Project Overview
Copthall Tunnel is one of five green tunnels being constructed as part of HS2 Phase One, the new high‑speed rail line connecting London to the West Midlands. Located in the London Borough of Hillingdon, the nearly 1km long cut‑and‑cover tunnel sits between the twin‑bored Northolt Tunnel and the Colne Valley Viaduct, forming a crucial link between two of HS2’s most significant structures.
The project is being delivered by the Skanska Costain STRABAG Joint Venture (SCS JV), HS2’s main works contractor for the London tunnels section. Kilnbridge was appointed to deliver the full reinforced concrete structure for the tunnel.
Kilnbridge’s package covered the complete RC structure, including the base slabs, tunnel walls, roof slabs, five 28‑metre tall ventilation shafts, integral drainage and waterproofing. The tunnel measures 14 metres wide and 11 metres high, with 1.3 metres thick walls and 1.5 metres thick roof and floor slabs.
In constructing the structure, Kilnbridge installed more than 40,000 tonnes of reinforcement and placed over 70,000m³ of concrete, making this one of the largest RC undertakings within the HS2 London works.
The volume of materials required careful planning in terms of logistics. To maintain programme certainty and avoid supply‑chain delays, SCS established a dedicated on‑site concrete batching plant, ensuring continuous production throughout the works. Alongside this, Kilnbridge and SCS created an on‑site rebar fabrication facility, enabling just‑in‑time reinforcement delivery and reducing reliance on external suppliers.
These innovations were essential in managing the unprecedented material volumes and maintaining consistent productivity across the programme.
Constructing the tunnel structure presented significant engineering challenges due to its geometry, thickness and the need for efficient sequencing.
Kilnbridge designed and implemented bespoke roof and wall traveller systems, which allowed the wall and roof slabs to be cast simultaneously. This system slowly travelled the length of the tunnel, with the roof and walls being cast in situ.
This approach accelerated the construction programme whilst reducing cranage requirements and lowering labour intensity, delivering notable health and safety benefits at the same time. It also ensured consistent quality and tolerances across the entire tunnel structure.
The roof and wall travellers became defining features of the Copthall tunnel construction methodology, demonstrating Kilnbridge’s ability to design and deploy bespoke solutions for complex environments.
The works required close coordination with the projects earthwork contractor, who was responsible for placing circa 1.2 million cubic metres of spoil as backfill around the advancing structure. This required the two teams to work closely to devise a programme that saw both teams progressing in parallel.
The wider HS2 programme also influenced sequencing, as Copthall Tunnel connects directly to the Colne Valley Viaduct – a project also delivered by Kilnbridge – and the Northolt Tunnel, both major engineering undertakings in their own right. This placed additional emphasis on interface management and programme integration across multiple contractors and disciplines.
As a green tunnel, Copthall Tunnel will ultimately be buried and landscaped with trees, shrubs and planting to blend into the surrounding environment. Excavated material was retained on site and reused to reduce lorry movements and minimise environmental impact, aligning with HS2’s sustainability commitments.
Outcome and Legacy
Through disciplined planning, innovative engineering and a commitment to safe and efficient delivery, Kilnbridge successfully completed the reinforced concrete structure of the Copthall Tunnel to the highest standards.
The project stands as a testament to Kilnbridge’s capability to manage large‑scale civil engineering works, solve complex logistical challenge, and collaborate seamlessly with other contractors.
The integrated logistics strategy, bespoke roof traveller and coordinated approach to earthwork all contributed to a delivery that reinforces Kilnbridge’s reputation as the UK’s leading structures specialist, capable of executing complex reinforced concrete structures at exceptional scale.
