Performance of Relief Well Systems along Mississippi River Levees

One of the failure mechanisms that can affect the safety of a dyke or another water-retaining structure is backward erosion piping, a phenomenon that results in the formation of shallow pipes at the interface of a sandy or silty foundation and a cohesive cover layer. The models available for predicting the critical head at which the pipe progresses to the upstream side have been validated and adapted on the basis of experiments with two-dimensional (2D) configurations. However, the experimental base for backward erosion in three-dimensional (3D) configurations in which the flow concentrates towards one point, a situation that is commonly encountered in the field, is limited. This paper presents additional 3D configuration experiments at two scales with a range of sand types. The critical gradients, the formed pipes and the erosion mechanism were analysed for the available experiments, indicating that the erosion mechanism is more complex than previously assumed, as both erosion at the tip of the pipe (primary erosion) and in the pipe (secondary erosion) are relevant. In addition, a 3D configuration was found to result in significantly lower critical gradients than those predicted by an accepted calculation model calibrated on the basis of 2D experiments, a finding that is essential for the application of the model in the field.

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“…Mansur et al, 2000;Vrijling et al, 2010). Sand boils often occur at locations where the flow concentrates towards the surface (USACE, 1956).…”

Section: Introductionmentioning

confidence: 99%

Developments in modelling of backward erosion piping

et al. 2015

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“…with respect to the initiation of piping is generally defined by the equation: The critical gradient (i c ) is the gradient needed to initiate erosion in the soil and usually varies from about 0.80 to 1.0 (Mansur, Postol, and Salley, 2000), depending on the unit weight of the soil. The exit gradient (i e ) is the gradient at the point of erosion calculated with Finite Element Analysis and it is a function of the subsurface geometry and the permeability of the subsurface soils.…”

Section: Deterministic Factor Of Safetymentioning

confidence: 99%

“…The failure mechanism to be considered has been explained by Mansur, Postol, and Salley, (2000), Guy et al (2007), McCook (2007, and Halpin and Ferguson (2007), and will be explained later on.…”

Section: According To the California Department Of Conservation (2007mentioning

confidence: 99%

“…It is the author's belief that this failure mechanism is not the actual underseepage failure mechanism that occurs in this type of levee section. As mentioned, the proposed (actual) failure mechanism has been explained by Mansur, Postol, and Salley (2000), Guy et al (2007), McCook (2007), and Halpin and Ferguson (2007), and is as follows: 1) high pressures in the sand (pervious substratum) cause the blanket (impervious substratum) to heave resulting in a crack in the blanket, 2) concentrated flow in the crack results in high gradients in the sand at the base of the crack, 3) erosion of the sand initiates, and 4) erosion progresses towards the river in the sand (backward erosion).…”

Section: Failure Mechanismmentioning

confidence: 99%

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Reliability Based Underseepage Analysis in Levees Using Monte Carlo Simulation

Polanco

Rice

2011

Geo-Frontiers 2011

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A new method for assessing the potential for unsatisfactory levee performance due to underseepage is presented. Specifically, the method assesses the potential for the initiation of piping (the internal backward erosion of the foundation or embankment caused by seepage). Current assessment methods consist of deterministic seepage analyses and simplified reliability methods. Deterministic methods produce either a maximum hydraulic exit gradient or a Factor of Safety against piping but they do not account for high levels of uncertainty in soil properties and subsurface geometry that are inherent to many levee analyses. The most common simplified reliability approaches thatare currently being used to analyze levees with regard to underseepage apply the First Order Second Moment (FOSM) Taylor Series method using the US Army Corps of Engineers "Blanket Theory" equation as the performance function. These methods take into account the uncertainty of the soil properties but are limited to simplified subsurface geometries and often do not model the actual mechanism responsible for levee failure due to underseepage piping.iii The proposed new method uses a Monte Carlo simulation to calculate the probability of unsatisfactory levee profile performance and can take into account complexities in subsurface geometry that cannot be assessed using the simplified reliability methods. The relationships between uncertainty of the soil parameters, the subsurface geometry, and the Factor of Safety against piping are defined through parametric variation analyses of a finite-element seepage model. The results of the parametric analyses are used to develop a series of equations that define the relationship between the various input parameters and the factor of safety. Using these equations, probability density functions for the various input parameters, and the computer program @Risk which interfaces with Excel, a Monte Carlo analysis is performed to calculate the probability of unsatisfactory performance which represents the probability of initiating erosion given a river flood level. The results of the analysis represent a single node of the event tree. In order to assess failure potential, other points in the event tree will need to be assessed with calculations or judgment since it is only the first phase that is currently considered to be in the process of piping. The new method is demonstrated using actual data of levee profiles from the Natomas Basin in Sacramento, California as a case study.The case study highlights the benefits of reliability-based analyses over the Factor of Safety and demonstrates the importance of subsurface geometry in reliability calculations.(172 pages) iv ACKNOWLEDGMENTS

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“…The Katrina disaster in Louisiana and frequent problems faced by levees along the Mississippi, Sacramento, and Red rivers have heightened the public concerns. Mansur et al [17] reported the sand boils caused by piping along the Mississippi river. Sills et al [22] inferred that piping was the probable reason for failure of some levees in New Orleans that were not overtopped during the Katrina hurricane.…”

Section: Introductionmentioning

confidence: 99%