Backward erosion piping: Initiation and progression

Backward erosion piping is an internal erosion mechanism during which shallow pipes are formed in the direction opposite to the flow underneath water-retaining structures as a result of the gradual removal of sandy material by the action of water. It is an important failure mechanism in both dikes and dams where sandy layers are covered by a cohesive layer. Sand boils can indicate that backward erosion is present and they are observed regularly during high water and floods. Although failure resulting from backward erosion piping is not common, several dike failures in the US, China and the Netherlands have been attributed to this mechanism. Given the impact that climate change is expected to have, prediction models for backward erosion piping are becoming increasingly important in flood-risk assessment. The prediction models available until now, such as Bligh’s rule and the Sellmeijer model, were validated in the research programme ‘Strength and loads on flood defence structures’ (SBW: Sterkte en Belastingen Waterkeringen) in the period 2008-2010 using small-, medium- and large-scale experiments. These experiments showed that an empirical adjustment of the Sellmeijer model was required to take the effect of the sand type into account and that validation was not possible for loose sand types because the erosion mode is different in those conditions. However, the absence of a theoretical basis makes this proposed empirical adjustment unsatisfactory because it lacks robustness. The main question addressed by this dissertation is how to explain and predict the pipe-forming erosion processes in uniform sands. A review of the literature, in conjunction with additional experiments, showed that the critical head in pipe formation leading to dike failure depends on either pipe initiation or pipe progression. In some experiments, the critical head for pipe initiation exceeds that of pipe progression and equilibrium is therefore prevented. The experiments in which no equilibrium was observed allowed for the development of a model for pipe initiation. It was possible to relate the observed differences in critical gradient caused by scale, sand type and configuration to the fluidisation of sand very close to the exit, where the local gradients are high. In the field, pipe progression is likely to determine the critical gradient. The Sellmeijer model predicts the progression of the pipe on the basis of the equilibrium of particles on the bottom of the pipe. The literature, and an analysis of the pipe width, depth, gradient and erosion process in experiments, indicate that pipe progression relies on two processes: primary erosion, which causes the removal of particles at the pipe tip, and secondary erosion, which causes the erosion of the pipe walls and bottom. Although the Sellmeijer model does not include primary erosion, it does function well for sand layers with a 2D exit configuration in which there is no variation in the grain size along the pipe path. The adaptation of the Sellmeijer model that was found necessary to account for the effect of sand type can be replaced by using the original model in combination with a variable bedding angle based on incipient motion experiments from the literature. The Sellmeijer model does not predict the critical gradient well for 3D configurations such as flow towards a single point, or for heterogeneous soils. Variations in the grain size in the pipe path were found to result in significantly higher critical gradients than expected, whereas a strong concentration of the flow towards the exit led to a fall in the critical gradient. 3D numerical calculations and the inclusion of primary erosion in the Sellmeijer model are needed to predict piping under these conditions.Geoscience & EngineeringCivil Engineering and Geoscience

Evaluation of Dutch backward erosion piping models and a future perspective: (Paper and Abstract)

The prediction of backward erosion piping is important for safety assessment of dikes in the Netherlands, where subsurface conditions are prone to this erosion mechanism. In the current assessment methodology, the adapted Sellmeijer rule is in use. In combination with the national safety philosophy and uncertainty in input parameters, this model results in high failure probabilities. This paper evaluates the Sellmeijer model and the recently developed Shields-Darcy model alongside recent developments in research on modelling of backward erosion piping, leading to a future perspective

Physical measurements of the backward erosion piping process: (Paper and Abstract)

A novel laboratory device is presented, in which the process of backward erosion piping is observed in cylindrical sand samples oriented horizontally. The cylindrical shape of the testing device constrained the location of the erosion path to the top of the sample, thereby allowing pore pressure measurements to be made in both the eroded pipe and the surrounding soil. Additionally, the pipe depth and width were measured. From the measurements, the local hydraulic gradient upstream of the pipe tip and the critical shear stress in the bottom of the eroded pipe were calculated. Results indicate that the local critical hydraulic gradient measured over a distance of 10 cm upstream of the pipe is not influenced by experiment scale. Further, the measurements suggest that the sediment transport in the eroded pipe can be adequately modelled using classic sediment transport theory for open channel flow

Piping in loose sands \u96 the importance of geometrical fixity of grains

Piping is one of the possible failure mechanism for dams and levees with a sandy foundation. Water flowing through the foundation causes the onset of grain transport, due to which shallow pipes are formed at the interface of the sandy layer and an impermeable blanket layer. In the past, the mechanism has been investigated predominantly in densely packed sands, in which the process was observed to start at the downstream side (backward erosion). Recently performed experiments in loose sand (van Beek et al. 2009) showed a different failure mechanism (forward erosion). In this article additional experiments of piping in loose sands are described for investigating the relevance of the forward process for practice. In these experiments the type of process was found to be dependent on the presence of shear resistance between sand box cover and top sand grains, that causes grains to be fixed geometrically. Without this shear resistance the process was found to be forward, whereas with this shear resistance the process was found to be backward oriented. The change in degree of fixity and relative density as a result of loading is investigated with electrical density measurements. The experiments show that the forward process is not relevant for levees in practice, in which the cohesive blanket layer causes the sand grains to be fixed properly

Piping: Over 100 years of experience: From empiricism towards reliability-based design

Backward piping is the process of channel formation in a sandy aquifer under river dikes. During high water periods this process manifests itself by the formation of sand boils. A long history of cases and experiments has contributed to the insights into this phenomenon and has improved the ability to predict the safety of levees.Geoscience & EngineeringCivil Engineering and Geoscience

Pipe measurement in small-scale backward erosion piping experiments: (Paper and Abstract)

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 or silty foundation and a cohesive cover layer. Although the pipe depth reveals a lot of information on the backward erosion process, it has never been measured systematically. In this study we used a contactless laser triangulation sensor to measure the pipe depth during and after small-scale backward erosion experiments with a circular exit for three poorly graded sands with mean grain sizes varying from 0.155 mm to 0.544 mm. The pipes prove to be extremely shallow and the pipe depth close to the pipe tip is just large enough to let a particle through. As the pipe grows, the pipe depth increases due to scour and reallocation of grains, allowing for a higher flow rate and more grains to pass. Furthermore, the pipe often consists of a shallow part in the middle and deeper parts at the outside

Overview geotechnical model tests on dike safety at Deltares

Three on-going dike safety studies (on: macro stability, piping and flow slides) in the Netherlands make use of geotechnical physical models. A short outline of these projects is presented; the physical models chosen are described and discussed. The three studies use different physical models, depending on the research questions at the beginning of the model test series, the heterogeneity that is anticipated in the field, the scaling laws and the knowledge level. The paper describes why a certain model was chose

Progression Rate of Backward Erosion Piping: Small Scale Experiments

Most research on backward erosion piping (BEP) focuses on the critical conditions leading to failure. This paper studies the development of piping over time once the critical conditions are exceeded, which is useful to estimate time to failure. A commonly used small scale rectangular box setup is modified in order to monitor pore pressures and pipe pressures with a high spatial and temporal resolution. The experimental program includes three different sand types to study the effects of grain size and compaction, and different degrees of hydraulic loading. The results indicate that the transport of particles in the pipe affects the progression rate, and that the progression rate is related to the bed shear stress in the pipe.Hydraulic Structures and Flood Ris