How does one replace a rolling gate track located 20m below water level?
This question turns to be actual in many locks with rolling gates, as maintenance or replacement is required more and more in the Belgium sea locks after long years of service. When those activities are local, one might tend to use a habitat and work under pressure. However for the replacement of the rails over the whole length of the gate such a working method is not the most ideal way of handling things.
The suitable way to do these kind of replacements, is the use of a ‘drooglegkuip/caisson’. Hereby a closed steel frames (shell/hull/shed) is placed on the bottom over the length of the rails in the lock chamber (not in the gate recess) This closed steel frame is the caisson.
At the recess the caisson is open and one can walk freely from the recess into the caisson. To seal and have a watertight contact, the caisson is placed against the vertical abutments. Also at this side, the recess closure is put on top of the caisson and also against the vertical abutments. At the far end of the caisson, at the other side of the recess, the caisson has a closed end and makes contact with the bottom. Finally, once the caisson and recess closure are in position, both can be pumped dry. Before the placement of those two steel structures, the lock gate is brought already into its maintenance position in the recess.
The major advantages of this system are the atmospheric and dry conditions in which the work can be done. There is no need for divers or working under pressure to replace the rails. It’s evident that this has a large impact on the quality of the work (alignment, installation of the anchors, checks & verifications, …)
In the past SBE has transferred this theoretical idea of a caisson into a practical set of steel elements. All together this set forms a modular system that can be installed in all the Belgium sea locks.
The system consist of three standard elements, each with a different length. Changing the configuration (using 1, 2 or 3 elements) allows to cover every width of every sea lock in Belgium. The only parts that need to be made individual for each lock, are the two end pieces. As variations in the details exist from one lock to another. For example at the abutments of the recess or in the bottom profile on which the closed end part rests.
The modular system offers a major saving in material use. As only the small end parts need to be fabricated individually to every lock. The standard parts can be reused for each lock. Another benefit of the caisson is the hoisting beam over the length of the steel construction. This beam helps in transporting the rails or other material from the recess to the far end of the caisson.
While working with the caisson safety is an issue. So once workers are into the caisson, ship traffic is not allowed in the lock.
Meanwhile the caisson has proven its service in different Belgium sea locks, such as the Kallo lock, Zandvliet lock, Boudewijn lock and the Vandamme lock.