Immersed and floating tunnels across Lake Zuerich

Options investigated to determine potential road networks for Zuerich in the year 2020 have included a ring road around the city. This would enable through traffic to avoid the existing city congestion. One of the impediments to such a scheme is Lake Zuerich, a long narrow and deep lake that extends into the heart of the city from the south. The beauty of the lake, nestled between steep hillsides, is treasured by the local residents, so that potential bridge crossings are therefore highly undesirable. Options for potential underwater crossings have therefore been investigated. The geology of the lakebed indicates that bored tunnel crossings should preferably lie within or below the lakebed glacial clay layer, rather than in the unstable chalk above it, increasing the necessary depth of bored tunnels and hence increasing considerably the lengths of approach tunnels. While it is unlikely that seismic loads would affect tunnel design to any great extent, many areas along and in the lake slope steeply and are unstable. Mapping of the lakebed also shows a number of holes in the bed of the lake, some extremely deep, that may once have been full of ice but have since filled with soft material. These issues must be considered in determining appropriate tunnel solutions. The proposed alignment across the lake would be 2 km long, mostly totally exposed within the water column (i.e. ''floating''). The maximum water depth at that location is 51 m. The question of whether to have a tunnel exposed in the water at all (a "first" of its kind), and if so at what depth, could be complicated by the fact that the exposed tunnel may form an obstruction to the hydraulic regime of the lake which is considered excellent drinking water. A very extensive zone of lakebed deposits is unstable material 30 m thick that has failed and is still moving, extending 500 m into the lake from the eastern shore. Any floating or immersed tunnel crossing this area will only be possible if this material is stabilized, at least in the immediate vicinity of any tunnel supports and where the tunnel passes through the failed layer into the underlying stable layers beyond. The cost of remedial work is likely to be extremely high and it is innovative, but is expected to be possible. Floating tunnels of the length being considered would either be positively buoyant and tied down to anchors at bed level, or would be negatively buoyant and sit on bents, either pile-supported or on spread footings. The bed of the Lake Zuerich is not far below the tunnel, so that the best method of holding the tunnel in place appears to be setting it on piers and caissons, with or without piles. Using such a method of support would effectively make the tunnel into an underwater bridge. As such, none of the techniques required to execute the construction would be new; existing techniques would merely be applied slightly differently than hitherto, despite the absence of access by sea-going vessels. A more northerly alignment between the portal at Brunau and the interchange at Burgwies could still have interchanges that meet the lakeshore roads each side at the same points as the originally proposed alignment without entering any areas with water depths in excess of 20 m so that a traditional, fully buried, steel or concrete immersed tunnel 1083 m long can be used. It could be particularly advantageous to construct the immersed tunnel and the adjacent junctions many years before the whole ring road scheme now under consideration is built; it might help to relieve the severe traffic congestion that currently exists in the downtown area of Zuerich. It would also be possible to spread the investment in the immersed tunnel over a longer period by building half of the immersed tunnel, all of the land transition structure, and half of the land tunnels now, reserving space for building the other half later. This article is produced by kind permission of ILF Beratende Ingenieure AG and ARGE ZUERIRING. (A). "Reprinted with permission from Elsevier". For the covering abstract see ITRD E124500.

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  • English

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  • Accession Number: 01011632
  • Record Type: Publication
  • Source Agency: Transport Research Laboratory
  • Files: ITRD
  • Created Date: Dec 19 2005 3:22PM