical gradients adopted in current design practice.
The collected data provide important input parameters for the calibration of analytical and numerical models, typically implemented to investigate the sand boil evolution and then to assess the backward erosion piping risk at real scale.
Backward erosion piping (BEP) is one of the major causes of seepage failures in levees. Seepage fields dictate the BEP behaviors and are influenced by the heterogeneity of soil properties. To investigate the effects of the heterogeneity on the seepage failures, we develop a numerical algorithm and conduct simulations to study BEP progressions in geologic media with spatially stochastic parameters. Specifically, the void ratio e, the hydraulic conductivity k, and the ratio of the particle contents r of the media are represented as the stochastic variables. They are characterized by means and variances, the spatial correlation structures, and the cross-correlation between variables. Results of the simulations reveal that the heterogeneity accelerates the development of preferential flow paths, which profoundly increase the likelihood of seepage failures. To account for unknown heterogeneity, we define the probability of the seepage instability (PI) to evaluate the failure potential of a given site. Using Monte-Carlo simulation (MCS), we demonstrate that the PI value is significantly influenced by the mean and the variance of lnk and its spatial correlation scales. While the other parameters, such as means and variances of e and r, and their cross-correlation, have minor impacts. Based on PI analyses, we introduce a risk rating system to classify the field into different regions according to risk levels. This rating system is useful for seepage failures prevention and assists decision making when BEP occurs.
