Stainless steel supports new electrification system in Severn Tunnel

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Over six weeks in September and October 2016, Network Rail—the company that manages most of the UK’s rail infrastructure—installed an innovative system for electrification of the railway track through the Severn Tunnel. This seven kilometre long, 130-year-old tunnel links the west of England to South Wales. Instead of traditional overhead wires, the system in the tunnel consists of an aluminium rail (which carries the electrified contact wire), supported from the tunnel roof by drop tubes and registration arms. Made by Swiss company Furrer+Frey, the Rigid Overhead Conductor Rail System (ROCS) is more robust and efficient than overhead wires, with reduced maintenance requirements. It is also more compact than the traditional wired system and can be used in tunnels where headroom is constrained.

The environment inside the Severn Tunnel is aggressive due to the ingress of chlorides from the saline water in the Severn Estuary and dust deposition from freight trains carrying coal which pass through the tunnel. For this reason highly alloyed stainless steel was selected for all of the structural steelwork supporting the ROCS in order to provide an 80 year design life with minimal maintenance. Tunnels by their nature are difficult environments in which to undertake construction and maintenance activities and consequently the additional material costs of stainless steel are acceptable when the system is viewed from a whole life cost perspective; a large part of the capital cost is obviously also due to the labour and plant costs.

M16 anchor rod fixings made from super austenitic stainless steel (UNS N08926/N08367) support the auto transformer feeder cables (at 2m centres) and the baseplate/stovepipe for the ROCS (at 8.5m centres). The rod fixings were resin anchored into the tunnel brickwork to a depth of 315mm. Super duplex stainless steel 2507 (UNS S32750) was used for the baseplate and stovepipe which holds the registration arm and for the cantilever structural components of the registration arm for the ROCS.

Austenitic stainless steel grade Type 316L (UNS S31603) has been used for the supporting steelwork for the ROCS in five other tunnels with less aggressive environments, as part of the Great Western Electrification Project, which electrifies the main line railway from London to South Wales.

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