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Western Dam Engineering Technical Note May 2016 19 Is Your Embankment Dam under Pressure – Underseepage Impacts Introduction A simple truth: every embankment dam that retains water also seeps water. This is due to the nature of soils and rock that constitute earth dams and porous dam foundations that have a potential to allow flow of water. The question will always be — Does the mechanism, location, and volume of seepage cause concern for the safety of the dam? How does one identify acceptable versus unacceptable seepage? These are lofty questions that dam engineers, top academics, and research agencies have been studying for decades, resulting in a continuous evolution of the understanding of seepage mechanics of earth dams during the past 100 years. The majority of U.S. dams are more than 50 years old and were designed and built prior to the current understanding of internal erosion control. One of the common challenges of evaluating an existing structure is recognizing what the surface expressions may be telling us about the internal mechanisms at work, (i.e., when a potentially hazardous condition may be developing). This concept is especially applicable to earth dams that are composed of and founded upon natural materials. Giving dam owners, operators, engineers, and regulators the knowledge and tools to monitor, detect, and evaluate observed seepage is an enduring task. This article introduces the extensive, yet critically important topic of seepage and focuses on the mechanics, monitoring, and investigation of seepage through the soil foundations of earth dams. More importantly, the article provides guidance as to when observed foundation seepage should be a concern. Future articles will focus on other aspects of earth dam seepage including embankment seepage, emergency response to seepage incidents, and long-term remedial measures. Basic Seepage Knowledge Seepage is defined as the flow of a fluid (water) through the porous space within a soil mass. Although on a micro scale the seepage follows an irregular path around the solid particles, engineers generally think in terms of the average linear flow path. Much work has been completed on the topic of seepage, and Cedergren’s Seepage, Drainage, and Flow Nets [1] is an excellent book on the characteristics of flow under and through dams. Most current seepage approaches are based on his assessments. Seepage that simply results in water loss is not necessarily a dam safety issue. It can, however, be an economic or societal issue, as retaining or storing water is the dam’s primary function. If the structure is not good at that, then cost and social impacts of the water loss may warrant improvements. Clean seepage from a dam may result in an inadvertent creek or pond downstream of the dam, unintentionally feeding distant aquifers, or causing settlement of nearby roads or structures—but these may not significantly affect the integrity of the dam structure itself. When seepage velocity is great enough, erosion of the soil can occur because of the frictional drag exerted on the soil particles. Dam safety engineers are focused on seepage that erodes the soil material of the dam or its foundation. This type of seepage-induced erosion can be considered in two broad categories; surface erosion and internal erosion. Surface erosion due to seepage initiates at the point of the seepage exit. The scour caused by the exiting flow of water can progressively destabilize embankments by removing materials which provide external support to the structure. Internal erosion occurs within the soil mass as soil particles within an embankment dam or its foundation are carried downstream by seepage flow. Internal erosion can progressively deteriorate the integrity of the structure by eventually creating large voids or “pipes” within the dam or foundation, ultimately leading to loss of the reservoir.