HS2 tunneling resumes after pool of bubbling foam appears near West London construction site

A pool of brown bubbling foam temporarily paused work on a section of the HS2 construction project after the unusual sight appeared on a rugby pitch in Ruislip.

Images showed a circle about five metres in diametre which appeared to be the result of a leaking borehole.

HS2 said investigations were ongoing and believed tunnelling machines may have gone through a small pre-existing borehole.

A statement added: "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."

Earlier this month giant machines digging HS2’s longest tunnel passed the halfway point. HS2 said the pair of tunnel-boring machines had each made five miles of progress on their 10-mile journeys under the Chiltern Hills. Once completed, the twin-bore tunnel will be used by high-speed trains travelling to and from London. The 170-metre-long machines, named Florence and Cecilia, are working continuously, operating as self-contained factories.

More than 1.3 million cubic metres of chalk and flint, enough to fill 500 Olympic-sized swimming pools, has been excavated.

It will be used as part of a chalk grassland restoration project at the south portal of the tunnel near Denham, Buckinghamshire alongside the M25 motorway.

Florence was launched there in May.

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