A dredged-out lock chamber

In 11 weeks’ time, 600,000 m3 of sand has been dredged out of the lock chamber by cutter suction dredger 'Biesbosch'. The lock chamber is now at a depth of 19m below NAP. The bottom of the lock will be made of gravel and concrete which, when completed, will give the lock a maximum depth of 18m.

Wessel Mak, Project Manager at Van Oord, is responsible for the dredging of the lock chamber. He has been involved in the construction of the new sea lock since 2014 and gave us an update on the dredging operation.

Wessel Mak told us, "We started our dredging operation of the lock chamber on 29 June. After 11 weeks, the lock chamber was at the required depth. The edges of the lock chamber had been cleaned and the walls sprayed clean by divers. The only remaining item now is the construction of the bottom of the lock."

The whole process

Dredging the lock chamber was an important and extensive process. Wessel explained what the work involved, "We dredged the lock chamber with the cutter suction dredger and a crane pontoon. The cutter suction dredger moved back and forth by means of its anchors. Meanwhile, the cutting head cut loose the sand in the lock chamber. The cutter suction dredger then sucked up the sand. The sand was pumped through pipes, containing a mixture of sand and water, leading from the lock chamber to the spray pontoon. From the spray pontoon, the mixture was pumped into the spray depot situated in the fairway of the outer approach channel to the North Lock. From here, the sand was dredged up again with the trailing suction hopper dredger and then transported to sea. At sea, the sand was dispersed at a designated location."

Dredging is a recurring cycle continuing 24 hours a day. Mak added, "Dredging is always a process that goes on 24/7. That's what dredgers and dredging materials are made for. Dredging continued at night to reduce inconvenience for stakeholders, such as local residents and users of the lock, as much as possible. The crane pontoon dredged the areas directly next to the lock chamber walls. Just like the divers who cleaned the walls, the crews of the crane pontoon also worked 24 hours a day for 5 consecutive days. Dredging activities were planned as much as possible during the day, with extensions into the evening and the night.

The lock chamber has now been dredged to its required depth and the gravel/concrete bottom will be poured in the autumn. Wessel explained, "The lock chamber has to be delivered neatly at its required depth. We dredged up to 19m below NAP. The next step will be the pouring of a layer of 30-50cm of gravel with a layer of underwater concrete on top of that." At around 30,000 m3 this will be one of the largest concrete pours of the new sea lock.

Communication as a challenge

In such a large and complex process, there are many things that had to be taken into account. You are not the only worker at the lock, you see, Wessel explained. He added, "Communication, in particular, was one of our most important challenges. Our work was not limited to the lock chamber, but we were also active at sea to distribute the dredged material. It was important to coordinate this process very accurately. We therefore had daily meetings during which we planned the work ahead. The dredging of such a large lock chamber was really unique!” Wessel, proud of his contribution to the new sea lock, added, "I think you can only build the largest sea lock in the world once in your entire life.