Incipient motion of sands near the pipe tip.

Backward erosion piping constitutes a major threat for dikes on sandy or silty foundations during high water and floods. This erosion mechanism results in the formation and development of shallow hollow spaces (pipes) at the interface between the impermeable top layer and the aquifer. Erosion at the tip of the pipe determines the progression of the pipe towards the upstream side. A valid criterion should be developed to predict the critical local gradient at which erosion at the pipe tip occurs. Based on the failure mode of the tip of the pipe that the erosion near the toe of the scarp of the pipe tip causes the collapse of the scarp, in combination with the seepage field near the pipe tip, analyses are conducted regarding the equilibrium of forces on single particles at the toe of the scarp and a group of particles of the scarp. The conditions for incipient motion for particles at the pipe tip are determined. And a novel criterion for predicting the critical local gradient at the pipe tip is developed, which is essential for assessing the risk of pipe progression in practice. [ABSTRACT FROM AUTHOR]