New rail bridge – in four days flat
Construction of Bern’s new RBS underground railway station began in July 2017. As the current train station is already stretched to its limits, a new underground facility is to be built under Bern main station over the next eight years. The large construction site is accessed via a shaft directly beneath Platform 1. For the trains to run as usual, a temporary rail bridge was built across the shaft in May 2018 – in a record time of just four days. This feat was accomplished by team of experts from Basler & Hofmann, who spent two years planning and coordinating this crucial moment.
The construction site on Platform 1 is unusually quiet. Voices are hushed; not even a cigarette is lit. At any moment the critical operation commanding everyone’s absolute concentration will begin. One of the two mobile cranes is moved a couple of centimetres at the last minute. Its metal feet have to have a firm grip of the surface. Along with the second crane, it is about to lift and position a 24-metre steel framework weighing 14 tonnes. The problem is not the weight of the framework, which is borne by the two mobile cranes, but the lack of manoeuvring space at the west end of Platform 1 at Bern main station. The overhead line is dangerously close – and it must not be touched or damaged at all cost.
Two years’ planning
A double-decker Intercity train is pulling out on the next platform. On Platform 1 a large hole gapes in the track: it is the building pit of the Laupenstrasse access shaft – one of the planned accesses to the underground construction site of the new RBS underground railway station. Yet the platform may only be closed for four days to allow the building of a temporary rail bridge in this short time. It will safely carry trains across the plunging, more than 30-metre-deep diaphragm walls of the shaft, which will then be excavated to a depth of 17 metres. Today is Saturday. “By Wednesday, trains will be running as usual,” says Martin Schäfer. He is a construction engineer at Basler & Hofmann, and, as site manager of this section, he is responsible for the upcoming manoeuvre.
Two years of co-planning on the part of the Basler & Hofmann team, RBS and SBB is about to be realised in the space of this one weekend. As Karola Wunder, construction engineer at Basler & Hofmann and responsible for liaising with SBB, remarks: “A huge coordination effort was necessary to ensure that the bridge can be built within such a tight schedule.” Organising the closure of the platform and deactivation of the overhead line in close cooperation with SBB, for instance. Special machinery, such as a large track gantry crane, had to be reserved, and deployments of various SBB specialist services had to be precisely coordinated so that the rails, overhead line and all the other railway systems would be back in place and ready to go by the end of the weekend. Execution was therefore planned minutely down to the last quarter of the hour. Night-time construction at Platform 1 in preparation for the bridge has been going on for six months. The construction crew removed the rails on Friday evenings and replaced them again on Sunday evenings to work below the platform. Now, at last, the bridge is about to be built – and the crucial moment about to take place: the assembly of the steel framework that will support the bridge.
Meanwhile, the framework, waiting on an articulated lorry, is lashed to the mobile cranes. The start signal is radioed by the site manager. The cranes slowly lift the steel structure into an upright position. The 24-metre structure is hanging above the building pit, just below the overhead line.
A new train station in eight years
The building of the temporary bridge is part of a large-scale underground project at the heart of Bern main station: The RBS underground railway station is already stretched to its limits by around 60,000 passengers daily. A situation that will become more acute in the upcoming years. Based on figures from 2016, an increase of around 40 per cent is forecast by 2030. Bern central station is being completely reorganised by 2026 in order to cope with these passenger volumes: the RBS underground railway station is to be relocated, and in the future will run parallel to and directly beneath SBB Platforms 2 to 7, letting passengers easily change from long-distance to local trains via a new mezzanine level. The future RBS underground railway station will have two large underground concourses, each with two tracks and a 12-metre-wide central platform. Basler & Hofmann holds overall responsibility for the new underground railway station.
The two mobile cranes have now positioned the framework over the support slabs, one at either end of the more than 20-metre-wide shaft, which the bridge will span. Site workers place themselves around the support slabs to position the framework exactly. “That was the toughest part,” says site manager Martin Schäfer, relaxing visibly. The next few hours will be devoted to mounting the crossbeams. At the side facing Platform 2, they will rest on massive, previously erected diaphragm walls, which at the same time close off the shaft. At the other side, the crossbeams are bolted to the framework. Concrete elements, ballast and finally the rails will then be placed on top.
Overall, construction work for the new RBS underground railway station under the rail bridge will take eight years. When the bridge is taken out of service in 2026, it will no longer span merely a shaft, but a brand new train station.
Stakeholders
Client: RBS
Planning syndicate: PG RBSverbindet (Basler & Hofmann, Emch+Berger, B+S, Theo Hotz Partner
Time-lapse video of the rail bridge installation