Fugro investigates piping under dikes on tidal sand

If the quantities are small, this is not a problem. But a larger stream of water carrying sand can seriously weaken a dike. During the last review round many dikes were rejected based on the current calculation rules, among other things because of the risk of piping. The necessary detailed practical knowledge about this failure mechanism is still lacking, which makes additional research necessary.

Dikes on tidal sand may be stronger than thought

Fugro, Deltares and Wetterskip Fryslân already conducted a practical test on the Friesian Wadden Sea coast in 2020. This showed that sand deposited by the sea appears to be much less sensitive to piping than sand deposited by rivers. The current calculation rule used to assess and design dikes is derived for river sand. If tidal sand proves to be stronger, fewer dikes need to be reinforced or the reinforcements can be carried out more cheaply, with less impact on the surroundings and the environment.

Commissioned by the water board Hollandse Delta, Fugro and Deltares have now started a large practical trial in the Hedwigepolder in Zeeuws-Vlaanderen; building on the results of the trial in Friesland. The Hedwige project aims to determine the actual strength of tidal sand against piping. This knowledge will then be translated and made applicable for assessment and reinforcement projects of flood defenses. The Hedwigepolder piping test is part of the international Living Lab Hedwige-Prosperpolder project. The initiator, the Department of Mobility and Public Works/ Hydraulic Laboratory, is jointly facilitating dike strength tests and crisis exercises with STOWA.

Grant from Flood Protection Program

Because the Hedwigepolder is being returned to nature, an extensive practical trial can take place there. The High Water Protection Program (HWBP) has made over 5 million euros available to Hollandse Delta Water Board for this purpose from the Knowledge and Innovation Agenda. Initial preparations began in mid-2020. Fugro and Deltares have already determined a test location based on soil structure and subsurface properties, where there is a suitable, naturally deposited tidal sand layer. The test will be carried out in 2021 and results should be available in 2022.

Innovative research method

Fugro is conducting the geotechnical and hydrological investigation on the basis of which the trial design is being prepared, in close cooperation with the project partners. An important aspect is the permeability of the tidal sand. This is measured with the innovative AMPT® probing technique, which Fugro has developed itself.

Gert-Ruben van Goor, project manager at Fugro: 'AMPT stands for anisotropic mini-pump test, an efficient complement to Hydraulic Profiling Tool or HPT soundings. The advantage of this method is that it provides a much more detailed picture of the structure of the subsurface. Permeability can be measured both horizontally and vertically more accurately than with other, large-scale techniques. This not only maps the different layers in the soil, but even the variation in permeability within individual soil layers. This allows us to compile a very faithful digital 3D model of the subsurface. Deltares uses our measurement data for a 3D groundwater flow model, which provides accurate insight into the water flows through the sand package. This subsequently provides better results for the piping risk analysis, on the basis of which Fugro and Deltares will prepare their recommendations following this test.'

Savings of 100 million euros

An average dike reinforcement costs about 6 million euros per kilometer. If the risk of piping can be determined more precisely on the basis of this test, it may mean that fewer kilometers of dike need to be reinforced or that dikes need to be made less wide than previously thought.

Marco Boer, project manager at water board Hollandse Delta: 'The Hedwigepolder offers us a unique opportunity to carry out this innovative practical trial in close cooperation with many partners. As water board Hollandse Delta, we like to encourage and facilitate innovative projects. We hope and expect that dikes with tidal sand in the subsoil will need to be reinforced less often. That could save an estimated 100 million euros for the whole of the Netherlands and limit the impact on the environment and surroundings.'

Substantial environmental benefits

An additional benefit is that a substantial reduction in environmental burdens can be achieved by requiring less space and less material for dike reinforcement. Among other things, this translates into fewer transport movements and less noise and vibration nuisance at dike reinforcement locations. It also results in substantial savings in CO₂emissions from trucks and earth-moving equipment. In short: less impact is created on the immediate surroundings and the environment.