Backwards Erosion Piping: A Historical Review and Discussion of Influential Factors

Backward erosion piping refers to the process by which seepage forces gradually erode cohesionless material from the foundation of dams and levees, thereby forming shallow pipes at the interface of the granular material and a cohesive cover layer. Piping is often expressed at the ground surface downstream of an embankment in the form of sand boils. If allowed to continue, the piping process may undermine the entire structure, leading to failure. As a primary failure mode of concern for dams and levees in both the United States and the Netherlands, the probability of piping failure is often assessed as part of routine risk assessments. Different methods of assessing piping potential have been published in the literature. Additionally, numerous papers have been published reporting laboratory test results on the piping potential of various soils. While useful, the correct application of the methods in the literature requires an understanding of the context from which each method was developed. Likewise, correct application of the laboratory testing data requires an understanding of the various test conditions, namely density, exit configuration, soil characteristics, and scale effects, that significantly impact the laboratory results. This paper provides practitioners with the information required to independently determine whether the various assessment methods and laboratory test data are applicable to their specific projects. The background, advantages, and disadvantages of each piping assessment method are discussed through a review of the evolution of piping knowledge over the past century. The influences of experiment scale and exit configuration on critical gradients are illustrated through comparisons of differing laboratory test data. Lastly, a brief discussion of ongoing piping research is provided to highlight current knowledge gaps to be aware of while assessing risks of backward erosion piping.