Dyke reinforcement Sterke Lekdijk – Salmsteke sub-section
*District Water Control Board
ABT’s work on the Salmsteke subproject focuses on the further development of the Prolock Filter Screen as an innovative vertical piping solution and on the design of the structural dike reinforcement with a cofferdam near a listed residence.
First Innovation Decision Sterke Lekdijk
Based on the successful innovation project in July 2023, the management team of Sterke Lekdijk took the Innovation Decision to apply the Prolock Filter Screen at the Salmsteke subproject. This marked the combination of substantive technology and the decision-making process: a milestone within the Sterke Lekdijk project and for ABT and Mourik after months of preparation for the decision. Innovation project leader Vasco Veenbergen: “To have such a technology successfully developed from concept to actual application in a dyke within three years is only possible through strong cooperation, a solid chain approach and innovative strength!”
Innovation project Prolock filter screen
ABT was behind the further development of the Prolock filter screen. Vasco: “Supplier Profextru B.V. already had the idea which we embraced back in the tender phase in 2019. We recognised the potential of the filter screen with regard to sustainability and future-proofing. The main challenge here is in installability and being able to sustainably secure the quality of the filter construction in the use phase.”
The filter screen is a solution against ‘piping’. Piping is the process whereby seepage water washes away sand under the dyke at high tide, making the dyke unstable. Seepage is groundwater rising to the surface due to high water pressure. The filter screens do not influence the groundwater flow, but instead hold back the sand. As a result, the ‘pipe’ no longer extends under the dyke.
Once sub-project Salmsteke had been awarded to Mourik, we first examined whether the concept was feasible from a theoretical point of view. To this end, a desk study and model calculations were carried out. The concept turned out to be promising. We then made scale models and tested them in the hall of Mourik Techniek. Experiments were carried out with containers of sand and water to confirm the results of the model calculations. First on a small scale, the so-called ‘coffee cup test’, in order to carry out many tests in a short time. This involved testing the limits of the concept by deliberately allowing the filter screen to fail. This provided valuable insights on the scope of application. The design of the test set-up was worked out in close coordination with Mourik Techniek. Mourik Techniek then manufactured the container.
The medium-scale test was the highlight of the series of scale tests. Using a bin of 4m long, 1.3m high and fitted with a 3m high spindle to simulate high water in the river, the filter screen was tested to see if it could actually stop a pipe. Coloured sand and numerous sensors were used to monitor the course of the test. With a sizeable sand entraining well at the outflow opening of the tank, we found that the Prolock Filter Screen perfectly stopped pipe growth through the tank. Since then, several parties have expressed their enthusiasm to be able to carry out tests in the test facility. A great way to share knowledge transparently in order and get innovations started together.
Off to the clay
After the successful trials, it was time for field work. These field trials were carried out by contractor Mourik and ABT was closely involved in the preparation and execution. Vasco: “In the case of innovations, it is essential to experience the practical aspects in order to understand the concept in its entirety. Actually putting your feet in the clay, means that you immediately see aspects of implementation that can influence the design.”
Low maintenance and circular
The second successful field trial of the filter screen took place in spring 2022. Vasco: “This system consists of tubes that we clean in advance and fill with filter sand. We use a camera to monitor the tubes so we can spot any blockage.” Extensive research has been carried out into the risk of clogging. The filter screens are low-maintenance and will last for 100 years. They can then be recycled into new screens. It is a fully circular product.
Based on the written Design, Assessment and Maintenance Guideline of the Prolock Filter Screen, ABT is currently (summer 2023) preparing the Final Design and Implementation Design. To this end, we use the in-house developed computer tool AD2A, in which both the filter design and the testing of the screen for dike forces are fully automated. The added value is the higher reliability of the design and flexibility in making changes.”
If all goes well, construction of around 800 metres of Prolock Filter Screen will start in 2024.
Cofferdam at monumental Veerhuis
ABT also designed a cofferdam at Salmsteke. This was necessary because the monumental Veerhuis is located outside the dyke and had to be protected. The cofferdam was designed by design leader Thomas Lankreijer: “As water safety team, we like to work on projects involving interaction between soil, construction and the environment. HDSR’s established concept of the cofferdam at roughly 1 metre from the monumental house made things extra challenging for us.”
For this reason, part of the steel sheet piling is pressed rather than vibrated. This is easier said than done, as the space available for the large scaffold is very little. Together with Mourik and the subcontractor, the entire construction phasing was included in the design. In addition, Mourik had the design reviewed externally for sound risk management. “This is something we can only support. We all benefit from a smooth execution process,” Thomas said.
ABT provided the integral design of the cofferdam. Besides the geotechnical calculations and consideration of the construction phasing, all structural aspects with purlins, weld details and anchoring were also worked out.
Currently (summer 2023), groundwork is being prepared around the monumental Veerhuis for the construction of the cofferdam. The work will be carried out in the second part of 2023.