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Western Dam Engineering <br /> Technical Note <br /> <br /> August 2016 <br /> <br />15 <br />Internal Erosion: Issues Just <br />Below the Surface <br />Introduction <br />Internal erosion is one of the leading causes of <br />embankment dam failures, second only to overtopping <br />failures. Internal erosion occurs when embankment or <br />foundation soil particles are transported downstream <br />by seepage flow. The erosion begins when the seepage <br />force exceeds the erosion resistance of the materials. <br />Erosion resistance is a function of several soil <br />properties as described later in this article. Modern <br />earthfill dam design usually incorporates embankment <br />zoning that includes a low permeability core as the <br />seepage barrier, upstream and downstream shells for <br />structural support, and an internal filter and drainage <br />system to intercept seepage and reduce the probability <br />of material transport. The use of filters in modern <br />dams has been proven an effective and reliable <br />method to protect against internal erosion. That is, <br />when the filter system is designed and constructed <br />properly. However, many older and smaller dams are <br />not adequately zoned to provide the preferred level of <br />protection against internal erosion. <br />Internal erosion issues can become apparent on first <br />filling or after many years of operation. During first <br />filling, the materials are subjected to new seepage flow <br />and gradients that had previously not been <br />experienced. Aging dams may experience changes in <br />forces due to continued settlement from reservoir <br />drawdown cycles, deterioration of pipes and <br />structures, or exposure to an extreme flood or seismic <br />event. In some cases, internal erosion is such a slow- <br />moving failure mode that signs of it occurring take <br />years or decades to first appear. <br />Internal erosion has been a topic of much interest, <br />research, and publication over the past 30 years, and <br />continues today. This is a potential failure mode that <br />cannot be completely analyzed using numerical <br />formulae or models. As our understanding of internal <br />erosion mechanisms evolves, valuable information on <br />dam and soil behavior is becoming available to help in <br />assessing internal erosion risks. This article summarizes <br />key information needed to gain a fundamental <br />understanding of internal erosion mechanisms and <br />presents a high level summary of some key parameters <br />that influence the likelihood of internal erosion <br />occurring and progressing. Two recent publications are <br />excellent references for more comprehensive details <br />on the topic: see references [1] and [2] at the end of <br />this article. Look for future articles in Technical Note <br />that will discuss methods of seepage remediation, <br />mitigation, and emergency preparedness. <br />Internal Erosion Process <br />Internal erosion occurs when seepage through voids <br />within a soil or rock mass exert hydraulic forces <br />sufficient to detach and transport particles. The loss of <br />material from within an embankment or foundation <br />can lead to significant deformation and eventually <br />breach of the dam. The process is often localized along <br />a crack, defect, or high seepage velocity zone that <br />expands as erosion progresses. There are four general <br />seepage paths by which internal erosion can occur: <br />1. Through the embankment <br />2. Internal erosion of the embankment into or <br />along the foundation or abutments <br />3. Through the foundation <br />4. Along or through penetrating structures <br />(conduits or structural walls) <br />For all failure paths, the typical series of events to <br />describe the mode of failure from initiation to <br />complete breach, known as an event tree, has been <br />developed and is generally described as follows [1]: <br />1. Reservoir is at or above threshold level. <br />2. Initiation of erosion – a defect exists that allows <br />soil particles to be transported out of the <br />embankment or foundation by seepage flow. <br />3. Continuation – particle transport is not hindered <br />by a downstream filter. <br />4. Progression – eroding material leads to sloughing <br />of slope or the formation of a pipe through a <br />continuous stable roof and/or sidewalls. <br />5. Progression – Constriction (i.e., cutoff wall or rock <br />joint) or upstream zone fails to limit flows. <br />6. Progression – No self-healing by upstream zone <br />(e.g. crack stopper or upstream shell fails to clog <br />developing void. <br />7. Detection and intervention are unsuccessful.