Colne Valley Viaduct, London

HS2 trains will only spend a minute or so above ground between the tunnels that take them out of London and through the Chiltern Hills – but a lot happens in that minute. They will soar past a tapestry of ancient woods and wetlands, skimming across four lakes and the River Colne. They will traverse roads, new cycle routes and footpaths. And they will race over the Grand Union Canal – the future of British transport briefly exchanging glances with its industrial past.

This whole scene will unfold on the longest rail bridge in the UK – a title that had been held by the Tay Bridge in Scotland for the past 140 years. The Colne Valley Viaduct, now structurally complete, is an epic piece of engineering. Following a gently curving 3.4km route through the Colne Valley, it is made of 56 reinforced concrete piers, spaced up to 80m apart. Above these sits a box-girder bridge deck, ranging in height from 3m to 6.7m and built from 1,000 unique concrete segments – an assembly that required its own on-site precast factory and a launching gantry so big that onlookers assumed it was the bridge itself.

Despite its colossal scale, the viaduct had to tread lightly on this protected slice of greenbelt, and enhance rather than overshadow the landscape. The project team – a joint venture of Bouygues Travaux Publics, VolkerFitzpatrick and Sir Robert McAlpine known as Align JV, and its design consultants Jacobs, Ingerop-Rendel, Grimshaw and LDA Design – had to minimise the carbon impacts too, in line with HS2’s commitment to cutting construction emissions by 50% by 2030, against a 2016 baseline. This presented a challenge, but also an opportunity: a structure that can withstand trains hurtling at 200mph will inevitably need a lot of materials, but across 3.4km any design efficiencies quickly add up to huge savings.

For architect Grimshaw, one of the biggest questions was how to make cohesive architecture from a form measured in kilometres rather than metres. “It’s a really complex site, and probably has every constraint you can imagine in terms of building a viaduct – the regional park, lakes, roads, reservoirs, the Grand Union Canal,” says Chris Patience, principal at Grimshaw. “There was a real risk that it would look different at various points along the route, with multiple engineering solutions all dealing with different issues.”

Their answer was a common design language, with a strict grammar that would apply across all structural elements. Gradually, a lexicon emerged based on folded planes of fair-faced concrete, with triangular geometries and defined edges. This seemed to resolve a number of problems, not least how to make massive structural objects appear lighter.

“We probably tested about 50 different ways of expressing form,” says Patience. “We found that facets were the most efficient way of breaking up visual mass because of the way they emphasise the contrast between light and dark. That allowed us to play with the silhouette and highlight particular geometries.” While more muscular elements retreat into shadow, the eye is drawn towards the free-flowing lines of the underside of the deck – the only curves in the entire structure.

The folded forms were also the most efficient way of supporting the load: “We initially looked at including more curved elements, but you end up adding concrete. The facets are optimised for structural performance.” This language was developed into a family of four structural components. The two main elements are a wide V-shaped pier, which carries the 80m spans over water, and a straighter, more upright form to support shorter 60m spans through the woodland.

“We wanted the structure to be responsive to the different contexts,” says Patience. “Over water, you want to maximise the views through the landscape by creating really wide spans. But in the dense woodland, the straighter piers push the structure up above the canopy, maximising clearance below.” Foot and cycle paths weave in and out of the piers – in all, 3.5km of new routes have been created as part of the viaduct project.

The other two elements are an expansion portal, a type of double pier that supports movement joints in the deck at 900m intervals, and a fixed buttress, which disperses the braking load from the trains into the ground. As with the main piers, these components are used in response to specific site features, says Patience. One of the most prominent locations is the Grand Union Canal, where the buttresses form a gateway on either side of the tow path. “It’s quite an amazing moment – the converging of entirely different speeds of travel. You’ve got canal boats, barges, and now high- speed rail.”

Each pier stands on four to six rotary bored piles, embedded up to 55m into the underlying chalk bed. They were cast using steel formwork – standardised to enable reuse. All of the in-situ concrete, which contains 70% GGBS, has been left as struck. Partly, this was because of the requirement for a 120-year design life, so there are no additional finishes to compromise durability or increase maintenance needs. But it is also another way of bringing cohesion. The architects introduced a single variation: a bush-hammered effect created using a flexible formliner, selectively applied where the structure meets the ground or water.

These rougher surfaces are recessed by 25-50mm, with a chamfered edge helping to navigate the turn into the adjacent face. This use of texture helps to mediate between the vastly different scales at which the viaduct is experienced. In the woodland sections, the rougher surface offers a finer level of interest to passing walkers and cyclists, and subtle horizontal definition to the hammered finish conceals day joints from the eagle- eyed. At the other end of the scale, where the viaduct is seen from across the water, the texture adds another layer of contrast, helping some planes to retreat into shadow as the structure melds into the water.

At the top of the piers, mechanical bearings take the full load of the deck and allow for thermal expansion and contraction. They also introduce visual separation between the main structural elements, conveying a sense of lightness. This is not disingenuous – the designers worked hard to minimise the load. The box girder locates most of the structure directly below the track, improving efficiency, while the deck width was chipped away until it stood at 13.4m, 1m narrower than in the outline design presented to Parliament in the High Speed Rail Bill.

“We probably spent about a year reducing the cross- section of the deck as far as we possibly could,” says Patience. “Across 3.4km, even saving 10mm makes a huge difference.” There are also knock-on effects – the piles, for example, are 10-15m shorter than in the outline design. “The more weight we put on the deck, the bigger the piers, the bigger the foundations, the more carbon – and the bigger the structure is visually.”

A continuous parapet wall envelopes the deck in a clean, uninterrupted horizontal band of fair-faced concrete. This reprises the language of faceted forms, with a fold line breaking up the scale of the outer face and providing a slender band of shadow along the top edge. These elements were precast in 3m-long panels and fixed into the deck reinforcement on site. To further reduce the weight of the deck, the wall sections were made from ultra-high performance fibre-reinforced concrete (UHPFRC), enabling thicknesses of just 55mm. UHPFRC also has low levels of porosity, which will slow the effects of weathering over the viaduct’s lifetime.

The project’s embodied carbon is 28% – or 63,300 tonnes – lower than the outline design. Often, it is hard to visualise what numbers like this really mean. But here, the absence of carbon is perceptible: there is simply less viaduct. A monumental piece of infrastructure has been broken into light and shadow, the dynamic forces of high- speed rail distilled into pure form. The HS2 project has often been depicted as something like the construction equivalent of Sisyphus pushing a boulder uphill for eternity. At Colne Valley, it skims like a stone over water.