COLNE VALLEY VIADUCT – Denham/Harefield • UK • 2025

THE SITE
The Colne Valley Viaduct is a railway bridge that will carry High Speed 2 over the Colne Valley Regional Park and the Grand Union Canal. It runs between Harefield in Hillingdon, Greater London and Denham in Buckinghamshire. The 2.1-mile-long (3.4-km) gently curving structure, to the northwest of London, will carry HS2 trains travelling up to 200mph.

Tarmac is a long-standing legacy key account for Saint-Gobain Construction Chemicals (SGCC) brand Chryso, supplying nationwide admixtures for the client for over a decade. Concrete was chosen as the primary material for the viaduct’s construction due to its numerous advantages, including durability, versatility, and cost-effectiveness. The use of concrete allowed for the creation of complex structural elements, such as the V-shaped piers and the ‘Robust Kerb,’ which would have been challenging to achieve with other materials. Additionally, concrete’s inherent strength and resilience make it ideal for supporting the heavy loads and dynamic forces associated with high-speed rail travel.

THE CHALLENGE
The project began in early 2021 with the installation of 292 foundation piles, each averaging 60 meters in depth. These piles were essential for providing the necessary support and stability for the viaduct’s massive structure, with operations split between land and marine locations. The viaduct’s design is notable for its gentle curve, which follows the natural contours of the Colne Valley. This design minimizes environmental impact and enhances the aesthetic appeal of the structure. The viaduct features 56 pier supports, each weighing approximately 370 tonnes, and required 1,000 unique precast segments for its deck. These segments were cast in a temporary factory on the main Align C1 compound, ensuring precise specifications and high-quality construction.

THE SOLUTION
For the viaduct pier operations, a highly flowable C40/50 concrete was designed for the conventional straight piers,
while a C55/67 self-compacting concrete was specified for the V-shaped piers. Both mixes incorporated high levels of supplementary cementitious materials (SCMs) to limit heat evolution and control risks associated with early age thermal cracking. These mixes were also designed to provide a high-quality visual finish. Mock-ups were cast at the main C1 compound months ahead of the works to confirm suitability for construction, including full compaction of the self-compacting concrete design in heavily congested reinforcement conditions, and to assess the visual finish.

The viaduct’s precast segment concrete mix design was a highly flowable C55/67 concrete that needed to meet early age compressive strength requirements of 10 MPa at 10 hours and 20 MPa at 20 hours. To achieve these challenging early age strength requirements, the concrete mix design included a high-performing accelerating admixture. Extensive on-site trials were completed over a year ahead of the first segment being cast, with a minimum concrete temperature of 25 degrees identified as a key parameter to meet early-age strength requirements. The on-site batching plant was equipped with a specialist hot water and hot air system to heat the aggregates and water, ensuring that concrete could be supplied 365 days a year, at night, at the target concrete temperature.

The construction of the viaduct involved several unconventional calculations and specifications to address the unique challenges posed by the project. For the piling operations, a high-performing C40/50 flowing concrete was designed in collaboration with Align JV and Keller VSL to minimize bleed and extend workability over four hours, aiding the placement operation.Due to the constrained nature of delivering concrete onto the viaduct deck to construct the robust kerb, Tarmac supplied all 8,000 m3 using their fleet of specialist 4 m3 Minimix trucks. This provided a cost-saving solution to the customer and a simple placement solution compared with long-range pumping options being considered for the works.

BENEFITS
Concrete played a pivotal role in the successful completion of the Colne Valley viaduct, demonstrating its versatility and adaptability in modern engineering projects. The material’s ability to be moulded into various shapes and forms allowed for the creation of intricate structural elements, such as the faceted formwork and V-shaped piers. Concrete’s durability and strength ensured that the viaduct could withstand the dynamic forces and heavy loads associated with high-speed rail travel. Moreover, the use of supplementary cementitious materials (SCMs) in the concrete mix provided additional benefits, such as improved thermal performance and reduced environmental impact. SCMs, including fly ash and slag, help to lower the carbon footprint of concrete production while enhancing its mechanical properties. This approach aligns with the project’s commitment to sustainability and environmental stewardship.