Strength of clay dikes examined more closely
Deltares and Waterschap Drents Overijsselse Delta are investigating the strength of clay in dikes on behalf of the HWBP. In full, the strength of the initial unsaturated zone. This is the part in the dike that is usually dry, but becomes wet during a high water level on the river and thus becomes saturated. The study is currently being conducted along the IJssel River (near Herxen). The results are applicable to all clay dikes in the Netherlands.
Important information
Project name: Research strength clay dikes
Date: Until end of winter 2025
Client: Hoogwaterbeschermingsprogramma (HWBP)
Location: Along the river IJssel, near Herxen
Size: Involves several measurement locations, including:
3 main measurement locations
8 ‘light locations’ spread across the Netherlands
Soil stronger than thought
What does previous research show? The soil in this zone is stronger than we initially thought. This means the levees are more stable than expected, requiring less reinforcement. This insight helps us understand how to better build and maintain clay dikes, especially since soil strength changes through the seasons, depending on precipitation and evaporation.
In winter, for example, the moisture content in the dike increases, which lowers the shear strength (the resistance to soil shifting). But even in winter, this shear strength turns out to be higher than expected. This is good news because many stability analyses assumed lower strengths, which may now mean we can work more efficiently.
Despite the fact that the strength of the soil appears to be greater than previously assumed, there are still some unknown factors that need further investigation. Therefore, the study has been expanded to include a third measurement site, the IJsseldijk near Paddenpol, and 8 new “light sites,” within the management areas of different water boards, representing a wide range of common clay types.
Innovative techniques for better insights
The field survey is being conducted by MOS Soil Mechanics, part of SOCOTEC, using several innovative techniques. One of these is the probing method with a dielectric cone. This allows us to determine both the amount of water in the soil (degree of saturation) and the way in which water is drawn up into the soil (capillary tension) insitu while simultaneously carrying out a class 1 probing. Two soundings are conducted every 5 to 10 weeks per measurement site so that we measure a year around. Special measurement series at the Paddenpol site at Herxen involve slow soundings in which up to 200 times slower than the standard speed is probed. This helps to understand how the speed of measurement affects the determination of soil strength. Probing 200 times slower than standard was an extremely interesting challenge for Mos Grondmechanica (SOCOTEC) where in-house knowledge, skill and years of experience in conducting probing came in handy. Several adjustments were made to the sounding unit and software for this purpose, with the aim of obtaining reliable sounding data at different sounding speeds. In addition, Field Vane Tests are performed at different depths and locations to determine the undrained shear strength in-situ.
Furthermore, Mos Grondmechanica (SOCOTEC) is performing machine drilling within this project for soil identification and obtaining (un)disturbed soil samples. Several laboratory tests, consisting of classification and strength tests, are performed on these soil samples at Mos Grondmechanica and Deltares. The strength tests are carried out unconsolidated and undrained (UU) to test the soil samples as much as possible at the initial situation, the unsaturated zone.
Cost savings
This project builds on previous research and will eventually lead to guidelines that can be used nationwide by dike managers. The goal is to use these to make dike reinforcements even smarter and more sustainable, so that the safety of the dikes in the Netherlands is guaranteed and costs can be saved.
The study will run until the end of next winter. The results are expected in 2026.
