Hinged 'green' concrete blanket 

The hinged concrete deck of Verkalit blocks on the Lauwersmeerdijk between the Peasumer Lannen and the Cleveringsluizen (province of Friesland) will get a sequel in Groningen on the project dike reinforcement Lauwersmeerdijk-Vierhuizergat. Because water board Noorderzijlvest also opts for this inventive concept that has proven its added value in Germany for many decades. Thanks to the unique hollow and doling system, stability is guaranteed, and the package can even move along with the profile of the dike. Nature also finds its way between the blocks, ultimately creating a beautiful green dike.

Unlike the usual dike covering with concrete columns, Verkalit is precisely not a column but an interlock system, says Hans Kamminga of LBN Concrete Products. "And the latter is precisely the strength of Verkalit." He clarifies this with an example. "With a normal concrete column of any manufacturer, the stability is realized by split between the blocks. During a storm, there is a chance that the split will wash out. This endangers the stability of the dike. After every storm, the dike revetment must be checked and, if necessary, supplemented with gravel. With Verkalit you don't have this problem, because the stability is obtained through the hollow and dagger system that interlock. Stability is guaranteed at all times, and an inspection after every storm is therefore no longer necessary."

Water Law

While Verkalit blocks have been used as dike revetment in Germany for over thirty years, by Dutch standards it is relatively new. Or yes new, LBN Betonproducten promoted the concept to the Dutch market years ago. "The product was then extensively tested and trialled, including in the Delta flume at Deltares in Delft," says Hans. "It also passed the Stone Test, as prescribed in the Water Act. Subsequently, a pilot project was realized in Kreyl on the IJsselmeer of 2,000 m2, and since 2019 it has been applied on a large scale, including previously in Friesland and now so in Groningen on the Lauwersmeer dike. Not least because of the construction consortium's good experiences with the product. The blocks are produced from recycled concrete in Burgum in Friesland. The transport distances are therefore minimal."

Higher specific gravity

In total, the Lauwersmeerdijk will be lined with about 80,000 m2 of Verkalit blocks. "Each block (30x30x30 cm) weighs 65 kilograms," according to Hans. "Because a 30-centimeter deck was chosen, we had to increase the specific gravity of the concrete to 2,650 kg/m3. The blocks are applied mechanically by the company Boot G.W.W., and they are very deft at it. They have developed their own clamp with which they seal 2 m2 in one movement. Thus they achieve a production of no less than 1,000 m2 per day with one crane. That really is a major achievement. With other systems you can't achieve that by a long shot. Because the blocks interlock, they create a hinged concrete blanket that moves with the profile of the dike. It thus compensates for inevitable weak spots in the structure of the dike, and makes damage perfectly visible."

On a small section at the head of the dike, the Verkalit blocks were applied in varying heights of 20 and 35 centimeters. "Here the wave is driven at an angle and there is a chance that it will rise too high. With the grid profile, we ensure that waves are additionally inhibited," Hans explains. LBN Concrete Products produces the blocks in heights from 15 to 40 centimeters. "They are of course delivered under KOMO certification and the thaw and salt tests show that they are extremely suitable for saltwater applications. Finally, the great thing is that nature has room to develop between the blocks. On the parts that have already been laid, you can see a green glow developing. So at some distance, it will soon look like a natural, green dike, but one that can withstand violent storms."