Double composite viaducts
Efficient design for complex crossings
Several viaducts on the HS2 route use an innovative double composite steel–concrete design. This improves structural efficiency and reduces carbon emissions.
This approach is built on experience from European high-speed rail networks where engineers have developed similar bridge solutions for long-span railway structures.
What is a double composite viaduct?
A double composite viaduct refers to how the structure works internally – not how many structures sit side by side.
In a conventional single composite steel bridge, a steel box girder carries the structural load. A reinforced concrete slab sits on top to form the deck. The steel and concrete act together structurally.
In a double composite design, reinforced concrete is placed both above and below the steel box girder. These materials act together in different parts of the structure to resist bending forces.
The lower concrete layer is particularly effective over the piers, where forces reverse direction. This improves overall structural efficiency and allows engineers to use less structural steel, increase span lengths and improve stiffness for high-speed rail loading.
Design cutaway
Learning from international high-speed rail
Double composite steel–concrete bridge decks are widely used on European high-speed railways, particularly in France, where engineers have refined efficient long-span bridge solutions over several decades.
By adapting these proven methods for UK conditions, we’re drawing on established international practice while building new skills across the UK supply chain.
Where we’re using double composite design
Wendover Dean viaduct
The 450 metre-long viaduct is located south of the town of Wendover in Buckinghamshire and will be more than 14 metres in height.
Small Dean viaduct
The 345 metre-long viaduct just south of Wendover will carry the HS2 line across the A413, Small Dean Lane and the Chilterns rail line.
Westbury viaduct
Set low in the landscape to the east of Brackley, Westbury Viaduct, cross the floodplain of the River Great Ouse.
Construction in practice
Construction techniques vary depending on location, but typically include:
- Installing deep piled foundations and reinforced concrete piers
- Fabricating steel girders off-site
- Launching or lifting steel deck sections into place
- Casting the upper and lower reinforced concrete elements
- Installing track systems and railway equipment
At the viaducts near Brackley, engineers have used incremental launching – pushing large sections of deck into position across the landscape. This reduces disruption to roads, railways and environmentally sensitive areas below.
Latest progress
Cutting carbon through smarter design
Design optimisation across these five viaducts has reduced embodied carbon by between 52% and 66% compared with more traditional bridge solutions.
By combining steel and concrete more efficiently, engineers have reduced the overall quantity of high-carbon materials while maintaining structural performance for high-speed rail.
Longer spans also reduce the number of piers required, helping minimise environmental impacts across floodplains and transport corridors. Lessons from early designs have been applied across multiple structures, improving efficiency further.