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

Developments in modelling of backward erosion piping

One of the failure mechanisms that can affect the safety of a dyke or another water-retaining structure is backward erosion piping, a phenomenon that results in the formation of shallow pipes at the interface of a sandyor silty foundation and a cohesive cover layer. Themodels available for predicting the critical head at which the pipe progresses to the upstreamside have been validated and adapted on the basis of experiments with two-dimensional (2D) configurations. However, the experimental base for backward erosion in three-dimensional (3D) configurations in which the flow concentrates towards one point, a situation that is commonly encountered in the field, is limited. This paper presents additional 3D configuration experiments at two scales with a range of sand types. The critical gradients, the formed pipes and the erosion mechanism were analysed for the available experiments, indicating that the erosion mechanism is more complex than previously assumed, as both erosion at the tip of the pipe (primary erosion) and in the pipe (secondary erosion) are relevant. In addition, a 3D configuration was found to result in significantly lower critical gradients than those predicted by an accepted calculation model calibrated on the basis of 2D experiments, a finding that is essential for the application of the model in the field

3D character of backward erosion piping: small-scale experiments

Backward erosion piping is an important failure mechanism for cohesive water retaining structures which are founded on a sandy aquifer. At present, the prediction models for safety assessment are often based on 2D assumptions. In this work, the 3D character of the phenomenon is demonstrated on the basis of small-scale experiments. Our approach reveals the correlation between the occurrence of piping initiation and progression and the width of the physical model, which is a measure for the inclusion of the third dimension (to be regarded in a real dike situation as the influence zone of a crater). In addition, it was found that the model width has an impact on pipe characteristics and pipe development. Therefore, our results enable a better understanding of the complex physical mechanism related to backward erosion piping and thus can lead to a significant improvement in the safety assessment of water retaining structures

Backward erosion piping through vertically layered soils

Backward erosion piping is an important failure mechanism for water-retaining structures, a phenomenon that results in the formation of shallow pipes at the interface of a sandy foundation and a cohesive cover layer. This paper studies the effect of two sand types on backward erosion piping; both in case of a homogeneous sand layer, and in a vertically layered sand sample, where the pipe is forced to subsequently grow through the different layers. Two configurations with vertical sand layers are tested; they both result in wider pipes and higher critical hydraulic gradients, thereby making this an interesting topic in research on measures to prevent backward erosion piping failures. Grain size analysis shows that the finer fraction is more likely to be eroded and also indicates that grains usually do not settle once they are eroded

Influence of sand type on pipe development in small- and medium-scale experiments

Up to now, the phenomenon of backward erosion piping in uniform sands covered by a cohesive top layer has been predicted using a 2D approach. The Sellmeijer model (Sellmeijer 1988) includes the 2D-flow towards the pipe, Poiseuille flow in the infinitely wide pipe and equilibrium of grains on the pipe bottom to predict the critical head.Arecent series of experiments indicates that the original Sellmeijer model fails to predict the critical head correctly for coarser sand types, and an empirical correction has been proposed to correct for this (Sellmeijer et al. 2011).A theoretical solution is not presently forthcoming. In this study the pipes formed in small- and medium-scale experiments have been analysed for different sand types. It was found that the width of the pipe increases with increasing grain size. This indicates that a 3D-approach is required to correctly model the piping process and to solve the influence of grain size on critical head. 3D numerical simulations of the piping experiments provide insight in the pipe hydraulics and difficulties encountered upon modeling of pipin

The effect of sudden critical and supercritical hydraulic loads on backward erosion piping : small-scale experiments

Backward erosion piping is an important failure mechanism for cohesive water-retaining structures which are founded on a sandy aquifer. This paper studies the effect of the sudden application of critical and supercritical hydraulic loads in small-scale experiments. For supercritical loads, it is clear that a part of the upper sand bed goes in suspension before being eroded, resulting in a relatively dense sand–water mixture being transported through the eroded pipe. A linear relation is found between the average velocity at which the pipe grows from downstream to upstream and the applied supercritical load. The pipe cross section is measured at the end of each test. The variations in pipe width, depth and cross section give more insight into the hydraulic regime within the pipe. The clear correspondence for gradual loading and sudden critical loading, and a smooth transition to higher supercritical loads, indicates that the fundamental erosion mechanism may be the same. Finally, grain size analysis of the eroded sand shows that the finer portion of the sand is eroded in case of subcritical or critical loading, while the coarse grains are eroded in case of supercritical loads