HS2: The Colne Valley Western Slopes

Satellite imagery of the Colne Valley Western Slopes worksite in early April 2021, from Sentinel. The tunnel portal is at the top-left (NW corner), just before the M25 motorway. © Sentinel Hub.

High Speed 2 (HS2) will leave London by curving away from the Chiltern Main Line, across the Colne Valley on what will the the UK’s longest railway viaduct. It crosses the Greater London boundary before briefly coming back to ground level and then on into the Chiltern Hills in a long tunnel.

The place between the viaduct and the tunnels is called the Colne Valley Western Slopes. It is here that Align JV, who are building the viaduct in one direction towards London, and the Chiltern tunnels in the other direction towards Birmingham, have created a huge construction site to assist in building both. The site covers several square miles of what were arable fields, and is highly visible from Harefield, a village on the other side of the valley:

The Colne Valley Western Slopes construction site, from Harefield village to the north-east.

The Colne Valley Western Slopes site is very large for several reasons – it houses factories to construct the concrete segments both for the tunnels and the viaduct deck and also the launch podium for the tunnel boring machines (TBMs). It will also be able to comfortably accommodate the chalk spoil coming from the TBMs. Having the production and spoil deposition both on site will substantially cut down on lorryloads bring concrete in and chalk out of the site. A private “junction” has been created on the adjoining M25 motorway, to provide a suitable route for the lorries that do need to arrive.

The upper part of the construction site (focusing on the tunnel itself) viewed from the south-west. The two TBMs (tunnel boring machines) are being reconstructed by the two huge cranes on the left, prior to launching the Chiltern Tunnels boring later this spring. The large factory on the right is the concrete tunnel lining creation plant.

The opportunity is being taken, post construction, to not just restore the arable farmland beforehand, but instead create a large-scale nature habitat – to partially offset HS2’s undeniable impact on the natural environment. HS2 have recently unveiled their detailed plans for the site, which include reconnecting two public paths that have been severed, and plans for new forest to help screen noise and improve biodiversity. A number of set-piece views of the line are also being designed.

Plan for the restored and added path network on the Colne Valley Western Slopes site. Drawing © HS2.
The middle part of the site. A balancing pond, to manage drainage, is being constructed on the right. The concrete “roadway” running through the centre of this picture, with a crane on it is the route of the railway. It will launch the tunnel boring machines to the left, and the viaduct construction girder to the right. The lower part of the site (to the right of the photo) will be mainly used to place the spoil from the tunnels.

A common design style for the tunnel portals, the viaduct, and a new road bridge to accommodate a rerouted road, Tilehouse Lane, that sits in the centre of the site, has been adopted. The design is modern and sophisticated, without distracting from what should return to a natural, rural environment post-build.

HS2’s common design for the three structures in the area – white concrete structures with distinct (~30 degree) angles, forming the tunnel entrance, bridge parapets and viaduct ends and piers. Drawing © HS2.
HS2’s plan for the eventual look of the site (looking from the south-west). The diversion of Old Shire Lane, to pass under the start of the Colne Valley Viaduct, causes the U-shaped path seen at the forefront. Tilehouse Lane’s new alignment results in the new bridge, crossing the line in the distance. The tunnel portal is at th far end. Drawing © HS2.

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