Tunnelling work on the HS2 project in Ruislip, a London Borough of Hillingdon, was halted on Saturday after a mysterious five-metre square pool of bubbling foam emerged from the ground.
The area, located on a rugby pitch, was immediately cordoned off, and danger signs were erected, while an investigation is underway to determine the cause of the incident.
Although local residents referred to the phenomenon as a “sinkhole”, a spokesperson for HS2 clarified that it was a slurry pool.
SCS, a Skanska Costain Strabag joint venture, is constructing 13 miles of twin-bore tunnels as part of the HS2 route to its southern terminus at Euston.
After Environment Agency officials arrived at the scene, the tunnelling work resumed, but the area is still being closely monitored. The brown, bubbling substance that initially appeared on the surface has been cleared. However, on Sunday afternoon, lumps of white foam were visible on the playing field, which is owned by Hillingdon council and leased to Ruislip Rugby Club.
To prevent subsidence resulting from the leak, the contractors have sealed off the area and are conducting additional ground monitoring. They believe that the leak was caused by an existing borehole, which caused slurry to travel up the borehole and pool on the surface.
Additional ground anchors are being installed to protect against subsidence, and the area will continue to be monitored over the next few days.
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.
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.”
According to HS2 sources the borehole is now sealed behind one of HS2’s tunnel rings.