The Colne Valley Viaduct is expected to be a ‘beacon’ of HS2 design quality (Hayes Davidson/Knight Architects)

New motorways and high-speed railway lines are designed to bring transformational benefits to everyone however, because of their scale and the quantity of structures involved, the planning and design of these major projects bring challenges in stakeholder engagement that are not experienced in smaller projects.

Often, planning permission is granted long before information on the design of structures is communicated to the public. This was the case for phase one of the UK’s new High Speed 2 railway line between London and Birmingham, which will be 230km long and include 400 bridges and viaducts and 41km of tunnels.

Recognising that the bigger the project, the greater the need to demonstrate the benefits to society, HS2 first published a design vision, to communicate its commitment to high quality design principles. This was followed by a series of ‘design requirements’ documents, each defining what good bridge design looks like, and finally a number of ‘specimen designs’ encompassing key structures as well as common design elements. These documents promised clarity to the public and set a strict quality benchmark against which the industry could measure its subsequent work on design and construction, while still allowing freedom in construction techniques.

The most important specimen design to date is for the landmark Colne Valley Viaduct near to Denham in Buckinghamshire. In granting permission to the first phase of the works in February 2016, the Parliamentary Select Committee described this structure as the ‘most significant visible engineering feature of the HS2 Phase One route’, holding ‘international significance’. The committee concluded that ‘sympathetically and imaginatively designed, the viaduct can become a suitable symbol for the country’s future HSR network’.

Developed by specialist bridge designer Knight Architects, with HS2’s engineering design partner Atkins, the specimen design for the 3.4km-long twin track viaduct was developed over several months in extensive consultation with the Colne Valley Regional Park Panel — a stakeholder group including HS2, the Department for Transport, the local planning authorities, county councils, Natural England, Environment Agency, Canal & River Trust, local wildlife trusts and the Colne Valley Regional Park Community Interest Company. The panel members’ mandate includes a requirement to work collaboratively to achieve a distinctive and sensitive outcome in the Colne Valley.

HS2’s design vision is based on the core design principles of ‘people, place and time’ and the Colne Valley Viaduct specimen design addresses each of these, with a particular focus on engaging communities and designing for a sense of place. In this case the ‘place’ is a 111km2 green valley whose combination of watercourses, lakes, meadows and woods makes the area valuable for recreational uses and environmentally sensitive in terms of landscape and wildlife. It will also be a distinctive experience for train passengers, being located between the 16km-long Chilterns Tunnel and the 14km-long Northolt Tunnel.

Aerial perspective of the route

The landscape context can be broadly characterised into areas of water and woodland. The former provides generous, open views with scenic properties while by contrast the woodland areas are defined by dense planting and enclosed spaces, with tree-lined roads permitting only restricted views of the structure which will pass through it. It was considered important that the design of the Colne Valley Viaduct responded specifically to these strongly divergent characteristics.

Equally important was the need to inform stakeholders of the defining structural and technical characteristics of high speed rail. Such rail viaducts are subjected to much heavier vertical and horizontal loads than road bridges, and must comply with strict deflection and vibration limits to guarantee passenger comfort and traffic safety. High speed rail bridges typically have a robust appearance and shorter spans than those carrying other types of traffic, which makes the design of elegant structures particularly challenging, especially when visual elements such as noise barriers are also required.

A successful design must negotiate both the site and HSR-related constraints and the development of a specimen design within a process of stakeholder engagement allowed a site-specific solution which was also technically robust.

The Colne Valley Viaduct is expected to represent the very best in contemporary design, becoming a ‘beacon’ of HS2 design quality. To many people this initially conjured images of the dramatically tall Millau Viaduct — yet the very low level alignment in the Colne Valley demanded a distinctly different design approach.

The bridge’s vertical alignment will be noticeably low at between 10-15m above ground level. This creates a significant design constraint as the structure will pass through rather than over the woodland, limiting the viaduct’s legibility. The visual heaviness of a low structure could easily overwhelm the pedestrians below.

By contrast, when over the open, calm water, the reflections of a low bridge offer a picturesque landscape opportunity. Horizontally, the viaduct will trace a gentle curve along the Colne Valley, creating fluid, free-flowing lines that sit well within the natural setting and will be enjoyable for passengers travelling upon it.

The viaduct will carry two rail tracks on a slab track base, with a cant due to curved plan. A derailment containment system is required as well as evacuation and maintenance routes on each side with a continuous protection barrier. Overhead line electrification supports are set outside of the rail lines and significant acoustic barriers (the equivalent of up to 4m high) will be required on at least on one side of the deck, along most of the viaduct length.

A viaduct with a low vertical alignment must be well proportioned, with span, deck-depth, deck-width and soffit clearance the main parameters influencing bridge proportions. Overly short spans detract from the drama and elegance of the structure, but longer spans require also larger deck depths and lower soffits, which conflicts with the openness of views and the space below the bridge. Finding a harmonious balance between span, depth and clearance is critical in the success of the design of a low viaduct. Numerous layout options were tested against the site and HS2 constraints, ranging from a very conventional viaduct with 50m to 60m-long typical spans to a tall, multi-span cable-stayed viaduct that was used to explain the drawbacks of the ‘beacon’ design that many people had in mind at the beginning of the exercise.

The selected design is a multi-span concrete structure whose typical deck arrangement in the woodland areas smoothly evolves from standard 30m-long spans into a series of 105m-long spans over the lakes, each of which has an arched soffit and triangular openings where the section is deepest.

This solution responds directly to the different character areas and obstacles that have to be crossed. It employs a standard deck section throughout the length of the structure, which enables economical construction, and the elegant sequence of arches exploits the reflection on the water to create a memorable feature.

A viaduct with a low vertical alignment must be well proportioned, with span, deck-depth, deck-width and soffit clearance the main parameters influencing proportions (Hayes Davidson/Knight Architects)

Concrete was chosen as the main material for the bridge due to its appearance, maintenance benefits and the inherent mass and damping it provides. The proximity of the deck to ground level and the shallow depth of the lakes also suit this approach.

A traditional box girder section with side cantilevers locates the structure below track level, would limit the clearance below the soffit and would make the deck seem very deep; a perception that would be made worse by the addition of the robust kerbs, noise barriers and parapets.

A U-shaped ‘through structure’ would improve these two aspects, raising the soffit and overlapping structure and other deck elements, but this would be at the significant cost of restricting the views for passengers and requiring piers to be either very wide, or in pairs, because of the need for edge support.

The proposed solution combines the benefits of these two extremes, with its cross-section being somewhere betwee a trough and box girder; actively responding to the HS2 constraints and the peculiarities of the site.

The height of the webs allows unobstructed views from the train and helps mitigate wheel-generated noise. It has a reduced depth below the tracks, offering increased ground clearance with enough torsional stiffness to allow compact and transversely-slender piers. The visually opaque woodland offers opportunities to locate the atypical substructure elements required for points of fixity and expansion joints out of public view.

The critical importance of stakeholder engagement in smoothing the progress of major projects is increasingly understood. In the case of HS2 this was recognised not only by the client team but also by the Department for Transport, the accountable governmental body, which observed: “If you get communities engaged, they feel they’ve understood the choices. Design can help to mediate uncomfortable positions”.

The use of a specimen design is highly valuable to articulate the art of the possible and to define how good a project will be, both to the project and to the public. This is particularly true when design is developed in close consultation with the promoter and the key stakeholders, as was the case for the Colne Valley Viaduct, where the classical combination of elegant arches, structural lightness and subtle proportions won widespread support as a fitting addition to the landscape.

Historically, stakeholder engagement has been a game of trust and the promise of quality has to be substantially fulfilled or the exercise may be seen as a cynical route forward, bypassing genuine scrutiny. It is hoped the use of specimen designs can help to lock in design quality for the long-term benefit of everyone.

Martin Knight is director, and Héctor Beade-Pereda is associate and project leader at Knight Architects