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A masterpiece of engi­neer­ing

Sitterviadukt Gruner Wepf AG, St. Gallen

At a height of 99 meters, the Sittertobel Viaduct is the highest railway bridge in Switzerland. It is around 365 metres long and was built from natural stone and steel between 1907 and 1910. The construction of the listed viaduct with the means available at the time is simply a masterpiece of engineering and architecture. The railway bridge that spans the Sittergraben at Gübsensee was last reinforced and repaired almost 40 years ago. Now, the bridge and the nearby Sturzenegg Tunnel are in need of total overhaul.

Gruner Wepf AG, St. Gallen, has examined the bridge and its condition intensively over a period of 16 years, checked the statics with various re-calculations and implemented some urgent/bridging measures. This 2019 repair project, for which Gruner Wepf AG, St. Gallen, was commissioned as project author for phases 21 through 33, is the result of these many years of examinations and calculations. In phases 41 to 53, the team from St. Gallen is now providing technical support for SOB's project and site management. The Sturzenegg Tunnel is being repaired by third parties.

Swiss Southeastern Railway (SOB) is investing CHF 19.5 million in renovating the tunnel and viaduct, and the structures are being made fit again for the next forty to sixty years. In the Sturzenegg Tunnel, the tunnel vault is being repaired and the carriageway and catenaries renewed. The tunnel profile is being adapted to enable double-decker trains to pass through the tunnel in future. With the Sittertobel Viaduct, the track system and catenaries are being replaced. Furthermore, the ballast troughs on the approach viaducts are being given a new seal and the ballast thickness increased. The steel structure of the fish-bellied girder will be partially replaced or reinforced, the sleeper beam replaced and the corrosion protection renewed so that the bridge can continue to withstand the high loads.



The team of experts is being challenged not only because of the complex statics, but also because of the tight time schedule for the renovation and the exposed location of the structure. For a period of five weeks (from 6 July to 9 August 2019) the line between Herisau and St. Gallen Haggen will be completely closed. As the bridge is cut off on one side by the simultaneous renovation of the tunnel, a scaffolding tower was erected on pillar IV to provide a further starting point for the work. This can also be used for the remaining work on the fish-bellied girder after railway operations are resumed. The renovation project for the viaduct run until spring 2021 and the tunnel are due to run until autumn 2019.

Work during the total line closure:
- Extending the old track system and partial dismantling of the catenaries
-  Partial renewing the viaduct’s steel structure, localised renewal of corrosion protection, fitting the new sleeper beam
- Renewing the sealing of the ballast troughs on the approach viaducts
- Fitting the new track system
- Assembling the new catenaries
- Repair work in the tunnel (by third parties)

The best thing to watch the video yourself to hear what Sarah Bitterli and Roman Schweizer from Gruner Wepf AG, St. Gallen have to say about the existing project:



Gruner Wepf AG, St. Gallen has been working on the following assignments:
- Since 2003: Inspecting the condition of the existing Sittertobel Viaduct
- 2008: Main inspection of the fish-bellied girder (including 1300 nodes/connections)
- Repair in 2019: Pre-project to construction project (including PGV) as project author
- Repair in 2019: Tender design as technical support to SOB
- Repair in 2019: Detailed design/implementation as technical support to SOB


Read more about the project and the historical structures at