The route leaves the Colne Valley Viaduct on the West Hyde embankment and enters the Tilehouse Lane cutting before entering the 10.6 mile long twin bore Chiltern Tunnel passing under the M25. The tunnel construction compound is accessed via two temporary slip roads to the M25. Tilehouse Lane has been re-aligned over a new overbridge. Chalfont Lane is closed for at least five years and a temporary link road connects to Hornhill Road. Five PRoWs will be closed for the duration of the construction period.
Two specially designed tunnel boring machines (TBMs) began work in May 2021. They are named Florence and Cecilia. Florence is boring the eastern or up line tunnel and Cecilia the western or down line tunnel.
Slurry removed from the tunnel is treated and used for landscaping and construction of the West Hyde embankment. The concrete batching plant provides concrete for the Colne Valley viaduct, the Chiltern Tunnel and the five shafts to the Chiltern Tunnel. Tunnel lining segments and viaduct sections are manufactured on site. The compound provides overnight accommodation for staff working on the TBMs.
The tunnel portal is the tunnel’s entrance. The porous portals are made from concrete and are designed to allow the passage of air from the tunnel to reduce both air pressure changes and the noise generated when trains enter and leave the tunnel.