The influence of the leakage length on the initiation of backward erosion piping: (Paper and Abstract)

The earliest relation describing the risk of backward erosion piping, from Bligh in 1915, calculates an overall gradient over the dike. This gradient should be lower than a certain value depending on the material of the dike foundation for the dike to be safe. Later developments rationalize this number, showing the influence of grain size, permeability, density and added the influence of the thickness of the aquifer. However, the principle remains the same. The actual overall gradient is compared to a critical gradient. This contribution will show, using analytical groundwater flow calculations that for the beginning of backward erosion piping, not only the overall gradient but also the leakage length of the semi-confined aquifer, usually present at the landward side of the dike, is of importance. This leakage length determines the piezometric head on the landward side of the dike. This is not very new, since this is also used in the ‘blanket theory’ developed and used in the USA, but a slightly different approach results in some new conclusions. It will also be shown that the influence of the leakage length on the pipe progression and in ultimo on the breach of the dike is less. This means that with a long leakage length the difference in overall head between pipe initiation and breakthrough is larger than for the situation with a shorter leakage length