Compared to the highly visible viaduct that strides through the Colne Valley and the army of concrete piers marching towards Birmingham鈥檚 new HS2 terminal, the structures of the Delta Junction have been keeping a low profile.
Contractor Balfour Beatty Vinci joint venture (BBV) has been steadily progressing Lot N2 on the eastern flank of England鈥檚 second city for five years with very little fuss, building a 10km network of railway viaducts, embankments and cuttings. Individual bridges may have attracted attention with their high-profile construction methods, but the scale of the overall scheme is itself worthy of note.聽
From a bridge engineering perspective alone, the presence of 13 major viaducts, of three different structural types built using a myriad of construction methods, sets it up as a text-book example of how infrastructure can be woven around existing transport links with little disruption. The site is supplied by a huge precasting yard that since June 2023 has turned out nearly 3,000 deck elements at a peak rate of 36 every week.
The Delta Junction consists of three main rail corridors: the Birmingham Spur, which peels off the main HS2 line to carry trains to and from Curzon Street station terminus in the centre of the city; the Delta Mainline, intended to carry trains direct to Manchester; and the North Chord, which brings northbound trains from Curzon Street back to the main line.
Each of these corridors has to skip over, under and around rivers, railway lines, streets and motorways while making the least possible impact on the environment and other travellers.
This existing infrastructure was the starting point for decisions on what type of structures would be employed where, based on the span lengths required and the appetite for road or rail possessions.
Where long spans are necessary (Water Orton viaducts hop over the M42 motorway on 45m-long spans) a significant effort has been made to maximise repetition of these span lengths and structural types so that the most efficient use could be made of the specialist equipment necessary for precasting and construction.
Hence Delta Junction features no fewer than six precast segmental viaducts, two of them well over a kilometre long. To accommodate the varied physical and environmental demands, the contractor is using three different methods to build these structures.
Like most of the bridges on the project, they are named in pairs 鈥� Water Orton One and Two on the North Chord, the longest of the six at 1.3km and 1.4km respectively; River Tame East and West at the north end of the Delta Mainline; and Coleshill East and West, which form the junction interchange where the Birmingham Spur peels off the main line. The main line has four rail lines while the other chords of the triangle each carry two lines, so viaducts vary in width depending on where they are located, some carrying single tracks and others double.
A further four crossings are composite viaducts 鈥� with steel beams launched or craned into place and concrete decks cast in situ 鈥� and there are also three low-level viaducts built on in situ concrete piers, with decks formed of precast, prestressed concrete beams and deck planks.聽
Most of the spans for the precast segmental viaducts are being erected by balanced cantilever construction using a temporary mast and cables to transfer the balancing forces, and a pillar-jib, or 鈥榮wivel鈥� crane to manoeuvre segments into place. It鈥檚 the first UK project where this type of construction has been used, although it is relatively common elsewhere.
The first and last spans on each of the precast segmental viaducts are built by assembling the segments onto a 42m-long, 6.5-tonne steel frame made of two steel beams before stressing them together. The steel frame compensates for the lack of a balancing span.
As BBV construction director David Waite explains: 鈥淲e crane the segments into place on the beams and then we prestress the span together, jack it up to remove the truss, then lower it down to its final position.鈥�
The upcoming Christmas blockade of the railway has been in planning for more than two years now, so its arrival will be a particularly welcome milestone for the team. The main goal during this possession is to continue the Water Orton viaduct over the local railway, a process that is only permissible with the line closed.
The balanced cantilever method involves a temporary steel mast that rises 22m above the deck, and a 14m tall 鈥榮wivel鈥� crane fixed close to the leading edge of the deck. With such large-scale hardware on display, there is obvious potential for distracting drivers, something stakeholders want to eliminate.
鈥淲e are still building the last few piers at Water Orton, using the traditional in situ method with steel shutters,鈥� reports BBVJV senior project manager Nicolas Gallone. 鈥淭he aim is to reach the north side of the Birmingham/Nuneaton railway line ahead of the blockade which starts on 23rd December for 125 hours,鈥� he adds.聽
The third construction method for these segmental viaducts will come into use next summer when the six Water Orton Viaduct spans that carry HS2 over the M62 will be wheeled into place during a weekend motorway closure.
As Gallone explains, this operation will have to be meticulously executed: 鈥淲e will use the steel truss again to assemble these spans alongside the motorway so that they are ready for the road closure.鈥� Once jacked up to working height they will be driven on self-propelled modular transporters to the pier locations, transferred onto the piers and stitched together.
Maximising the use of the larger spans was BBVJV鈥檚 unique selling point, Waite claims. 鈥淥bviously you could use a form traveller to construct these spans, but Vinci鈥檚 proposal was to use precast segmental viaducts and keep repeating that span length as many times as you can.
鈥淥nce you have got the precast yard set up it makes sense to look at where else you can use it. On Coleshill Viaduct we could have used a 30m span but as it鈥檚 in a flood plain, it鈥檚 best to keep the footprint of the bridge as small as possible to maximise the flood capacity.鈥� Having a modular design made it possible to adapt to different span lengths.
Four 鈥榢its鈥� 鈥� each of which includes a steel mast, a swivel crane and a self-propelled modular transporter 鈥� are in use for the precast segmental viaducts. They are supplied with deck elements from the Kingsbury precasting yard at Lea Marston, which is 3.5km from the nearest and 6km from the furthest bridge. Laydown space is available at each viaduct, sufficient to have enough units in hand to build two spans, Gallone says.
聽The first of the precast segmental viaducts to be built was the 472m-long single-track River Thame West crossing, which was completed in February 2025. Work started on the viaduct piers in 2023, and concrete deck construction followed in the spring of 2024.
Once the single-track viaduct deck was completed, work switched to erection of the deck for the double-track crossing. The public service vehicle bridges are concentrated at this interchange, including the two longest, so naturally the kit and personnel resources are gradually being relocated here as the decks of the shorter structures reach completion.
鈥淥n the Tame viaduct where the deck is complete, we are now doing the third wave of activity 鈥� building everything above the deck, adding the architectural precast parapets, stitching them, casting the kerb and adding all the walkways, cable boxes and waterproofing and so on,鈥� says Gallone.
鈥淎t the moment we have two kits of equipment on the Coleshill East single track viaduct and the Coleshill West double track viaduct,鈥� he says. The intention is to move these two kits to Water Orton in the new year so all four kits will be available for a final deck construction push.
Since production began in 2023, the precast yard has turned out some 90% of the 2,742 units needed for the segmental decks, with the last ones due to roll off the production line at the start of 2026. But the yard won鈥檛 be dismantled just yet; it will be adapted to produced precast deck planks for the superstructure of another of the joint venture鈥檚 contracts 鈥� the viaducts that carry the railway into Curzon Street Station.
Meanwhile, the precast concrete beams for the Delta Junction viaducts are being cast at a purpose-built factory in Melton Mowbray.
鈥淎t Kingsbury we have nine shortline beds side by side, and each of them has four positions, so in a normal week we are able to manufacture up to 36 segments,鈥� Gallone explains. 鈥淓ach position can be used both for typical segments and specific ones, such as those for the pier top or abutment where the arrangement of reinforcement and post-tensioning ducts is very specific. The density of rebar goes as high as 460 kg/m3 of concrete, so the items that need to be embedded have to be placed into the
聽 form with great accuracy,鈥� he adds. Match casting is used, hence each deck element must be carefully labelled to identify its final position.聽
Such density of inclusions demands a concrete mix with microaggregates that can be easily compacted into the gaps. According to Waite, the yard has not had to scrap a single segment after concreting has begun: 鈥淲e鈥檙e very proud that we鈥檝e never lost a segment that we鈥檝e started casting!鈥�
Careful management of the shortline beds is crucial to keep the pace of production, and Gallone notes that the use of the Maturix real-time concrete monitoring app gives engineers the confidence to authorise striking and moving of units without relying on strength test results from external laboratories.
鈥淓very day you want to be able to cast another segment, so the concrete needs to be cured and with sufficient strength to allow it to be lifted out and placed in the storage lines by the gantry crane, releasing space for the next one,鈥� he says.
鈥淲e developed the maturity curve according to the different temperatures, the core temperature of the concrete, the size of the cast element and so on, and use thermocouples cast in the elements to relay data to the app so we can correlate them with those curves. Our engineers can then decide when to strike formwork.鈥�
The yard is also set up to prevent the ambient temperature from affecting production, as Waite reveals. 鈥淲e can tent it over, and we鈥檝e got heating in there so we can carry on working through the winter,鈥� he says. 鈥淥ur batching plant also has a chiller on board 鈥� it鈥檚 worth having it here as we are using so much concrete, not only for the casting yard but also to supply other assets.鈥�
A second system, called Verifi, is used to analyse the characteristics of the concrete in the drum and eliminate the need for a slump test with every batch. 鈥淲e still occasionally do samples and tests to confirm that Verifi is correct, but not for every batch,鈥� Gallone says. 鈥淰erifi has speeded up the process of pouring the concrete and Maturix has speeded up the process of striking and moving units.鈥� Both Maturix, developed by Danish start-up Sensohive, and Verifi, a US innovation, are now owned by Saint-Gobain.
New equipment has also played a central part in improving safety in the construction of the bridges, including the development of a special spreader beam for lifting deck elements, and a robotic unit for handling the jacks that are used for stressing the strands inside the deck.
The segment lifter is an automatic spreader beam designed to fit to the openings cast in the top of the precast deck units. It is deployed both on the gantry crane in the yard and on the pillar jib cranes on site, enabling the concrete units to be lifted and handled easily and safely.
鈥淐onventionally that was done by someone climbing up to fix chains and lifting tackle onto the segment,鈥� says Waite. 鈥淭hat process has been eliminated, making the operation safer and quicker.鈥�
The robotic multi-strand jack handler is even more impressive, eliminating the awkward manual handling that is usually necessary to carry out the post-tensioning operation within the box girder.
Gallone explains: 鈥淲e use very heavy compact multi-strand jacks that weigh up to 1.1 tonnes and our staff have to handle these jacks inside the box girder, moving them from span to span and lifting them to carry out the stressing of the bundles of up to 31 strands.
鈥淭he robotic unit is a chassis on articulated bogies with an articulated arm and the whole unit is radio remote controlled. It weighs almost 2.2 tonnes in total, and is moved inside the deck to the required position, where the arm is deployed by the hydraulic cylinders spaced along it. The multi-strand jack that is used to stress the strand bundle is fixed at the end of the arm.
鈥淲e have four of these machines, one for each kit, and they are remotely controlled by the operator,鈥� Gallone adds. Conventionally this procedure would involve jacks being manually handled by operatives in confined spaces using a block and tackle.
The concept for the machine dates from the 1980s, reveals Gallone, but at that time the developers could not get it to work as they wanted it to, so it was shelved. Changes in technology, coupled with the potential for extensive use on HS2 provided the necessary impetus to revisit the idea, and HS2 marks its first official deployment.
The only disappointment for Gallone is that this nifty piece of kit is tucked away inside the box girder, depriving it of the attention it deserves.
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