Multi-scale experiments for a coarse sand barrier against backward erosion piping

Backward erosion piping poses a severe threat for embankments founded on sandy deposits. The physical mechanism by which a pipe progresses upstream beneath the embankment is conceptually understood, and assessments indicate high failure probabilities in the Netherlands. Rising sea levels and economic growth increase the risk of flooding, and the need for remediation measures. Conventional measures have undesired side effects, in terms of costs, maintenance, space requirements and sustainability. The coarse sand barrier (CSB) is a novel nature-based solution lacking these side effects. The CSB is a trench filled with coarse sand that is placed below the downstream toe of the embankment, which prevents the pipe from progressing upstream. The effectiveness of the CSB is based on its higher resistance against erosion and its effect on groundwater flow. This paper presents experimental work that was conducted to test the feasibility of a CSB for application at a pilot site in the Netherlands. This involved experiments with seepage lengths of 0·34, 1·34 and 15 m, using different barrier materials and background sands. A conceptual model is presented for the physical mechanism of pipe formation in the presence of a barrier. As the resistance of the barrier can be characterised by a local scale-independent strength criterion, laboratory results are applicable for design practice.