HS2 investigates slurry pool above Costain-Skanska tunnelling

HS2 is investigating how a roughly 6 square metre pool of bubbling slurry emerged on a rugby pitch in Ruislip, north-west London.

The brown foam emerged from the ground on Saturday (18 February), above a site where CSC – a joint venture between Costain, Skanska and Strabag – is boring 13-mile twin tunnels from Euston Station.

Boring work temporarily stopped after the slurry was reported, with contractors erecting fencing around the bubbling pool, which has now been absorbed back into the ground. HS2 emphasised that the slurry pool was not a sinkhole, contrary to some reports.

Any idea what this is @HS2ltd being forced to the surface after tunnelling in Ruislip? @Stoptheroute @hs2aa @CV_Action_HS2 @Hs2Rebellion #HS2 pic.twitter.com/qkUttb0Pd4

— Ross Cluskey (@hector_moriarty) February 18, 2023

While HS2 is investigating the exact cause of the pool, it believes the slurry was a combination of water and conditioning agent used in the boring process which travelled up a narrow, vertical borehole as CSC tunnelled through it.

The vertical borehole is believed to be roughly a couple of inches wide and created during early groundwork investigations by HS2.

All boreholes created during HS2’s groundwork investigations should have been capped, but it is understood this borehole was not sealed. However, it is  now thought to have been closed by the boring underneath it.

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 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.”

And an update on HS2 emanations at Ruislip…….LISTEN!!! pic.twitter.com/4If0T9oqnr

— Mark Keir (@MarkKeir6) February 19, 2023