Developments in modelling of backward erosion piping

Abstract

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 sandy or silty foundation and a cohesive cover layer. The models available for predicting the critical head at which the pipe progresses to the upstream side 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.

Experimental and numerical investigation of backward erosion piping in heterogeneous sands

“…The process of backward erosion piping was investigated by means of small-scale experiments with circular outlet configuration, which have been described in detail in Van Beek et al (2015). The experiments were performed using a setup consisting of a rigid box filled with sand covered by a transparent plate, a water supply system and several riser tubes to measure pore pressure at various locations in the sand sample (Figs 2, 3).…”

“…Lengthening of the pipe restarted only after a significant increase of head and it developed towards the upstream side of the setup without any further head increase. In the other experiments stepwise increase of head resulted In order to evaluate the effect of macro-scale variation in grain size in the path of the pipe on piping resistance, experiments with macro-scale heterogeneity were compared to tests conducted by Van Beek et al (2015) in which equivalent homogeneous fine sand samples have been used. The critical head for piping progression of these experiments is plotted in Figure 9.…”

“…It was found that in the numerical simulation of heterogeneous tests the critical head calculated by MSeep at which channel formation took place in the downstream fine layer was higher than the measured values. Van Beek et al (2015) have shown that experiments having a 3D configuration result in considerably lower critical gradients than a 2D configuration. Because 3D groundwater flow conditions cannot be accurately simulated by a 2D numerical model, MSeep couldn't predict correctly the critical head of the tests.…”

“…Previous laboratory tests (Van Beek et al 2008, Van Beek et al 2015 have indicated a substantial increase of piping resistance in sand samples characterized by variation of properties in the path of the Experimental and numerical investigation of backward erosion piping in heterogeneous sands G. Negrinelli Figure 2. Small-scale experimental setup.…”

“…This model considers two-dimensional pipe and groundwater flow. It assumes that the main mechanism of pipe progression is secondary erosion, which is erosion of grains in the piping channel caused by the flowing of water through the pipe; it neglects primary erosion, which is erosion at the tip of the piping channel and which occurs, according to Van Beek et al (2015), when the local gradient at the tip of the pipe exceeds a critical value resulting in fluidization and transporting of the grains. The Sellmeijer model can estimate the critical hydraulic head across the structure that causes the complete development of the pipe and leads to failure.…”