Tunnelling on HS2 was halted for around two hours on Saturday after a pool of foam appeared above the TBM in Ruislip.

An HS2 spokesperson said the TBM was paused between 2-4pm as a safety precaution, and to dry the leak, and tunnelling then resumed.

“Upon discovering the small pool of foam in Ruislip, HS2’s main works’ contractor SCS (Skanska Costain Strabag) sealed off the area to investigate,” the spokesperson said. 

“The pool appears to have come out of a pre-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. There has been no impact on the programme schedule.”

The borehole is now sealed behind a tunnel ring. SCS will continue to monitor the area over the next few days and install additional ground anchors to ensure there is no subsidence.

HS2 stressed the incident, which appeared on Ruislip Rugby Football Club land, was not a sinkhole. The club was not affected as the borehole sits away from the pitch.

SCS is currently constructing 21km of twin-bore tunnels on the HS2 route to its southern terminus at Euston. The two TBMs for the 8km-long tunnels from West Ruislip to Greenford were launched last October. They will take 22 months to excavate the twin-bore Northolt Tunnel.