Cracking of the cover layer leading to backward erosion piping

Abstract

In the Netherlands, over 1300 km of the levees must be reinforced by 2050 as part of the Flood Protection Program. Innovative calculation techniques, driven by new knowledge, aim to optimize these reinforcement efforts. One of the dominant failure mechanisms that causes the required improvements is backward erosion piping. In assessing the probability of occurrence for this mechanism, the formation of an exit point in the cover layer is critical. The current assessment method does not explicitly account for the development of a crack in the cover layer. Including the cracking behaviour of the cover layer in the assessment framework would offer a more realistic and potentially more favourable evaluation of levee stability. Therefore, a comprehensive investigation was conducted, including a literature review, 13 geocentrifuge model tests, and two full-scale field tests. For the exit point formation of the cover layer, a physical description was written. This was primarily based on observed behaviour from the centrifuge tests. It was found that cracking started at the bottom of the cover layer, proceeding diagonally upwards to the toe of the levee. From there, an extended failure tree by backward erosion piping was drawn up. Next to cracking, other causes of exit point formation are possible, e.g. man-made holes or animal burrows. Field observations of sand boils were also incorporated into the analysis. This paper concludes with possible ways of an addition of the current assessment of uplift, differentiating between soil types, cohesion, tensile strength, and the thickness and volumetric weight of the cover layer.