Sittertobel Viaduct
A masterpiece of engineering
The original cost of the viaduct 110 years ago ran to around CHF 1.5 million. At around CHF 15 million, the present renovation of the spectacular structure is likely to cost ten times more. But it is money well spent: given the resources available at the time of its construction, the viaduct project was an architectural and engineering tour de force. The rail bridge across the Sitter gorge near Gübsensee lake was last strengthened and repaired nearly 40 years ago. Now, a full-scale refurbishment is needed.
Long years of experience and supervision pay dividends
Gruner Infrastructure Eastern Switzerland has closely monitored the bridge’s condition over a 16-year period, performed structural recalculations and promptly implemented any necessary temporary measures. The 2019 refurbishment project, on which Gruner Wepf AG, St. Gallen, was appointed as project designer for phases 21 to 33, marks the culmination of these long-term investigations and recalculations. In phases 41 to 53, the team from St. Gallen is now providing expert support for the SOB project and senior site management teams. The Sturzenegg tunnel rehabilitation is being handled by a third party. The track and overhead lines on the viaduct are being replaced. The ballasted decks on the side spans are being re-waterproofed and the ballast thickness increased. The steelwork of the inverted bowstring truss is being partially replaced or otherwise reinforced and the anticorrosion finishes renewed to ensure that the bridge can continue to accommodate the high loads.
Complex statics within narrow timeframe
The structural complexity of the viaduct is not the only problem facing the project team. The exposed location and tight schedule for the refurbishment have posed additional challenges: the line between Herisau and St. Gallen Haggen was fully closed for five weeks during the summer of 2019. As the bridge is cut off on one side due to the simultaneous rehabilitation of the adjacent tunnel, a scaffold tower with goods hoist was fitted to bridge pier IV to provide an additional access point to the works. After the resumption of rail operations, this will also be used for the remaining works to the inverted bowstring truss. The refurbishment works will continue until spring 2021.