Restoring the 'hidden little gem' of canal tunnels

Engineers will use a combination of traditional methods and modern technology to repair one of England's oldest canal tunnels.

Preparations have begun ahead of work starting on the Preston Brook tunnel which is on the Trent and Mersey Canal in Cheshire.

The Canal and River Trust is undertaking the detailed inspection using both traditional hands-on inspections alongside 3D mapping and drone footage.

Peter Simpson, principle engineer for tunnels and culverts at the trust, said the biggest challenge is gaining the "knowledge and understanding" of the condition of the tunnel before any repair work can begin.

The canal opened in the 1770s and is 1,138m (3733ft) long.

It is owned by the Canal and River Trust, along with 56 other tunnels across England - nine of them in the North West.

They range in age from the 18th Century Preston Brook Tunnel to the Liverpool Canal Link, which only opened in 2009.

Simpson said the tunnels are the "hidden little gem" of the canals network, as a number of them are not open to the public.

As well as the analysis of the tunnel itself, investigation work will be done on the tunnel's four air shafts.

One of them, which dates from the 1980s, will involve engineers abseiling down a 17m-deep shaft.

But that is not what he sees as the biggest challenge.

Simpson added: "It really is knowledge and understanding.

"We know when they were built, and roughly how they were built.

"But it's then building that knowledge again because over the years, information has been lost.

"Being a charity, we've only got a finite level of resource and so we have to make sure we've got a full understanding of everything before we commit to spending any money."

Susie Woodward-Moor, principal project manager, said it was important for tunnels to be inspected regularly.

She said: "Those inspections show us what further investigations need to take place, and what we're here doing is analysing and assessing what further work might need to take place to repair the tunnel in specific areas."

She said new technology, which creates a full replica of the tunnel, gave a "useful blueprint" of the condition of the tunnel.

"Not only does that allow us to determine what work we need to do now, but it gives us a really useful baseline for the future, to look at what's deteriorated, and look at what further repairs might be needed down the line," she said.

Part of the 3D modelling will allow the team to "unroll" the image of the tunnel into a flat image, which she said would make it "much easier" to assess the work.

Woodward-Moor added that after the investigation stage, the team would be working with local schools and colleges to showcase careers.

"We do want to encourage people into construction. It's a great industry to be in and we want people to understand what we do, looking after the network itself.

"It is an ageing asset that does need a lot of looking after. And we want people to understand some of that work that we do," she said.

Simpson added many of the lessons that were learned while building structures like the Preston Brook Tunnel had fed into modern projects.

"Even to this day there are tunnelling techniques that developed while building canal tunnels that are still used on major tunnels."

He said while projects like HS2 used "massive tunnelling machines" on a much bigger scale, the principles were similar to a time when it was "a mining shield and blokes with pickaxes".

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