Colne Valley Viaduct

Stretching for more than 3.4km (2 miles) across a series of lakes and waterways between Hillingdon and the M25, the Colne Valley Viaduct will also be the longest railway bridge in the UK. The viaduct will carry the new high-speed line across a series of lakes and waterways on the north west outskirts of London, and will be almost a kilometre longer than the Forth Rail Bridge.

The Colne Valley is an area of diverse countryside with areas of parks, green spaces and reservoirs. Along with the River Colne and the Grand Union Canal, the valley is a well-used public space offering recreational opportunities for visitors to enjoy.

Watch our webinar on building the Colne Valley Viaduct

Viaduct construction

The huge viaduct project is being led by HS2’s main works contractor Align JV – a team made up of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick.

Construction of the UK’s longest railway bridge began in early 2021, with an expert team of ground engineers sinking the first of almost 300 piles to form the foundations for the Colne Valley Viaduct.

In Spring 2022, an enormous 700 tonne bridge-building machine was launched on site. Known as a ‘launching girder’, the 160m long bridge-building machine is the only one of its kind in the UK and is used to lift the giant concrete deck segments that form the viaduct’s arches into position. Once each section is complete, the machine inches itself forward into position to build the next stage.

56 piers each weighing around 370 tonnes have been constructed ahead of the girder, with the girder moving from one pier to the next, installing the deck segments as it goes. One segment is put in place each side of the central pier, using a cantilever approach to balance the structure, as two half-arches either side of each pier are constructed simultaneously. Steel tensioning cables will be threaded through the segments to strengthen the bridge.

Uniquely shaped and weighing as much as 140 tonnes, 755 of the viaduct’s 1,000 deck segments have been installed creating 39 spans over land and water for 1.5 miles. To allow for the gentle curves of the viaduct as it crosses the valley, all the segments are slightly different shapes and made on site at a purpose-built temporary factory close to the north abutment.

The viaduct pre-cast factory where the segments are made at the peak of construction will cast around 12 segments every week using a ‘match-casting’ technique. This approach – where each segment is poured against the previous one – will ensure the whole deck fits perfectly when assembled on the piers. The mammoth 100m long viaduct precast factory, which is visible from the M25, has an internal volume of 105,000 cubic metres – making it larger than the Royal Albert Hall.

In November 2023 main construction of the bridge officially passed the half-way mark. Once construction is complete, the factory and surrounding buildings will be removed and the whole area between the viaduct and the Chiltern tunnel will be transformed into an area of chalk grassland and woodland as part of HS2’s ‘green corridor’ project.

Management of construction noise at the South Portal

This document gives information about work at our viaduct fabrication plant at HS2’s South Portal works site near West Hyde, Rickmansworth, and may be useful for residents who can hear our works activity.

Download our Management of construction noise at the South Portal frequently asked questions (FAQs)

Viaduct design

The design of the Colne Valley Viaduct was inspired by the flight of a stone skipping across the water, with a series of elegant spans, some up to 80m long, carrying the railway around 10m above the surface of the lakes, River Colne and Grand Union Canal.

Set low into the landscape, wider spans will carry the viaduct crosses the lakes, with narrower spans for the approaches. This design was chosen to enable views across the landscape, minimise the viaduct’s footprint on the lakes and help complement views across the natural surroundings.