Large-scale off-site: in HS2’s precast factories

HS2’s Colne Valley Viaduct and Chiltern Tunnel precast plants are responsible for some of the most ambitious precast work ever attempted in the UK. Joshua Stein reports

Scheme Central Lot 1 (C1) of HS2 Phase 1 – Colne Valley Viaduct and Chiltern Tunnel, Hillingdon

Customer HS2

Main contractor Align joint venture (Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick)

Contract value £965 million

Contract awarded July 2017

Notice to continue April 2020

Start of main works September 2020

IIt may only be a small part of the massive HS2 megaproject, but the South Portal site in West London is perhaps the largest currently working on the £107bn rail construction project pound sterling.

The task ahead is to build the 3.4km Colne Valley Viaduct – set to become the UK’s longest rail bridge – and the 16km twin-bore Chiltern Tunnel, the longest excavation on the road HS2. Both of these mammoth tasks have been cut into manageable chunks through the use of prefabrication, resulting in two of the largest off-site fabrication projects ever undertaken in the UK.

The Align joint venture – made up of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick – holds the £965m contract for this section of the route, the Central 1 (C1) lot of HS2 Phase 1.

Line up steel construction specialist Caunton Engineering to build two factories; one to prefabricate tunnel sections and the other to cut viaduct slices.

Both facilities are huge. The viaduct plant is 100 meters long and has an internal volume of 105,000 cubic meters, large enough to swallow London’s Royal Albert Hall. The nearby prefabrication plant for the Chiltern Tunnel, which has a U-shaped plan, is 125 meters long and has an internal volume of 110,000 cubic metres.

Around 2,000 people are employed at the Align sites, the majority in Hillingdon, and they were working hard when Building News visited in early April. The site has the impression of being autonomous from a university campus, with residence halls.

Workers who live beyond commuting distance have the option of staying overnight for a few days at a time in the “Snooze Box,” a purpose-built hotel on site, which was recently expanded to 160 rooms. NC Didn’t spend the night so can only speculate on creature comforts offered.

In large scale

The scope of the workforce corresponds to the scale of the business. The 3.4km viaduct is set to become the longest railway bridge in the UK, but not the straightest. Its course straddles a series of lakes and waterways in a great curve, meaning each of the 1,000 segments used in its construction has a unique shape.

Making so many unique concrete pieces, each weighing 60 to 140 tonnes, is certainly a challenge, says placement site manager Seb Lewis, who is employed by Align member Bouygues Travaux Publics. “The main factors of variation between segments are the height of the segments and also the angle on the bottom slab,” he explains outside the factory.

“We place the formwork for the next segment against the cage [of the previous one]so the new will be [bear] against”

Seb Lewis, Bouygues Public Works

To ensure that the segments fit together perfectly, the previously manufactured segment is used in the manufacturing process of the current segment. “We place the formwork for the next segment against the cage [of the previous one]so the new will be [bear] against that,” Lewis says. “If there is a problem with the finished face of the segment, then it can be countered with the next segment, as it is matched against it.”

The prefabricated segments are lined up outside the factory by the hundreds on the day of NCis the visit. Align plans to start assembling them this summer. A few steps away, in the tunnel’s prefabrication factory, there are variations to manage as the segments are produced.

Each of the 16,044 rings will be made up of seven different segments, meaning around 112,000 segments will be shaped across the plant. Most of the concrete segments will be reinforced with steel fiber, but approximately 1,200 rings will be double reinforced with steel rebar and steel fiber, allowing a higher level of post-tensioning to additional rigidity.

“The reason why some have ring rebar in addition to steel fiber is that they were designed for specific areas of the tunnel, such as cross passages or shafts,” says Marcos Roman, principal tunnel prefabrication engineer of Align. Given the varying requirements, digital techniques are used to ensure that each segment is shaped correctly.

Casting process

The molding process at the factory involves a 10-step carousel, each step takes 11 minutes. A manager must press a button to indicate that the work has been performed to the correct standard before the entire carousel can proceed to the next stage. The process operates continuously, 24 hours a day, five days a week, to maximize throughput. From the first stage of the production process, each segment is equipped with a barcode, so that Align can track when work has been done on each part and assign where each part should be placed in the finished structure.

“It tells you where the segment is in production, when the segment was produced, and you can track any deterioration or need for reinforcement, which will come later, once the tunnel is developed, using the barcode. This eliminates the need for paperwork and makes the details of each segment easily accessible,” says Roman.

The same digital pattern and barcodes allow each part to be tracked during manufacture and once the tunnel is complete. Once out of the carousel, the segments are kept inside the plant for 24 hours to avoid the risk of surface cracking due to temperature changes during the curing of the concrete. They are then stored outside for 14 days to gain strength, although the plant’s supply of segments is currently sufficient for the next two to three months of excavation work. The intention, says Roman, is to avoid any possible slowdown in labor once it has started.

Triple productivity

As HS2 and the construction industry as a whole step up their work in the wake of the pandemic, they share a common watchword: productivity. Efforts are continually made to improve productivity in all prefabrication processes. At the viaduct plant, a transfer cart is used to support the segments as they emerge, reducing the pressure placed on them, Lewis explains. This allows them to be moved earlier in the curing process, saving time overall.

Segments have been designed for specific areas of the tunnel

“Using the transfer cart, we only have to wait for the concrete to reach 12 megapascals [in compressive strength]instead of 36 megapascals, it therefore triples the productivity rate when it comes to developing the segments,” explains Lewis.

In addition, the pouring schedule was designed so as not to waste time waiting for the concrete to set. “We pay a night shift at 4 or 5 p.m. So by the time the guys from the day shift arrive, in the morning at 7 or 8 a.m., this concrete has reached the required strength,” he says.

At the tunnel factory, efficiency is enhanced by accelerating the heat curing process. At the final stage of production, each segment is baked at 45 degrees centigrade and 80% humidity. This combination has been found to speed up the concrete strengthening process so that hardening, which would normally take 24 hours at room temperature, can be sped up to just five or six hours.

“There are multiple benefits to the process here,” says Roman. “You save a lot of time to complete each segment, and you don’t have to worry about weather conditions either, because the whole process is done inside the factory.”

Safety is “a top priority”

Many safety risks present on the construction site can be swept away thanks to automation. Roman says the precast tunnel factory is 95% automated, which eliminates a host of safety issues. Removing the need for manual intervention as much as possible results in a safer and better workplace for everyone, he adds.

Roman explains that as each segment goes through 10 distinct stages, conventional precast methods would require a lid to be manually placed over each mold at each stage.

“The problem is that it gives you usability and security issues,” he says. “You have to manually lift the lids all the time, you have to clean the lids, and that can create a dust problem, especially during cleaning activities. What we did was we completely removed the lids of the carousel and we installed an automatic cover system.

“In the long term, we plan to have no MEWPs [mobile elevating work platforms]”

Marcos Roman, Align

“The automatic cover is designed to have a universal shape, which will not change the shape of the segment itself but fulfills the same requirement. By simply clicking a few buttons, the carousel operator can manage everything, and wash or dismantle the cover, thus we eliminate any ergonomic problem or any safety problem of the line. »

Likewise, at the viaduct factory, minimizing working at height is a “key objective,” says Lewis. According to the Health and Safety Executive (HSE), falls from heights remain the leading cause of accidents in construction. “In the long term, we plan to have no MEWPs [mobile elevating work platforms],” he adds.

For now, by using a stud lifting system and a 140-ton secondary crane to lift the precast segments, operators no longer need to work at height on the segments themselves, minimizing the danger to the workers.

think about the community

The engineering behind HS2 may be impressive and the Colne Valley Viaduct will represent an exceptional building feat, but it remains a bone of contention that locals will not directly benefit from the rail line when it opens – no new stations is planned in the vicinity.

The use of on-site prefabrication plants has greatly reduced the need for heavy goods vehicles on local roads, minimizing the impact on local communities. As Roman puts it, “Everything for this project is done in-house, the idea being that it doesn’t affect the wider communities.”

The tunnel work creates a fair amount of waste, much of it chalk, which is dug up during the construction process. This will be used on the site to create a 90ha slope, open to the public, made up of meadows and woods. Before work began, the site was agricultural land.

The idea, explains Align communications manager Rob Hutchison, is that no one will notice the amount of work being done on site. “We want people to cross the viaduct by train and notice the beauty of the area, not the fact that we were once at work here,” he adds.

Alice F. Ponder