Why is it costing £1.5m to investigate mine shaft under tram tracks?

Emergency investigation and repair work costing £1.5m is taking place at a mine shaft which has opened up underneath Sheffield tram tracks.

The Supertram route between Gleadless Townend and Halfway was suspended in August after a sinkhole appeared.

Engineering experts Volker Rail have been brought in to undertake work on the outbound track at Birley Lane after part of the tracks fell down the shaft.

South Yorkshire Mayoral Combined Authority (SYMCA), which controls the tram network, said original cost estimates were between £750,000 and £1m, but following initial site investigations, the expected bill is now nearer £1.5m.

The authority said there was no timeline for when the suspended blue tram route would be able to reopen.

A report by SYMCA read: "On 23 August, ballast under the outbound tram track between White Lane and Birley Lane tram stop disappeared down a hole, leaving the sleepers and rail suspended over a void.

"Tram services were immediately stopped while investigations commenced. To expedite the investigations Volker Rail were commissioned on an emergency basis.

"Investigations have been complicated by approvals to undertake any works in the vicinity of the probable mine shaft being required from the Mining Remediation Authority."

The Mining Remediation Authority said it was supporting the investigation work and was in regular contact.

"Permits for the investigation were granted last week to allow drilling to 60 metres," said a spokesperson.

"Investigations are still under way to establish what the feature is, and it is too early to say what treatment will be needed."

Prof Liz Laycock, from the school of engineering and built environment at Sheffield Hallam University, said it is feasible that engineers did not know about the mine shaft when Supertram was constructed 30 years ago.

"The coal authorities have got about 175,000 entries of various mines that they know about all over the country, so old mine workings are really common.

"Around Sheffield there's loads of old coal mining workings but there wasn't a requirement to keep any records until 1842, so there is always the chance of these mine shafts appearing.

"In a lot of areas the soil has been churned up over the years so there's not a lot of signs of where a mine shaft might be. You don't dig and find a huge hole, you dig on the capping over it."

She added that holes could appear when old backfill materials covering them, such as railway sleepers or planks rotted away over time.

"When the backfill starts to sag you know there's a danger that it's starting to move and it becomes obvious but until then, it's really difficult to see where these old shafts might be."

She said tunnels could run in different directions underground and could be unstable.

"They certainly will not send anybody down because it might contain mine gases, water or be at danger of collapse.

"The first investigatory methods will be bore holes and remote sensing using ground penetrating radar to try and find out the extent of the cavities underground."

Prof Laycock also said passengers shouldn't be worried about using the rest of the network.

"Trams rumbling over it will not have helped but you can get shafts opening up in fields with rabbits skipping over, so it's not necessarily tram vibrations causing this to open.

"Statistically it's very unlikely there'll be another shaft under the tram lines. It's very unlikely that Supertram will fall into a tunnel in some sort of Marvel comic way."

Replacement buses are running between Gleadless Townend and Halfway but councillors said buses on the 120 route are often full, leaving some passengers stranded at stops.

Sheffield South East MP Clive Betts said the combined authority needed to provide a better bus service.

He said: "A replacement bus is a service that can work for a few days, it doesn't stand the test of working for months which has been going on now.

"This replacement service has to be substantially improved and there is also the issue of people in wheelchairs as they can't always get on a bus as easy as a tram.

"Often there isn't space for more than one wheelchair on a bus too. Those sorts of situations are really difficult and do need addressing."

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