Initiation of backward erosion piping in uniform sands

Abstract

The process of backward erosion piping poses a threat to dams and dikes on foundations of nonplastic sands and silts. The available models for design and predictions focus predominantly on the progression of the pipe. However, sand boils in the field will occur as a result of the initiation of sand transport. Although criteria are available for predicting sand boiling and heaving in columns, there is no model describing the initiation of piping in situations where the exit flow is not uniform, as is the case in most backward erosion experiments and situations in the field. This study compared laboratory experiments in which the process of initiation leads directly to failure with analytical and numerical groundwater flow calculations and heave criteria. The aim was to develop a model for the onset of pipe formation. It emerged that the sand bed needs to be fluidised over a distance of at least 20 times the grain diameter from the toe of the structure for a pipe to initiate. The proposed model explains the scale effects of grain size and configuration on a critical gradient. This approach clarifies the processes governing pipe initiation and progression and it can be used to establish a conservative estimate of the critical head in uniform sands, which is essential for laboratory work on this topic and for the appraisal of sand boils in practice.

Developments in modelling of backward erosion piping

“…This paper focuses specifically on the type of backward erosion piping that occurs in the granular foundation of dams and dykes consisting of uniform silts and sands, and covered by a cohesive layer. The head loss across the structure results in a flow through the aquifer, and the convergence of flow lines near the downstream exit causes the sand bed to fluidise locally (Van Beek et al, 2014a). Subsequent erosion leads to the development of shallow pipes at the interface of the aquifer and the cohesive top layer.…”

“…Pipe initiation, which is marked by the transition from an intact sand bed to a sand bed with a short pipe caused by the initiation of sand transport, can be described by the flow conditions in the sand bed near the exit, and it has been investigated in Richards & Reddy (2012) and Van Beek et al (2014a). Once the pipe has initiated, equilibrium can be observed in pipe formation, requiring the increase in head difference H to make the pipe progress.…”

“…Hanses (1985), Sellmeijer (1988), Sellmeijer et al (2011) and Schmertmann (2000) have published models, which are briefly described by Van Beek et al (2014a). Hanses (1985) assumed that 'primary erosion' -erosion at the tip of the pipe -causes pipe lengthening.…”

“…Recent research (Van Beek et al, 2014a) shows that this is incorrect. Only experiments in which the critical head is dominated by the process of progression should be used for the validation of models in which pipe equilibrium is assumed.…”

Effectiveness of distributed temperature measurements for early detection of piping in river embankments

“…Imre et al, 2015) but in most cases the erosion process is slow enough to allow an effective intervention, which consists in building temporary sandbag rings around the sand-boils to stop or slow down the sand transport. The double possibility, of either fast or slow evolution of the mechanism, finds confirmation in theory (Van Beek et al, 2014a) and experiments (Koelewijn and Taccari, 2016).…”