Working below water level

News item
02-10-2019
Droogzetkist

A small part of the work on the new sea lock at IJmuiden requires a special approach. In the case of the inside of the lock chamber, the work has to be carried out in the water and even below water level. For this work, building consortium OpenIJ uses a temporary cofferdam solution placed against the combi wall of the lock chamber. A temporary cofferdam is a steel structure that enables contractors to perform their in-water work in a dry condition.

Droogzetkist, the Dutch word used for this temporary cofferdam solution, is not found in the leading Van Dale dictionary of the Dutch language. ‘That is no surprise’, said Supervisor Luuk Groot and Planner Rob Kanters of building consortium OpenIJ. ‘We more or less invented that word ourselves’. Groot explained why a droogzetkist is necessary here. 'Most of the work on the lock can be carried out on land by ‘dewatering’ - removing excess groundwater from the construction site. Dewatering, however, is not possible at (part of) the inner lock head and also no option for the concrete pouring operations for the smooth concrete panels attached to the raw walls of the lock chamber. These are on-site and in-water operations and that is how we came up with our temporary cofferdam solution’.

Leaks

The temporary cofferdam is attached to the combi wall of the lock chamber and then sunk to its required depth. For the sinking operation, the ballast tanks of the temporary cofferdam are filled with water. A crane assists in lowering the cofferdam. Once at its required depth and exact location, the temporary cofferdam is sealed and made watertight and after the water has been pumped out of it, concrete workers can carry out their work in the open air below water level. ‘Safety is of the utmost importance here', Kanters pointed out. ‘The temporary cofferdam will never be hundred percent watertight, so a small amount of leakage is allowed. During the preparation stages, we tested the rubber seals for a period of nine months before we were satisfied. If necessary, we will use divers to stop any leaks under the water line that cannot be sealed from inside the cofferdam and any leakage that still might occur will be pumped out by a deep well pump’.

Outside the fairway

The 14.5 metre long temporary cofferdam was lowered into the water for the first time in September 2018. The temporary cofferdam will have to be hoisted eleven times into and out of the water to carry out work below the water level at eleven locations. That is a lot of work. Would a larger size cofferdam not have been more economical in this case? Kanters explained: 'Of course we have thought about this, but a larger size cofferdam would simply have been too heavy and, moreover, the strain on the structure would have become too high'.

The activities involving the temporary cofferdam form no obstacle for ships and companies in the Amsterdam port region. That is because they are taking place inside the new sea lock - an area which is still outside the normal set of fairways to Amsterdam.

Hoist, relocate, and sink

Each hoisting, relocating and sinking operation of the cofferdam takes a week. Each of the eleven locations requires five weeks of steady work. ‘At the moment we are at location number six', Groot explained. ‘We continue working with the cofferdam throughout autumn until we have reached location number eight. Work at the last three locations cannot start before other activities there have been completed. We have scheduled another three more cycles of six weeks at each location for the end of 2020.’