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Western Dam Engineering Technical Note May 2016 20 Internal erosion has led to more catastrophic dam failures than any other mechanism except overtopping. Internal erosion is itself a broad category, as it encompasses several mechanisms including backward erosion piping, concentrated leak erosion, contact erosion, and suffusion/suffosion. Describing all of the initiating, continuation, and progression mechanics of internal erosion is beyond the practical scope of this article. Three excellent references for further reading on internal erosion mechanisms are [2], [4] and [9]. Look for a summary of internal erosion mechanisms and associated potential failure modes in a future article. Foundation Seepage and Backward Erosion Piping The focus of this article is specifically on seepage through a dam’s foundation and its impact on dam safety. Seepage through the foundation of an embankment dam is sometimes referred to as “underseepage.” Internal erosion through the foundation due to underseepage is especially dangerous because there may be only subtle evidence that it is taking place until the situation has progressed to near failure. This is because the exit point may not be readily visible, as it could be a distance downstream of the dam, be underwater, or be hidden by terrain and/or vegetation and be undetectable to the casual observer. Underseepage is controlled by the natural conditions of the foundation and any engineered features in the design and preparation of the foundation. Engineered features for foundation preparation may include removal of unsuitable soils (organic, low strength, permeable, etc.), in-situ soil improvements (dynamic compaction, deep soil mixing, etc.), cutoff walls and trenches, surface treatment of bedrock contact, and grouting. Foundation treatment was often limited or absent for small to medium sized dams, especially those constructed before about 1970, as a result of not fully understanding or recognizing the potential adverse impacts of seepage flow through, into, or out of the foundation. Backward Erosion Piping Although there are several internal erosion mechanisms that can occur through the foundation, the most common is known as backward erosion piping, or just “piping.” Backward erosion piping is erosion of soil by seepage water that initiates at an exit point and progresses backward (upstream) toward the reservoir (generally in a horizontal direction as shown on Figure 1). Particles of soil are carried away by the seepage, until eventually a tunnel, or pipe, is formed from the downstream exit point to the reservoir. A continuous layer susceptible to piping, beneath a layer or structure capable of forming a roof, is needed for backward erosion piping to lead to dam failure. In addition, an unfiltered exit point is required for this type of erosion to initiate. The typical series of events to describe the mode of failure from initiation to complete breach, known as an event tree, has been developed and is generally described as follows [9]: 1. Reservoir at or above threshold level 2. Initiation – Erosion starts 3. Continuation – Unfiltered or inadequately filtered exit exists 4. Progression – Continuous stable roof and/or sidewalls 5. Progression – Constriction or upstream zone fails to limit flows 6. Progression – No self-healing by upstream zone 7. Unsuccessful detection and intervention 8. Dam breaches There are two idealized foundation conditions most commonly considered when evaluating the potential for excessive foundation seepage. McCook [5] and Pabst et. al [6] are two excellent references that provide understandable and concise descriptions of these two conditions and methods to estimate factors of safety against the initiation of erosion. The terms ‘heave’ and ‘uplift and blowout’ are sometimes used interchangeably; however, as described below, the mechanisms are physically different. The use of ‘heave’ should be limited to cohesionless soils (no confining layer) and the use of ‘uplift and blowout’ to foundations with a confining layer.