Copthall Green Tunnel

Copthall Green Tunnel will be located within the London Borough of Hillingdon, between Harvil Road in the west and Breakspear Road South in the east. The 880-metre-long tunnel sits between the twin bored Northolt Tunnel and the Colne Valley Viaduct.

The Copthall Tunnel is one of five green tunnels on the HS2 route, which are shallower tunnels that will be finished with trees, plants, and shrubs on top – helping them blend into the natural landscape. Materials from the excavation are kept on site and then reused on the tunnel surroundings, reducing movement of spoil.

Tunnel construction

The team excavated 20 metres down from ground level before installing piles in the portal to support the tunnel in the shallower sections. The tunnel is being constructed with a base slab, walls, and a roof using reinforced concrete.

The strong box structure for the tunnel is formed using tied steel bars with concrete poured around – allowing trains to pass through and green landscaping above. To create the roof of the box tunnel, a 40-metre-long temporary deck is positioned between the box walls to support the pouring of the concrete roof. Once the roof has gained sufficient strength the deck is moved down 20 metres to allow construction of the next section of roof.

As part of the works in this area, Harvil Road is being realigned and will travel over the western end of the Copthall Tunnel. The deck for the road realignment has already been installed and the realigned Harvil Road reopened in February 2024.

Greener integration

Original designs for this section of the route showed a cutting – Copthall Cutting – as opposed to a bored tunnel. However, after a review the decision was made to change the design to a cut and cover tunnel to reduce lorry movements in the area and the visual impact of the railway.

The new design means material, approximately 1.21 million cubic metres, can be re-used on site resulting in a substantial reduction in the number of lorries required to take the material away whilst using the local highway network. The cut and cover tunnel also ensures a better, greener integration of the railway within the surrounding landscape, reducing its visual impact.

West London habitat creation