Amey, the Forth Bridges Operating Company, has completed an unusual project to infill a disused railway tunnel underneath the approach roads north of the Forth Road Bridge.

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.

The tunnel runs underneath the A9000 and B981 on the northern approach to the Forth Road Bridge. It is 420 metres in length, 4.3 metres wide and 5.1 metres high, with a vaulted roof and brick lining. Both ends had been sealed off and the adjacent cuttings filled in, so the only remaining means of access was via a vertical shaft at each end.

Amey engineers carried out a structural inspection in February 2016, finding that parts of the tunnel were degrading and in need of preventative maintenance to ensure continuing structural integrity. Due to the limited depth of cover above the tunnel, a failure could potentially have had an impact on the roads overhead.

Two options were considered: an ongoing programme of inspection and maintenance, or a one-off project to infill the tunnel with a low cost material. The infill option was chosen as it would eliminate the need for future inspections or maintenance and so prove more cost-effective in the long term.

After considering workforce safety, overall cost and the need to avoid disruption to the local community, it was decided to fill the tunnel with expanded polystyrene (EPS) blocks 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.

The EPS blocks were pre-cut to a size and weight that allowed easy manual handling on site. This allowed work to be carried out from the access shaft at the north end of the tunnel, keeping construction traffic out of North Queensferry for the majority of the works. Another advantage of the lightweight blocks was that they could be delivered in large lorry-loads, significantly reducing the number of vehicle movements required.

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.

The tunnel was lined with a hydrocarbon resistant membrane, before a total of 21,342 EPS blocks were installed, built up gradually in steps to allow safe working at height.

Local primary school children from Burntisland and Lauriston were invited to fill two time capsules with items of their choice. These were then buried in the tunnel amongst the blocks.

Once the body of the tunnel was infilled the access shafts were filled with concrete to seal the tunnel and prevent damage to the blocks, with work reaching a conclusion in late March 2018.

Mark Arndt, Amey’s Operating Company Representative for the Forth Bridges Unit, 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.”