Work halted on HS2 after mystery pool of bubbling foam appears

theguardian.com:

Work on HS2 was halted on Saturday after a mysterious five-metre square pool of bubbling foam emerged above the ground where tunnelling was being carried out.

The area on a rugby pitch in Ruislip, to the west of London, has been cordoned off, danger signs have been erected, and an investigation is under way. Local residents described it as a “sinkhole”, but a HS2 spokesperson said what had emerged from the ground was a slurry pool.

The spokesperson said that tunnelling work has resumed at the site after it was halted on Saturday. It is understood that Environment Agency officials came to the scene and are monitoring the situation.

The incident began when a brown, bubbling substance emerged from the ground. HS2 said the foam has now been cleared, but on Sunday afternoon lumps of white foam could be seen emerging from the grass on the playing field, which is owned by Hillingdon council and leased to Ruislip Rugby Club.

SCS, a Skanska Costain Strabag joint venture, is constructing 13 miles of twin-bore tunnels on the HS2 route to its southern terminus at Euston. The West Ruislip tunnel is being created using a Tunnel Boring Machine (TBM).

When the contractors saw what was happening, they sealed off the area and started extra ground monitoring with the aim of preventing subsidence resulting from the leak. Additional ground anchors are being installed to protect against subsidence.

The contractors believe the leak was the result of an existing borehole, which caused slurry to travel up the borehole and pool on the surface. They will continue to monitor the area over the next few days.

The chair of the rugby club, Jatin Radia, said: “Local residents have been up in arms about the HS2 project. When something like this happens, it