Fugro comes up with sustainable optimized approach

The Waal dike can take it again for a while

Several years ago it was determined that about 15 km of the Waal dike between Wolferen and Sprok near Nijmegen did not meet safety standards. After thorough preparation, the De Betuwse Waard construction consortium - the contractors Ploegam, Dura Vermeer and GMB - began work on the necessary reinforcement at the end of 2021. From the exploration phase, Fugro was part of the construction team.

Parts of the Waal dike in this area are insufficiently stable or sensitive to piping. The clay lining of the dike is subject to high requirements because of possible wave overtopping. The project must ensure that the entire dike meets the stricter requirements of the High Water Protection Program (HWBP) for another 50 years.

The main technical challenges in this project are effectively solving the piping problem, the limited space for applying reinforcement measures, making the dike flood-proof and working within Natura 2000 requirements. Right from the tender phase, consultants from Fugro's Water sector have been closely involved in the preliminary planning on the basis of which the tender was won. This involved an extensive measurement program, including probing, drilling and electromagnetic measurements.

Piping risk

On almost 90% of the dike section there is a risk of piping: small channels in the ground created by differences in water pressure in front of and behind the dike. If these "pipes" carry sand and erode as a result, they can weaken the water barrier. The additional research focused on determining as precisely as possible the erosion resistance and permeability of the clay revetment and the clay layers under the foreland of the dike. Therefore, electromagnetic measurements, among other things, were carried out.

Durable plastic heaveschields 

As a result of the additional investigation, a piping solution was not required for much of the project. For the high-risk locations, Fugro proposed a solution with heavy-duty sheet piling and optimized its design. Part of this sheet piling is made of plastic and part of it of steel, depending on the stability requirements at the site. Recycled plastic screens are shorter and lighter, and reduce CO₂-emissions by over 90% compared to a steel sheet pile wall.

Thinking in optimizations

Marijn Hop, Hydraulic Engineering Consultant at Fugro: 'From the initial ground design, we made a detailed plan design. Fugro always thinks in terms of optimizations: more nuance in rules, more research to arrive at an even better and more efficient solution.'

Probabilistic analyses 

'In order to use the land as effectively as possible, you have to be as precise as possible in the design. To do this, we use new software and new calculation methods. For example, we have performed probabilistic analyses to show that costly reinforcement - which would normally have been applied preventively to prevent the dike from sliding during high water - is not necessary. We calculate not only what we need, but also precisely what we don't need. Digitalization is therefore indispensable for sustainable and resistant work.'

Permeability of soil layers

The permeability of sand layers was determined using 66 HPT soundings and mini-pump tests. To make this method suitable for Dutch soils, Fugro has developed the HPT-AMPT^®-test or Aquisense™ was developed. This involves injecting water under high pressure into the soil above the probing cone so that the pore pressure can be determined. This gives a picture of the relative permeability of the soil over the entire depth of the probe. By conducting this test simultaneously with two cones at some distance from each other, absolute values can also be determined.

Using area-specific material

To limit the crest height of dikes, dike design in recent years has focused on greater transshipment. Wolferen-Sprok is a clay dike, over which 10 liters of water per second per meter of dike is allowed to surge.

Landscaping

Werner Halter, principal consultant at Fugro: 'A less high and less wide dike was chosen here, fitting into the landscape. But the inside of the dike must not erode due to the overburden. In this section, trees and driveways are present right behind the dike, so it was necessary to consider in great detail what the consequences of the modifications would be. For example, the sand shear present could erode away during overburden. Fugro recommended applying erosion-resistant clay at specific locations. Research into the required clay properties made it possible to reuse local clay for this purpose. This saves a lot of transport from elsewhere and therefore saves costs and CO₂-emissions.