ABSTRACT Small-, medium- and full-scale experiments are performed to investigate the process of backward erosion piping and to validate the model of Sellmeijer. The observed processes are described in this article and are divided in four phases: seepage, backward erosion, widening of pipe and levee failure. Initially the erosion activity consists of very limited not noticeable sand transport. Increase of hydraulic head results in backward erosion, marked by the occurrence of continuously sand transporting sand boils. The transition from backward erosion to the widening of the pipe cannot be observed by the amount of transported sand or the flow rate. Failure can take place within a short time. The swift transformation of small sand boils to levee failure therefore appears to be extremely hazardous when determining the seriousness of piping underneath a levee.
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