2014 IR&D Annual Report

Development of a Numerical Approach to Modeling Internal Erosion in Embankment Dams and Levees, 20–R8463

Principal Investigators
Biswajit Dasgupta
Gordon Wittmeyer
Goodluck Ofoegbu

Inclusive Dates: 04/01/14 – Current

Background — Failure of embankment dams and levees by backward erosion or "piping" is a major safety concern. Internal erosion by piping typically occurs in non-cohesive granular soil (e.g., sand, silt) within the foundation and embankment of the dam or levee initiating at the toe of the dam when the fluid forces from focused seepage are sufficient to suspend and transport sand particles out of the embankment or foundation. The resulting cavity further concentrates seepage, more sand is eroded, and the cavity elongates, forming a pipe or tunnel that propagates upstream. The pipe may grow in size because of internal soil erosion from concentrated internal seepage. The pipe collapses when the pipe walls can no longer support the overburden pressure, causing the overlying earth embankment to fail in modes that range in severity from slight slumping to complete collapse. Observed dam failures from piping have been sudden with little or no prior indications.

Preventing or mitigating internal erosion as a result of piping or backward erosion process is an essential consideration when designing, inspecting, and maintaining levees and dams. Embankment dams with permeable foundations are common in the United States, and there is extensive experience in design, maintenance, and operation of these dams within the civil engineering profession; however, backward erosion remains a poorly understood failure mechanism. Existing internal erosion prediction methodologies, which are based on information acquired from laboratory or field tests, are not adequate for accurate prediction.

Risk assessment of existing dams is gaining importance in the United States and in the rest of the world to ensure safety from aging and from  beyond design-basis flood and seismic events. However, the current risk assessment evaluation methodologies for piping failure modes of embankment dams rely on empirical approaches developed based on existing data that may not be applicable to conditions of existing dams. The purpose of this research is to develop a computational approach to model the internal erosion phenomena and develop a process to simulate piping in embankment dams and levees to support risk assessment of existing dams in the U.S. and other countries.

Approach — Internal erosion of the soil embankment or foundation is a complex process that involves continuous three-dimensional (3-D) interactions among seepage forces and intergranular forces wherein piping initiates in the downstream area, erodes backward toward the reservoir, and increases in size, potentially resulting in dam breach and collapse. The research aims at developing a computational methodology for modeling backward erosion in embankment dams and levees by using both micromechanical and continuum modeling approaches. A particle-based micromechanical model will be used to simulate piping at the laboratory scale for a wide range of hydro-mechanical conditions in order to evaluate relationships among hydro-mechanical properties needed for a continuum description of the piping process. The resulting relationships will be applied to develop a continuum modeling approach for coupled fluid-mechanical interaction analysis for simulating backward erosion in full scale dams and levees.

The technical approach for accomplishing the objectives of the research project consists of several steps: (i) extend the capability of a commercially available particle flow code (PFC-3D) with coupled computational fluid dynamics (CCFD) for simulating seepage-induced erosion in a laboratory-scale experiment, (ii) use the model to replicate key physical features observed in small-scale laboratory piping experiments, (iii) establish criteria for the occurrence of piping for a range of particle sizes and particle size distributions, (iv) use the results to develop continuum-scale block or element failure criteria, and (v) simulate small-scale laboratory piping experiments using the continuum-scale code FLAC3D with block failure criteria to simulate backward piping. The simulated backward erosion must initiate on the downstream side of the levee and progress backward to the upstream side, forming a continuous "pipe" through the foundation material.

Accomplishments — Progress has been made in numerical simulation of coupled fluid flow and particle flow using PFC3D and CCFD software to evaluate feasibility of modeling laboratory-scale experiments. Simulation of basic erosion processes was demonstrated for onset of vertical piping due to liquefaction failure, and simulation of horizontal piping in a sand-box experiment is in progress. The simulations also are being used to explore representation of the physical forces involved in particle-fluid interactions to develop failure criteria for continuum damage of internal erosion. Simulation of coupled seepage flow and mechanical response in the granular media of a dam foundation is underway to evaluate the continuum damage concepts using two-dimensional continuum code FLAC.

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