3D character of backward erosion piping

Backward erosion piping is an important failure mechanism for cohesive water-retaining structures which are founded on a sandy aquifer. Nowadays, piping research and safety assessments are often based on experimental or numerical modelling using arbitrary model widths or even two-dimensional (2D) assumptions. This technical note shows the influence of this limitation through a series of small-scale experiments with varying model widths. The flow pattern proves to be highly three-dimensional (3D), influencing both the pipe geometry and critical gradients leading to piping failure. A 2D model is unable to capture the important aspects of the erosion mechanism and a correction factor needs to be applied if the minimum width for correctly simulating a 3D situation is not accomplished.