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Consulting Engineers

How can 20,000 polystyrene bricks stop a tunnel collapsing?

What can you do with a disused tunnel that’s starting to crumble – and putting a key road bridge at risk?

Engineers in Scotland decided on a novel solution – they filled a Fife tunnel with more than 20,000 polystyrene blocks.

The tunnel runs under the approach to the Forth Road Bridge and was originally part of the Dunfermline to North Queensferry railway line. It was a link to the ferry, until the opening of the Forth Bridge in 1890. After that it was used for freight, until it closed in 1954.

When the tunnel was inspected in 2016, structural engineers found that parts of the tunnel were degrading and they needed to put a plan in place to prevent further issues.

Both ends of the tunnel are sealed and the only access is via a vertical shaft. The advantage of polystyrene is that the blocks can be carried by hand, so workers were able to handle the lightweight material very easily.

A similar density to the packaging material in the picture above, the large, white, pre-cut polystyrene blocks are highly manoeuvrable. They were moved into place in the tunnel using a specially designed sliding rail.

The beauty of this solution is that if the tunnel ever needs to be reopened, the blocks can be easily removed.

Engineers used lightweight expanded polystyrene for this unusual project, and it demonstrates the usefulness of polystyrene in engineering and construction.

We’ve used foamed concrete in projects where the concrete may need to be removed at a later date. Polystyrene balls within the concrete make it lighter and easier to take out, and it can work well in foundations where loads are spread.

Foamed concrete can also be used to fill voids, and while it’s easier to remove than conventional concrete, it’s not quite as simple as carrying a polystyrene block out of a tunnel.

Our team of engineers are working on a wide variety of projects so if you need any help or assistance with the structural side of things, please do get in touch.

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