Amey has infilled a disused railway tunnel underneath the approach roads of the Forth Road Bridge using expanded polystyrene (EPS) blocks.
The 420m tunnel, which runs underneath the A9000 and B981 on the northern approach to the Forth Road Bridge, is 4.3m wide and 5.1m high, and was degrading. A failure could potentially have had an impact on the roads overhead.
The EPS blocks were manufactured to a specific compressive strength capable of resisting the weight of rock and tunnel lining in the event of a localised failure. Unlike with concrete or aggregate material, EPS blocks can also be easily removed if the tunnel ever needs to be reopened.
Once offloaded, the blocks were passed down the access shaft and transported along the tunnel to the work face hooked onto a specially designed sliding monorail system. When completed the access shafts were filled with concrete.
The tunnel was lined with a hydrocarbon resistant membrane, before a total of 21,342 EPS blocks were installed, built up gradually.
Amey operating company representative for the Forth Bridges Unit Mark Arndt said: “This has been an unusual and interesting project where we’ve learned something new about the history of the area as well as gaining the satisfaction of making a disused tunnel safe.
“The team deserves particular credit for developing innovative solutions that maximised workforce safety while minimising the cost to the public purse and the impact on local communities.
“It’s a real measure of success that most local residents were not even aware this work was taking place, despite the tunnel emerging within metres of homes in North Queensferry.”
The structure originally formed part of the Dunfermline to North Queensferry railway line, providing a link to the ferry service until the opening of the Forth Bridge in 1890 and continuing in limited use for freight until 1954.