HS2 completes first green tunnel using excavated material from nearby works

HS2 has completed civil works on its first green tunnel, the 880 m-long Copthall tunnel near West Ruislip on the western edge of London. The cut-and-cover structure was built using around 1.2 million cubic metres of material excavated during construction of the nearby twin-bore Northolt tunnel.

The design represents an important construction and material-management decision. The railway was originally planned to run through an open cutting at this location, but engineers opted for a covered structure instead. This allowed excavated material from the Northolt tunnel to be retained and reused on site, avoiding the need to transport it elsewhere by road. HS2 estimates that this prevented around 100,000 truck movements on local roads.

Copthall is the first of five green tunnels on the HS2 route between London and the West Midlands to be completed, meaning that both the tunnel structure and the earth covering above it are now finished. The next stage will involve planting trees, shrubs and other vegetation over the tunnel so that it gradually blends into the surrounding landscape.

A green tunnel is a cut-and-cover structure designed to reduce the permanent visual impact of a railway. Once the structural work is complete, the tunnel is covered with earth and landscaped, allowing vegetation to be established above the railway.

Copthall is more than half a mile long, around 12 m high and up to 16 m wide internally. It is the only single-bore tunnel on the new high-speed railway and has been engineered to accommodate the forces generated by trains passing at speeds of up to 200 mph.

The tunnel includes five natural ventilation shafts, extending as far as 17 m from the newly created landscape above to the tunnel roof.

The structure was constructed using travelling formwork and falsework systems that advanced in 20 m sections. This approach enabled the construction team to form the tunnel’s gently curved alignment while maintaining a consistent sequence of work.

Copthall tunnel is positioned between the Northolt tunnel and the Colne Valley viaduct, connecting two major HS2 structures within a constrained infrastructure corridor. The works also required changes to local infrastructure, including the realignment and reconstruction of Harvil Road and the construction of a new bridge to carry the road over the high-speed railway.

From an engineering perspective, the project demonstrates how tunnel design can be integrated with excavated-material management. Instead of transporting the material away from the site, the project team used it to create the earth covering over the completed tunnel. This reduced road haulage and provided the material needed for the final landscaped structure.

The project also illustrates the use of cut-and-cover construction in an environmentally sensitive location. Although this method requires extensive temporary excavation and carefully planned construction sequencing, the completed structure allows the railway to be covered and the landscape above it to be reinstated.

With civil works complete, the tunnel will now enter its landscaping phase. Over the next two years, the earth covering will be planted so that the structure begins to blend into its surroundings. Copthall provides an example of how structural construction, excavated-material reuse and landscape reinstatement can be combined within a single infrastructure design.