Colne Valley Viaduct – HS2’s largest bridge

HS2 is a massive undertaking with many huge contracts already let and underway. The largest bridge will be the Colne Valley Viaduct (CVV) – the most significant visible engineering feature on the Phase 1 route.

It will carry trains at 320kph (200mph) on slab track, extending for 3.37km and weighing 116,000 tonnes. To put that length in perspective, it is equivalent to the well-known viaducts at Glenfinnan, Ribblehead, Harringworth and Barmouth, and the Royal Border bridge, placed end to end. Built in concrete, the structure will have 57 spans of 45m, 60m and 80m in length. At a height of around 10m, it will follow an alignment with a horizontal radius of 5,280m – initially a right-hand, transitioning to a left- hand curve.

CVV will be within the Colne Valley Regional Park to the north-west of London which includes parkland, farmland and woodland, and large reservoirs which surround the River Colne and the Grand Union Canal. Broadwater Lake – a nationally important site – supports huge numbers of water birds.

This is one element of the 21.6km Contract C1 (Central 1) – awarded to the Align JV in 2017 – that also includes the 16.04km Chiltern Tunnel, north of the viaduct. The joint venture comprises Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick who bring their wide experience to the delivery of this demanding contract.

The site brings challenges of construction access, environmental and public sensitivity, and these have been addressed by the development of an elegant structure and through innovative construction methods for temporary and permanent works. Align’s Project Director, Daniel Altier, said: “I have no doubt that the viaduct will become one, if not the most striking element of HS2 Phase 1 once complete. The way it will be constructed is going to be equally fascinating for engineers young and old.”

Designing the structure

The High Speed Rail (London-West Midlands) Bill recognised that the design of the Colne Valley Viaduct should reflect its national significance. The Transport Select Committee said that “Having argued against a viaduct, local people deserve that its design be respectful and respectable, sympathetically and imaginatively designed.”

A specimen design was produced for HS2 by independent specialists Knight Architects, working with Atkins, in consultation with the Colne Valley Regional Park Panel, as well as members of the independent HS2 Design Panel. This formed the basis of tenders for Contract C1 and reflected the key principles of the HS2 Design Vision, together with guidance of the HS2 Open Route Structures Design Approach, Landscape Design Approach and Bridge Design Requirements. It outlined the design approach behind the viaduct and how it addressed the three core values of the HS2 Design Vision. In particular, it was fundamental that the viaduct should ‘tread lightly’ across this sensitive landscape.

The CVV will be seen by passengers on the Chiltern line, visitors to the park and from boats on the canal and reservoirs. Each will have different viewpoints and opportunities to see the structure. The design of elements reflects these contrasting views; those that face the pedestrian environment will be of a quality, scale and texture appropriate to the up-close and low-speed scrutiny it will be subject to.

The noise of passing trains could impact on wildlife, visitors and boaters so a noise barrier will be incorporated to reduce the spread of noise. The OLE structures will be bespoke to the CVV: as slender as possible and spaced to align with the viaduct’s substructure.

HS2 and Align have worked closely with Affinity Water and the Environment Agency to monitor water quality and agree working methods that will protect wildlife and potable water extraction during both temporary and permanent works.

Align JV are responsible for developing the final design and construction. The all-important architectural design was carried out by an integrated design team – known as Align-D (Align, Jacobs and Ingérop), working with Grimshaw Architects – and developed a structure that is expressive of power, speed and function.

Structural spans and pier forms vary along the length reflecting the changing landscape of the Colne Valley. Where the viaduct crosses lakes, the 80m spans between V-piers form a rhythmic sequence of low, slender arcs that skim lightly over the surface, preserving landscape views across the water. Elsewhere, through woodland, the spans are shorter and the piers more conventional. Using facetted concrete forms will add visual patterns and tactile interest to the CVV structure.
Barriers to each side will contain track and wheel noise, and comprise 1.65m noise-absorbing panels, 2m high self-cleaning acrylic panels to give vision at train window level, topped by a further 0.35m panel.

Rail braking loads could be significant. To resist these, four shock-absorbing units will be included along the length of the deck structure. In addition, there will be four expansion joints.

Enabling works

The enabling works for constructing Contract C1 are vast in extent and scope, and delivered by HS2’s enabling works contractor, Fusion JV, and by Align JV themselves.

A 160ha compound has been established between the north end of CVV and the Chiltern Tunnel portal. This accommodates the precast factories for CVV and the tunnel segments, as well as the offices, plant and welfare facilities. It will also include space for storing and treating the three million cubic metres of chalk slurry discharged by the TBM. This will eventually be used to form 90ha of new calcareous grasslands across the site.

Site investigation, landowner and stakeholder engagement have been ongoing for 3½ years and agreements are in place with each. Water main diversions have been carried out by Affinity Water to accommodate temporary and permanent works. A 275kVA overhead power line has been realigned and a strategic gas main has been protected from construction works loading by encasing it within a buried bridge structure and other gas mains diverted. In addition, Align have also diverted other water and HV services.

Impacts on ecology have been a key feature of the design and planning, with much of the site being within a Site of Special Scientific Interest. Badger and bat relocation has taken place and containment of invasive species will be a constant feature of construction.

Access along the contract has been designed to minimise highway traffic and a haul route is being constructed throughout; it will be complete by October. Included are two access points/highway crossings of the A142 near Denham Water Ski Club and at Moorhall Road, with a further site access through the former Hillingdon Outdoor Activities Centre in Dews Lane. The former is a busy route and is a regular diversionary route when the adjacent M25 is closed. Site accesses have been created at crossings, with the highway widened and strengthened and traffic control installed.
Load testing of test piles provided Align-D’s structural designers with important information on the ground conditions, resulting in a 10-15% reduction in the depth of the piles, with associated time and cost savings.

On-site precasting of deck units

The main deck of the viaduct will be precast in sections at a temporary factory in the compound before being assembled from north to south. The factory is within a steel shed covering an area of 40m x 105m; it is 24.75m in height – 26.75m at its highest point – a function of the need for the travelling cranes to lift precast units out and over from their formwork.
The factory is being operated by Align JV, although some specialist subcontractors are being engaged in addition. At present it is dedicated to this contract alone, but other HS2 applications may be identified for it.

Beginning in October and continuing until June 2024, the factory will produce 908 deck units and 92 pier head units. Each one is unique, with constant top cross-section but variable height of 3.5m to 6.7m. Weights will vary from 60t to 140t each. Units will be match-cast against its neighbour to ensure a close tolerance fit during installation and post-tensioning. Steel formwork will be adapted in height between pours. The external forms will be fixed in frames at 1.5m above the floor, with internal forms being inserted after the placement of the prefabricated steel reinforcement. The design of the formwork is built on the European experience of Bouygues Travaux Publics.

There will be three casting bays, two for the deck units and one for the pier head units. Two deck units will be cast in each 24-hour period, with accelerated curing to ensure a minimum of 14Mpa. Being more complex, the pier head units will be produced at one every three days. After 28 days, when fully cured, each unit will be transferred to the launching gantry using multi-axle vehicles.

Constructing the viaduct

The haul roads will follow the bridge alignment on land. To access the piers within the water and wetlands, temporary jetties are being built. Work got underway in May. These will be of steel construction, 12m in span, with four longitudinal beams carried by piers of driven steel tubular piles.

The working arrangement and design of the jetty have been agreed with Natural England to ensure the integrity of water and wildlife. Decking of the jetties will be concrete planking with sealed joints and a cross-fall to a drainage system incorporating onshore silt traps and oil interceptors. Booms will be placed in the water for emergency spill containment and extensive spill kits will be available on the jetty and in safety boats. All plant will, as far as possible, use biodegradable oils to minimise pollution risks and potential impacts on the ecosystem.

At pier locations, the jetty will be widened to include the whole working area, with decking sections removed as required for permanent piling and to construct cofferdams. 292 bored cast-in-place piles will be required to support the piers and abutments. These will be up to 55m in depth, with a diameter of 1.5m to 1.8m.

The land piers will be constructed conventionally with the facetted formwork erected above bored piles. The water piers are more complex in shape and access. Pile caps will be cast within temporary sheet-piled cofferdams. On these will be formed a pile base structure followed by the complex V-shaped piers. Soffit formwork will be constructed and, after steel fixing, upper formwork will be added before concrete is pumped to create the structure.

V-pier formwork

The spans will be erected between mid-2022 and summer 2024. Each will take 11 days to complete. The 150m long, 690t launching gantry will be launched to the first pier and will then place sections sequentially either side of the pier to maintain balance. Propping will also be installed alongside the pier, beneath the extending deck. The deck sections will be secured to the pier head by internal post-tensioned tendons in the top leaf of the units, followed by tendons in the lower leaf. Once complete the whole bridge will be further externally post-tensioned by tendons within the closed internal box of the deck structure.

Construction timetable

This is a very large project and the construction timescale reflects this:

• Spring-Summer 2021 – construct internal haul road
• Autumn 2021 – viaduct segment factory commissioned and viaduct launching girder assembly
• Spring 2022 – viaduct deck construction begins from north embankment
• Spring-Summer 2024 – viaduct deck construction completed
• Spring 2025 – Align JV demobilises and HS2 begins railway systems installations.

This bridge project is considerable in both scale and duration. It will undoubtedly become one of the iconic features of the HS2 route, both during construction and in service. Rail Engineer will follow its progress with interest and return to report as the structure begins to extend southwards.