Zemst lock – Restoration of a mitre gate
The Zemst lock is an important link on the Brussels-Scheldt maritime Canal. Every day vessels and pleasure craft make use of the lock to travel along the Canal, making the lock a deciding factor in the travel time of these vessels. In 2012 SBE has designed the replacement of the mitre gates in the middle lock head in order to reduce the leveling time between the lock chambers as much as possible. This Blog describes the challenges that came with this renovation project.
The lock has a total length of 250m and a maximum lift of eight meters. On the upper head a vertical lift gate is installed, whereas the lower head uses a mitre lock gate. The lock chamber can be divided into two smaller ones by intermediate mitre lock gates. This allows a faster locking cycle for small ships and reduces the water losses. On average it takes 17 minutes to pass the lock. Watch the video on the project’s page to learn more about the lock.
On weekdays, the lock is filled or emptied about 40 times per day (cfr. diagrams shown below). Although the lock is manned 24/7 most of the traffic occurs between 6 AM and 7 PM. In weekends the use is reduced to 15 times or even 5 on Sundays, where only pleasure craft passes the lock. The constant traffic through this important link on the waterway gave rise to the question if it was possible to reduce the levelling time on such a way that the lock was never out of use.
In 2012 SBE made a complete redesign of the middle mitre gates aiming to significantly reduce the leveling time of the small lock chambers. The old gates had each 2 butterfly valves of diameter 900mm whereas the redesigned gates have each 4 butterfly valves of diameter 1600mm. This is a reduction of the levelling time by a factor 6!
One of the biggest challenges of the project was designing a new gate which was compatible with the existing civil works. The position of the gate hinges (pintle and gudgeon) as well as the quoin blocks needed to remain unchanged. Moreover, the load transfer to the civil works could not change. Due to the larger butterfly valves it was difficult to install a floating tank with approximately the same floating capacity as in the existing gate, so a slight increase of the reaction forces due to self-weight was inevitable.
The client clearly demanded that the watertight skin plate of the new design had to be on the upstream position, as in the old design. This meant that, when subjected to a hydraulic head, the gate experiences a large uplift force which couldn’t be compensated by the self-weight itself. To ensure the vertical stability of the lock gate, it was necessary to consider friction as well at the level of the quoin blocks at the gate end post. Due to the large dimensions of these quoin blocks, a considerable bending moment needed to be introduced in the gate. A schematic overview of these loads acting on the quoin blocks is shown in the figure below. SBE had to be creative to detail the connection of the quoin blocks.
For more information about the Zemst lock, click here.