Hole appears above HS2 Chiltern Tunnel

15 May 2023

Investigations are under way after a surface hole appeared above an HS2 tunnel in Buckinghamshire on Saturday.

Initial reports said the hole above the Chiltern Tunnel was 5-6m wide but HS2 now says it is around 4m wide and 3-4m deep. The geology in the area is chalk.

An HS2 spokesperson told T&TI: “A ground movement has occurred within a field above the Chiltern tunnels. Investigations are ongoing, but this is likely to be linked to pre-existing ground conditions above the tunnels. The site has been sealed off and there is no risk to the public.”

The hole is above a completed section of tunnel so tunnelling operations have not been affected.

The Environment Agency has been informed and HS2 is discussing plans with the landowner.

At 16km long, the twin-bore Chiltern Tunnel is the longest tunnel on the high-speed rail line. The two 10.26m diameter Herrenknecht TBMs – Florence and Ceclia – were launched in summer 2021. They reached the halfway mark in February.

HS2’s main works contractor, Align – a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – is responsible for the Chiltern Tunnel and nearby Colne Valley Viaduct.

Earlier this year, tunnelling on the 8km-long tunnel from West Ruislip to Greenford was halted when pools of foam appeared above the tunnel on Ruislip Rugby Football Club land. The HS2 spokesperson told T&TI that tunnelling resumed “a few weeks ago” and main works contractor SCS (Skanska Costain Strabag) was continuing to monitor the area.