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, in a west London borough, 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 raises concerns about unintended consequences.
“This happened on a field, but what about houses which are being tunnelled under? How can HS2 reassure residents? The question is, is it safe? My job is to ensure that our rugby pitches are safe for both children and adults.”
Sarah Green, a local environment campaigner opposed to the HS2 project, said: “I’m standing here watching bubbles coming up all over the ground. You can hear the ground bubbling. HS2 are lucky that this has happened in a field and not underneath someone’s house. Government should cancel the HS2 project.”
One local resident who was out walking his dog and stumbled across the strange sight filmed it on his phone. “It was almost like a foam, but left a dusty dirt on my boots,” he said.
A second dog walker issued a warning in a local Facebook group, saying: “Our dogs did avoid it, but I would worry a child would investigate it and not sure how big the hole it’s bubbling out of is.”
A HS2 spokesperson said: “Upon discovering the small pool of foam in Ruislip, HS2’s main works’ contractor SCS sealed off the area to investigate. The pool appears to have come out of an existing borehole, causing foam to travel up and pool on the surface. The area is safe, the leak has been sealed, and the foam has been cleared.”
According to HS2 sources the borehole is now sealed behind one of HS2’s tunnel rings.