Deltares - R&D Highlights 2009

Contact: ulrich.forster@deltares.nl +31 (0)88 335 7203 vera.vanbeek@deltares.nl +31 (0)88 335 7228 andre.koelewijn@deltares.nl +31 (0)88 335 7338 Full-scale piping experiment (SBW) Piping, the process of retrograde erosion in sandy layers below clay dikes, is seen as an important failure mechanism in waterretaining structures in the Netherlands. Several computation models are available for estimating the potential occurrence of piping. The most widely-used models in the Netherlands are the empirical relation of Bligh (1910) and the semi-analytical model of Sellmeijer (1988), which describes the piping process in a more complete way. Sand transporting well The Sellmeijer model is based on the equilibrium of the forces of sand grains, fl ow in the developing channel (pipe) and fl ow through the aquifer. The model states the relation between pipe length and hydraulic head at which the sand grains are in equilibrium. Below the critical head the channel will stabilise. We drew on a series of small-scale, medium-scale and centrifuge experiments to refi ne the Sellmeijer calculation model. Four fullscale tests were conducted to validate diff erent aspects of the calculation model. The full-scale experiment was performed at the Smart Dike location in the north-east of the Netherlands. Instrumentation detail at piping location 40

Full-scale piping experiment Two large basins were created, fi lled with two types of sand (coarse and fi ne). A clay dike was built with a height of 3.5 m, slopes of 1:2 and a seepage length of 15 m was built on top of the homogeneously densifi ed and well-saturated sand layer. At the sand-clay interface, several rows of pore pressure meters monitored pipe formation. In addition, fi bre optics at the interface measured temperature and strain diff erences. Two of the four experiments tested additional monitoring equipment. On the basis of the observations in the full-scale experiments, piping can be broken down into four phases: seepage, retrograde erosion, clearing out and failure of the dike. The retrograde erosion phase is modelled by Sellmeijer. In this phase, channel formation is observed in the shape of sand traces (sandy spots without any crater formation), clean wells and sand transporting wells (sand craters). When sand transporting craters appear, the critical hydraulic head has almost been reached. The start of the next phase, clearing out (enlarging and cleaning of the channel from upstream to downstream), can be monitored only by using water pressure meters. The amount of transported sand increases signifi cantly only when the channel reaches the downstream side. The process of clearing out may directly result in failure as soon as the channel reaches the downstream side, but it may also result in the deformation of the clay dike, partially closing the channel and therefore extending the duration of this phase. Failure takes place as a result of a signifi cant increase in sand and water transport and the deformation of the dike. It has emerged that failure caused by piping is a real threat for dikes. Further reading V.M. van Beek, H.T.J. de Bruijn, J.G. Knoeff , A. Bezuijen and U. Förster (2010) “Levee failure due to piping: A full-scale experiment”, Fiſt h international conference on Scour and erosion, San Francisco Funding Research programmes “Strength and Loading on Flood Defence Structures” (SBW) and “Flood Control 2015”. Staatsbosbeheer made the test site available 41

ulrich.forster@deltares.nl, vera.vanbeek@deltares.nl, andre.koelewijn@deltares.nl,
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