Introduction
The lock in the Södertälje Canal is situated between Lake Mälaren and the Baltic Sea. It was built in 1924 and with a chamber length of 135 metres and a width of 20 metres; it is the largest lock in Scandinavia. The average differential head is 60 cm.
The construction of a new lock is necessary to meet the growing volume and size of marine traffic. Over the years, not only the amount of shipping has increased but also the size of the seagoing vessels. The new lock will have a width of 25.3 metres and a length of about 170 metres. The renewal of the lock is part of the large-scale Mälaren Project.
The renewal project consists of the extension and widening of the lock chambers, the construction of two new lock heads and lock gates plus a new bascule bridge. The client is the Swedish Maritime Administration and the project is performed by Züblin Scandinavia AB partnering with the client. Design and engineering of the lock heads including the sluice gates are performed by joint venture S3P, consisting of the two Dutch engineering firms MH Poly and Iv-Infra.
Special Lock Gate with a Special Construction Method
As requested by the client, a special type of lock gate will be used: a segment gate made of duplex steel (a type of stainless steel). This type of gate is rarely used for locks in The Netherlands. Duplex steel is more expensive than regular carbon steel, but it has the benefit that maintenance during the structure life is minimized. The gates are partly circular, rotate around a horizontal axis and are also used for the levelling of the chamber. In addition, the gates can hold water in both directions and can be rotated up and above water level for inspection and maintenance.
Another unique feature of this project is that the lock heads will be built at the side of the existing canal, after which they will be moved to their final position as complete structures. As a result, the canal can remain available for shipping as long as possible during construction.
Structural and Mechanical Design - Details of the Leaf Gates
The leaf gate has a skin plate formed as a cylinder segment, connected by 2x2 radial arms to two main axles situated in the machine buildings. At the back of the skin plate there are two buoyancy chambers. Here, main and secondary girders and stiffeners are placed. The buoyancy chambers ensure that the gate will tend to close automatically, although it will be locked in position when opened (or closed) for safety reasons. The circular shape of the skin has the benefit that the line of action of hydrostatic pressures stays close to the main axles and that the arms bring transfer of forces to it by normal force. To avoid ships damaging the gate arms, a modular fender structure is mounted on the lock head and between the gate arms and the canal.
The interface between the gate seals and their opposite surfaces in the lock head is complicated, because of the circular shape on the sides and it needs to be both watertight and durable. While opening, due to the innovative arrangement, the side seals will move away from their contact surface, instead of sliding along it, as such limiting friction and wear of the seals. While opening, the gate will temporarily be stopped after 5°, where an indented part of the skin forms a maximum opening to allow the water in the lock chamber to level with the water on the other side of the gate.
Each gate is moved by two hydraulic cylinders at each main axle. A main bearing near the gates arms and a secondary bearing inside the machine building supports each axle. Each axle assembly, the pulley plates, the main and secondary bearing, and the drive cylinder are integrated on one skid, which can be positioned and aligned in the desired position and direction, both at the start of the project and if necessary, adjusted in a later stage. This ensures that any eventual settlements over time will not lead to unnecessary high stresses, friction and wear in the main bearings.
Construction Method
To allow ship traffic to continue during most of the construction phase, the lock heads are constructed in building pits at the side of the canal. Construction of the lock head is thus carried out in dry conditions. The interface area for the gate seals consist of embedded duplex steel and are prepared in a 2nd stage concrete cast so that they can meet the stringent geometrical tolerances required for such areas. Mock-up tests with the drive system and with dummy structures for the seals will be also carried out in these dry conditions.
The original plan was to move the lock head from the construction pit to its final position by skidding, but after an evaluation of schedule, risks and costs, the alternative, based on floating was chosen. When the lock head is in place, it will be temporarily supported by four jacks at the corners. Hereafter, the gap between the canal bottom and the lock head will be filled up with concrete. Moving and sealing the lock heads on their final position is to be done within an outage of maximum one week each.
Interfaces
Building on top of the footprint of an already present lock and dealing with surrounding existing buildings and infrastructure means that the surrounding buildings, temporary structures and new structures which add to the lock function itself, like the bascule bridge, the moveable walkway and the machine buildings, all require specific attention to interfaces. There are many situations where interference can be critical, which requires the various design teams to work together seamlessly.
The presentation will provide more examples and details of the design and elaborate on the process followed to realise this project.