The Colne Valley Viaduct: HS2 and Britain's Longest Railway Bridge

This guide covers the Colne Valley Viaduct's design, construction, environmental measures, and the procurement opportunities it created.

‍

Colne Valley Viaduct: key facts and figures

At 3.4 km, the Colne Valley Viaduct is the longest railway bridge in the UK. The finished structure weighs roughly 116,000 tonnes and was announced as structurally complete in September 2025 according to New Civil Engineer.

Trains will cross at speeds up to 200 mph (320 km/h). The viaduct sits between two tunnels: the Northolt Tunnel carrying HS2 beneath west London to the south, and the Chiltern Tunnel to the north.

Parliament called it "the most significant visible engineering feature of HS2 Phase 1."

In 2024, it won the Engineering Award at the Royal Fine Art Commission Trust Building Beauty Awards.

Where is the Colne Valley Viaduct?

The viaduct crosses the Colne Valley Regional Park near the M25 motorway and the village of Denham, northwest of London.

More precisely, it runs between Harefield in the London Borough of Hillingdon and Denham in Buckinghamshire.

You can see a Google Maps view of the viaduct in the map below:

Along its 3.4km length, the structure spans three lakes, the River Colne, and the Grand Union Canal. The deck sits approximately 10 metres above land and water, high enough to clear the landscape while keeping the railway as flat as possible.

The Colne Valley Regional Park supports a wide variety of habitats and species.

This made it one of the most environmentally sensitive sections of the entire HS2 route.

That sensitivity shaped nearly every design and construction decision. For interactive views of other major projects, stations, and bridges see our blog covering the entire HS2 route map.

How was the Colne Valley Viaduct built?

Knight Architects and Atkins unveiled the concept design in January 2018. The idea behind it was simple but striking: the flight of a stone skipping across water.

Wider spans reach up to 80 metres where the viaduct crosses the lakes. Narrower 45m and 60m spans cover the land approaches on either side.

The V-shaped piers standing in water serve a practical purpose. They reduce the structure's footprint on the lakes and preserve sightlines across the valley. Rendel-Ingerop later developed the concept into detailed construction drawings, with Grimshaw serving as architect under the Align JV contract.

• Gentle horizontal curve: The deck follows a radius of 5,280m along its length, allowing high-speed trains to maintain speed without sharp turns.
• Faceted concrete surfaces: The exterior surfaces are textured rather than smooth, giving the structure visual depth and a tactile quality.
• Transparent acoustic barriers: 4-metre-high barriers run the entire length, reducing noise for the surrounding area while letting passengers see out between the two long tunnels.
• Bespoke overhead electrification: Custom equipment designed to sit lower and reduce visual clutter above the deck.

Construction began in March 2021, with main deck construction starting in May 2022.

The 1,000 precast concrete deck segments were manufactured on-site at a purpose-built factory, rather than being transported from elsewhere. Each segment weighs up to 140 tonnes.

A specialist launching gantry installed the segments from above.

This machine, one of the largest bridge-building machines in Europe, lifted each segment into place without needing heavy equipment on the ground below. That approach reduced disruption to the sensitive landscape and waterways.

The last deck segment was lowered into place on 5 September 2024, one month ahead of schedule.

Full civil engineering completion followed by the end of 2025. Align JV managed the entire construction programme.

Once construction wraps up completely, the factory and surrounding temporary buildings will be removed.

The area will then be transformed into chalk grassland and woodland as part of HS2's green corridor project.

‍

Colne Valley Viaduct cost and associated contracts

Align JV delivered the viaduct. This joint venture comprised Bouygues Travaux Publics, Sir Robert McAlpine, and Volker Fitzpatrick.

HS2 Ltd awarded Contract C1 in 2017 at £1.6 bn, covering both the viaduct and the adjacent Chiltern Tunnel.

The original Lot C1 estimate ranged from £800m to £1.3 bn; however, HS2 has not published a standalone cost figure for the viaduct alone, though it is one of the project's most complex and visible structures.

The Oakervee Review identified the civils programme escalating by approximately £6bn overall.

Structures like the Colne Valley Viaduct were central to that escalation, given the bespoke design, environmental constraints, and on-site manufacturing requirements.

The viaduct is now in the rail systems installation phase.

That means new procurement for track, electrification, signalling, and maintenance contracts, all of which represent separate tender opportunities for suppliers.

‍

Tracking Colne Valley Viaduct contracts on Stotles

The Main Works Civils Contracts (MWCC) notice, originally published on TED in September 2015, is where the Colne Valley Viaduct procurement trail begins. The contract detail view below shows the MWCC award with Lots C2 and C3 broken out at £1.49bn and £994m respectively, along with the procurement timeline and CPV classification.

‍

‍

‍

Even smaller packages tied to the viaduct are trackable.

The Colne Valley Viaduct Deck Waterproofing contract, awarded to BiP Solutions under the Align JV umbrella, is a good example of the sub-contract opportunities that sit beneath the headline figures.

The contracts list view below shows how Stotles groups related HS2 awards together, scored by relevancy, so suppliers can see the full picture, from the overarching MWCC framework, down to individual packages like deck waterproofing and GRP access stairs.

‍

‍

A project of this scale, consuming roughly £7bn a year of public money, demands transparency. Understanding exactly where that spending goes, which contracts were awarded, to whom, and at what value, matters for public accountability.

Stotles provides that lens, making HS2's procurement history searchable and structured rather than buried across OJEU archives and PDF notices.

For suppliers, that same visibility is a competitive advantage. In the below link, you can get a free guest pass to view the HS2 buyer page, tracking all live and upcoming opportunities in one place.

‍

Why the design and build drew criticism

The bespoke approach is exactly what critics point to when discussing HS2's cost overruns.

Paul Chapman is a complex project delivery expert at Oxford's Saïd Business School. He questioned the decision to produce 1,000 uniquely shaped segments in a purpose-built on-site factory.

Chapman argued that other countries would have designed a replicable, value-for-money product and streamlined production.

Britain, by contrast, built a bridge that won a beauty award, but with 499 other bridges on the route still to deliver.

The Financial Times described the completed structure as "the perfect example that boosting public investment does not raise economic growth."

Fare-paying passengers are not expected until the late 2030s. The viaduct will therefore stand largely unused for nearly a decade.

HS2 has consumed approximately £7bn a year of public money, about 0.25% of GDP. In the 2026–2029 spending review period, it will cost taxpayers two and a half times all other rail investment combined.

The criticism was not limited to cost.

A protest camp existed near the site from October 2017 until HS2 began evictions in January 2020. Activists occupied the area for over two years before construction could proceed.

A farmhouse was compulsorily purchased and a watersports centre relocated; both were demolished to clear the path.

Stop HS2 criticised the project for allegedly ignoring local residents' concerns. Buckinghamshire County Council requested a pause on preparatory work in April 2019 pending the Oakervee Review, though construction ultimately continued.

That tension sits at the heart of the viaduct story. It is genuinely world-class engineering. Yet it may also represent the bespoke, high-cost approach that made HS2 unaffordable.

‍

Environmental measures on the Colne Valley Viaduct

The Colne Valley Regional Park is one of the most environmentally sensitive landscapes on the HS2 route.

Construction measures included habitat creation, water quality management, noise mitigation, and ongoing ecological monitoring throughout the build.

The transparent acoustic barriers feature vertical lines visible to bats and wildfowl.

Those lines help reduce collision risk for wildlife moving through the area. Long-term environmental improvements include new areas of woodland and grassland, enhanced biodiversity, and improved public access within the park.

Chalk from the adjacent Chiltern Tunnel excavation, approximately 3 million cubic metres, is being used to create chalk grassland habitat. This habitat is being established on the Colne Valley Western Slopes.

Rather than disposing of the spoil elsewhere, HS2 is using it to restore a habitat type that has declined significantly across the UK.

‍

How suppliers track HS2 and construction opportunities

The Colne Valley Viaduct and Chiltern Tunnel together form one of the largest civil engineering delivery packages on HS2.

They represent just one of four Main Works Civils Contracts worth a combined £ 8bn+.

The full HS2 programme is estimated to generate 400,000 supply chain opportunities across Phase 1.

The Colne Valley Viaduct contracts shown above illustrate how a single structure generates opportunities at every tier, from a £1.6 bn main works package down to individual deck waterproofing awards.

• HS2 Ltd buyer profile: Open tenders, contract awards, and procurement patterns from the main client organisation.
• EKFB JV buyer profile: The adjacent section covering the bat tunnel and other major structures on Lots C2 and C3.
• HS2 Route-wide Framework: Upcoming works worth £500m–£600m across the route.

For the full HS2 series maps, see the HS2 Route Map for an interactive view of the alignment.

‍

‍