Tunnelling Forth

A leading civil engineer has proposed a method of constructing a tunnel on the Firth of Forth seabed that could prove a considerably cheaper replacement for the Forth Road Bridge than a new bridge.

John Carson, former head of Miller Civil Engineering has suggested that his plan to build concrete tubes on the dock and lower them to the seabed would be a more cost-effective and environmentally friendly solution to traffic congestion than the road-and-rail bridge currently favoured by the Forth Estuary Transport Authority (FETA).

The Edinburgh Chamber of Commerce has leant its support to the idea of creating an "immersed tube" running between the Capital and Fife. Previous geological evidence had suggested that tunnelling would be inappropriate, but this method would obviate the need for tunnelling.

No decision has been made on a new crossing, but the existing Forth Road Bridge could be forced to close by 2019, because of corrosion in the main cables, lending urgency to the search for a solution.

Carson, who has more than 30 years' engineering experience and was involved in the construction of the Skye Bridge, said, "The replacement bridge would have to be incredibly long. There isn't a bridge in the world of the length needed to cope with a double-deck, multi-modal design. An immersed tube can be built using local technologies. It would be quicker to construct, more environmentally-friendly and more convenient."

The cost of a new bridge has been put at between £500 million and £1 billion, while Carson claims a "tube tunnel" would cost about £600 million.