Characteristic of Zoutkamp and its surroundings is the complex soil structure with alternating overconsolidated sand layers and pot clay. This presents quite a few challenges for the New Water Works project in Zoutkamp, since a lot of soil is being disturbed for the construction of the new pumping station and the lock expansion of the existing monumental Hunsingo lock. In order to steer all this in the right direction, the help of CRUX, an independent consultancy in the field of geotechnics, geohydrology, soil and environmental influence, was called in.
CRUX is no stranger to both main contractor Strukton and structural engineer Nepocon. "We already know both parties from previous projects and were asked to work with them on the geotechnical part of the design," says Alwin Rutgers, senior geotechnical consultant at CRUX. "For the new pumping station and lock extension, we provided the cofferdam and foundation advice. We also prepared an integral deformation analysis in connection with the excavation work of the cofferdams. The focus here has been on the Plaxis 2D analyses of the soil deformations at the site of the building pits, the existing Hunsingo lock, swelling from the pot clay layer, the moment progression in the existing wooden piles of the historic Hunsingo lock and the new pile foundation of the pumping station and lock extension to be built."
The complexity of the soil conditions here is quite challenging, Alwin emphasizes. "In addition to the overconsolidated (strained) sand layers, we also have to deal with a very compact clay layer in the subsoil that also has a varying thickness from 10 to as much as 20 meters across the entire cross-section of the newly built structure. The excavation of the construction pit usually leads to a volume increase (swelling) of the cohesive layers and thus also the pot clay layer. This swelling can exert an additional load on the structural floor and piles of the new pumping station and lock extension with which it will be anchored in the deep sand layers. Also, this may have an effect on the foundation of the monumental Hunsingo lock."
Stirring in the hard sand layers can also have a negative effect on the lock. "Therefore, we prepared an integral deformation analysis to assess whether everything remains balanced in terms of deformations and forces, and whether the forces that occur on the foundation of the existing lock are acceptable. Through smart design, this was indeed found to be the case, but we did recommend, in terms of implementation, that visual inspections and monitoring be included as a management measure."
CRUX also worked out the cofferdam and foundation structures of both the pumping station and the expansion of the Hunsingo lock. "For both structures, we advised an excavation in the wet with the application of an underwater floor," Alwin explains. "Furthermore, stamped sheet pile walls were applied to absorb the forces. Because of the horizontal stability of the structure, part of the building pit of the pumping station was also anchored with grout anchors. Incidentally, we proposed not to vibrate the sheet pile walls. This in connection with the overconsolidated sand layers and its influence on the pile foundation of the existing lock. We also advised to first pre-till the soil at the pumping station, in order to reduce the resistance when the dam walls are pressed in by the hard sand layers. And this has worked out well in practice, as the building pit and foundation structures of the pumping station have been successfully installed."
Ground displacement combi piles were used at the pumping station and self-drilling anchor piles were used at the lock extension. Currently, the lock extension is still under construction. Because the sheet piles are not as deep there, pre-drilling is not necessary there and the sheet piles can be pressed in without measures. CRUX's job is therefore over, although Alwin says they are on standby for any advice or questions during execution. Despite the complex soil conditions, Alwin already looks back on a project that went smoothly. "At CRUX we have a lot of knowledge and expertise in-house. This also proved to be of great added value here in Zoutkamp. Geotechnical and environmental engineering is our core business. We have done this before, and that is precisely why we are called in throughout the Netherlands for this type of complex work. I think that the 'crux' of the collaboration with both Strukton and Nepocon is the joint approach during the design and execution phase, the pursuit of a robust and innovative design with an eye for the environment and the practical approach together. Especially the short lines between the work planner (Strukton) and the structural engineer (Nepocon) were very effective and we experienced them as positive."