Because it is used by very large maritime vessels, extreme forces are encountered along the seabed in front of the lock. This area is dredged to around 20 m below the mean sea level (MSL) to ensure adequate depth, and is protected by a layer of heavy rock armor placed on filter layers which fulfill hydraulic design requirements. To give the rock armor a greater resistance against scour, it is subsequently grouted using a special underwater grouting technique. A particular challenge in this project are rare but possible extreme low-tides which could result in excess pressures under the fascine mattress with consequent damage to the erosion protection. To overcome this problem, a granular drainage layer was planned under the mattress with relief wells to release excess pressure. However, such a granular layer would be very difficult to install under the conditions prevailing. Therefore, a uniquely engineered alternative solution was required. Enka Solutions was invited to support this engineering challenge.
THE ENKA SOLUTION
Protection of seabeds against scour is often achieved by the use of prefabricated fascine mattresses and the application of rock armor. These mattresses normally consist of several layers of geotextiles (nonwovens, and woven fabrics with loops) and bundles of willow branches (wieps) attached in a square pattern. This type of fascine mattress is well known in the Netherlands and has been used for decades to install filter layers below rock armor in all kinds of water conditions. With this as a starting point, a new solution was engineered in consultation with a team of project-aligned civil and hydraulic engineers, the contractor, and the project owner.
The solution can be summarized as follows:
- Use the standard engineered fascine mattress
- Integrate EnkaDrain ZB 350 between the different layers of the mattress
- Prefabricate the newly designed composite elements at the production facility of the supplier of the fascine mattress
- Fix different prefabricated elements together at the project site, then tow them to the planned position and submerge using specialized vessels
- Apply the grouted rock armor and relief wells
EnkaDrain ZB is a high-end drainage composite consisting of a very stable drainage core with heavy nonwovens connected to both sides. Including EnkaDrain ZB in this custom-made geocomposite mattress ensures safe transportation of rising groundwater towards the relief wells installed in case excess pressure occurs during extreme low tide conditions.
INSTALLATION BENEFITS & RESULTS
The new lock will be 500 m long, 70 m wide and 18 m deep, making it the world’s largest sea lock. Construction began in January 2016 and the new lock will be available for shipping in 2022. Its size ensures accessibility for the largest modern vessels to pass the North Sea Channel and enter the Port of Amsterdam. The unique engineered solution provides durable and safe bottom protection at the lock entrance and is ready to cope with extreme low-tide conditions. EnkaDrain ZB integrated in the protection system prevents damage to the erosion control system at extreme low tides, ensuring a safe construction for coming generations.
Additional benefits include:
- Replacement of 0.30 m conventional gravel drainage layer by an 8 mm drainage composite
- Reduction of dredging level over full area
- One-step installation on site
- Long lifetime expectancy thanks to EnkaDrain ZB’s stable drainage core and heavy filter layers incorporated
- EnkaDrain’s heavy filter layers prevent fine soil moving with the rising groundwater into the drainage core, and this ensures erosion protection from underneath