Why investigating Sheffield Supertram mine shaft costs £1.5m

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.”