Analysis of pipe progression during backward erosion piping in the presence of a coarse sand barrier

The coarse sand barrier (CSB) is considered as a promising measure to prevent failure of embankments due to backward erosion piping. In this method, a trench consisting of coarse sand is placed below the blanket layer at the downstream side of the embankment, across the possible path of the pipe to prevent development of a pipe. A pipe can progress upstream until it encounters the CSB, which prevents further progression of the pipe unless a significantly higher head drop (compared to the case without CSB) is applied. This results in a much higher safety level for the levee. The increased strength is due to the barrier’s higher resistance against erosion, and the relatively high hydraulic conductivity contrast between the barrier and the background material leading to a reduction of the hydraulic load in the barrier. The feasibility of this method has been investigated in a three-phase experimental programme at Deltares consisting of small-, medium- and large-scale experiments, confirming this method as a highly effective piping inhibiting measure. This contribution presents the results of one phase of this research, the medium-scale experiments, during which several laboratory experiments were conducted. In this paper, the piping process and observations on the pipe progression in presence of a CSB are presented and analyzed to get a better insight in the principle of pipe progression with respect to different barrier materials.