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Insight | Balfour preps Welsh rail tunnel for mega-cycleway revamp

The 128-year-old abandoned Rhondda rail tunnel in Wales is set to be transformed into Europe’s longest cycleway. 

Balfour Beatty has set the transformation on its way by carrying out the initial examination on its structure.

The work being carried out by the contractor was a precursor to the tunnel – together with the Abernant tunnel - receiving a £250,000 grant to carry out additional detailed inspections and analysis of the work needed to make the reopening of both tunnels a reality.

Balfour Beatty examining engineer Richard Storey carried out the initial examination of the 3.1km long masonry arch structure.

The structure was completed in 1890 and closed in 1968. Storey said 90% of it was in “incredibly good condition”but that the remaining 10% was the “interesting bit”.

Of this 10%, Storey said there were three to four specific areas which required special attention the masonry arch structure was unstable.

One such stretch is a 200m long section where the structure has been reinforced as mine workings above have caused parts of the masonry arch to fall in.

“They put in ribs in a classic ‘lash up’”, he said. “They were trying to keep the trains running quickly, so you get whatever you can there to keep it stable.

“As such they’ve used bull-head rails and curved them to form ribs to support the masonry arch. They’ve then filled in between with timber laggings to stop the masonry falling locally between the ribs.”

Storey said this area would be challenging and would require strengthening before opening as the timbers had rotted.

He said the challenge with the area would come from how it is to be strengthened. He said keeping the rails would add to the historic story, and act as a temporary support for a strengthening measure, but would create a “shadow” in a sprayed concrete lining.

He said the ribs could be removed but then the team could face a potential collapse as the face would then be unsupported.

“The temporary condition is the problem,” he said. “Certainly, over the last 30 years the mining which used to go on above has stopped so the risk has decreased significantly. There’s a decision to be made and that will come with a detailed design.”

Another area he said would need careful attention was a hinge fracture which caused “substantial” movement to the masonry and finally resulted in the closure of the tunnel in 1968.

Although he says there are two 5m long timber structures and one 7m long timber structures which were built to support the tunnel lining to prevent further collapse, he does not believe these have actually taken any load in the 50 years since they were installed.

“These crib areas I’ve looked at the historical photos and it looks the same now as it did then,” he said. “I don’t think it’s ever taken any load because there’s no packing, there’s no hard wood, that you’d expect to see.

“I think it’s a bit of a red herring and I would be happy to take them out tomorrow, but we’ve recommended remote monitoring is essential to ensure there is no further movement.

“The more data you get, the more secure you are in your assumptions.”

The team which takes on the restoration work will also have to contend with a 2.1m thick concrete wall around 200m from the entrance of the country end portal. Around a 400mm diameter hole made up of several cores was cut out during the examination, but its full removal will present a significant challenge said Storey.

“We were very surprised that the concrete was that good, the quality was fabulous,” said Storey. “It will require a non-percussive method leaving a 150mm ring around the edge so they don’t disturb the masonry.

“But what happens to it when you remove it will also have to be taken into account as over time it will have had a strutting effect to the arch. Again there will have to be monitoring to make sure there’s no movement.”

A full drainage survey will also have to be carried out as water was found to have backed up behind the wall to around 1.2m deep. It was also found to be pouring through the brickwork at the London end portal, however Storey said this was not actually causing any damage to the brickwork.

To examine the tunnel, the team used tapping poles which he said together with a tactile examination of key areas was a tried and tested method of finding damage. 

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