Piping (erosion)

Internal erosion is the formation of voids within a soil caused the removal of material by seepage. It is the second most common cause of failure in levees and one of the leading causes of failures in earth dams, responsible for about half of embankment dam failures.

Internal erosion occurs when the hydraulic forces exerted by water seeping through the pores and cracks of the material in the dam and/or foundation are sufficient to detach particles and transport them out of the dam structure. Internal erosion is especially dangerous because there may be no external evidence, or only subtle evidence, that it is taking place. Usually a sand boil can be found, but the boil might be hidden under water. A dam may breach within a few hours after evidence of internal erosion becomes obvious.

Piping is a related phenomenon and is defined as the progressive development of internal erosion by seepage, appearing downstream as a hole discharging water. Piping is induced by regressive erosion of particles from downstream and along the upstream line towards an outside environment until a continuous pipe is formed.

According to the International Commission on Large Dams (ICOLD), there are four general failure modes for internal erosion of embankment dams and their foundations:

The process of internal erosion occurs across four phases: initiation of erosion, progression to form a pipe, surface instability, and, lastly, initiation of a breach. Internal erosion is also classified in four types, dependent on failure path, how the erosion initiates and progresses, and its location:

Concentrated leaks occur when cracks form in the soil. The cracks must be below reservoir level, and water pressure needs to be present to maintain the open pipe. It is possible for water flow to cause the sides of the pipe to swell, closing it and thus limiting erosion. Additionally, if the soil lacks sufficient cohesion to maintain a crack, the crack will collapse and concentrated leak erosion will not progress to a breach. Cracks that allow concentrated leaks can arise due to many factors, including:

Longitudinal cracks arise from the spreading of the embankment, while transverse openings, which are much more common, are due to vertical settlement of the dam. The hydraulic shear stress τc required for the initiation of concentrated leak erosion can be estimated using laboratory testing, such as the hole erosion test (HET).

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