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Building the UK’s longest railway viaduct

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In the past few months, HS2 has been ramping up work on the UK’s longest railway bridge that will stretch 2.1 miles across a series of lakes and waterways just outside London.

As part of the work a second fabrication factory has become operational at its C1 compound near Denham in Buckinghamshire, which is operated by ALIGN, the joint venture made up of Bouygues Travauxs Publics, Volker Fitzpatrick, and Sir Robert McAlpine.

Challenging build

It is no easy task – this section of the high-speed railway includes the 3.4km Colne Valley viaduct, the 16km Chiltern tunnels, a couple of reservoirs and the world’s largest slurry treatment plant. It also its own access to and from the adjacent M25, keeping construction traffic away from local roads.

The 136-acre site, located in a large hollow on the edge of the Chilterns, was formerly a mix of quarry and low value farmland and is now a self-contained railway fabrication and construction site.

When construction activities are complete, more than 90 per cent will be returned to nature in a rewilding project using tunnel-bored chalk and chalk disturbed from other earthworks. The Align Project Director is Daniel Altier, an experienced engineering professional from Bouygues.

Prior to any construction work beginning, haul road are often created to achieve access to a worksite. This was not possible at C1 because the viaduct spans two reservoirs, so four jetties and associated piers had to be built across the water to enable cranes and piling rigs to start their preparatory work for some of the 56b viaduct supporting piers.

Segment production

The first fabrication factory, open for a year, is fabricating the 112,000 tunnel lining segments required for the twin bore Chiltern Tunnels. The second facility is the viaduct fabrication factory, which produced its first viaduct deck segments in February, completing seven by the by the media visit on 16 March. That left another 993 to go.

Each of the viaduct’s 1,000 deck segments will be individually numbered and uniquely shaped due to the vertical and lateral track alignment required to accommodate the high-speed slight curvature to form a perfect track deck. Each segment, weighing up to 140 tons, will be fabricated in its order of installation in what is called a Match Construction Technique.

Every segment is being poured and cast next to its neighbour to ensure the geometrical characteristics match the neighbouring piece. Installation of the first segments was planned to start in April using a 700 ton, 155 metre-long launching girder assembled on site.

Completion times

Nicolas Gay, viaduct precast factory senior engineer from Bouygues Travaux Publics, who comes from Lyon, expects the task to be completed in two years. The factory contains three huge assembly cells into which the bespoke rebar frames are placed before shuttering and moulds are put in place. It is an incredibly noisy environment where all staff have to wear ear defenders and it has several purposes away from viaduct segment production:

Work can be carried out under cover away from weather interference preventing production;

It reduces external construction noise pollution locally.

There are two external production lines outside the factory where the rebar frames are constructed. Seven huge jigs, each containing a unique rebar frame in various stages of production, were adjacent to a line of completed frames waiting their turn to be transported into the factory. The steelwork is stored in a compound alongside the rebar construction jigs and where preparatory work is carried out by steelworkers.

Once completed, frames are taken on a self- propelled lifting platform into a factory side door when their assembly slot calls. Once placed in the factory cells, flexible moulds and traditional shuttering is put in place held by jacks and rams.

When cement has been poured, the moulds collapsed and withdrawn from the segment, cleaned and checked, it is transported from the factory production line on another self-propelled rail-mounted platform to a temporary storage position outside. From there a 140-ton capacity crane will lift the segment to its storage position on a large concrete apron waiting for its visit to the 155 metre, 700 ton lifting frame, assembled onsite, and final installation.

Production began slowly in February to test that all processes, plant and machinery operated as planned. A stockpile will be manufactured so that viaduct installation will not be delayed once the process commences should any factory or supply chain disruption take place.

The unique size of each segment is also driven by the interior strength required for each concrete deck segment, which means a different individual depth depending on where the segment’s position on the viaduct will be. This is why the flexible moulds are used to create the voids in the rebar frame. Moulds can be made into any shape to match the individual design of the segment.

Segment finishing work is then carried out on every edge and checked to ensure accuracy and conformity to design while still in the factory cells. Tools and equipment are transported to the workforce within the factory by roof mounted 15-ton capacity cranes.

Easy maintenance

The production line is capable of turning out 12 segments weekly and was running at about 25 per cent capacity in mid-March, but speeded up after engineers were satisfied the planned activities were working as expected. The finished articles will contain three voids below the deck on which track will be installed. These will be used for services enabling easier maintenance and drainage while lateral strength will be assisted by pre-stressed tensioning cables running along the viaduct’s length creating internal strength.

The viaduct’s north abutment and four piers were in place in mid-March with the former’s embankment being made up to enable the launching girder to access the construction. Segments will be installed around piers to maintain physical balance at all times in cantilever style with the widest spans crossing the lakes with narrower ones elsewhere.