HS2 slides into the record books

In December, HS2 celebrated a world first as a team of around 450 people, led by HS2’s construction partner in the Midlands Balfour Beatty VINCI (BBV), slid a 12,600-tonne bridge a record 165 metres across a motorway in Warwickshire.

A video of the Marston Box Rail Bridge sliding into position over the M42 was the BBC’s ‘most watched’ at one point over the Christmas break. As the footage demonstrates, this was both an impressive spectacle and an engineering triumph that could lead to wider use of this method in the UK.

It was good news for motorists too. The project team worked closely with National Highways to eliminate two years of restrictions and the closure even ended a whole day earlier than planned.

“The Marston Box slide was a great example of what can be achieved when we work collaboratively and as one team,” says Sasan Ghavami, BBV’s construction director, who oversaw the project. “This complex and challenging puzzle was solved thanks to lots of different teams coming together, each bringing their own area of expertise and experience. It’s certainly a career highlight for me, as I’m sure it is for many others who contributed to this impressive engineering feat.”

Sasan Ghavami, BBV’s Construction Director

Taking place between Christmas Eve and Boxing Day, this was the first slide of its kind across a smotorway in the UK and specialist Freyssinet, which designed the slide, believes it to be the world’s longest. Box slides are not uncommon in the UK but are usually over or under rail lines; in North Africa and Europe, box slides over highways are less of a novelty.

This was an alternative design for the M42 Marston Box Rail Bridge, requiring BBV’s designer SYSTRA – as part of the Mott MacDonald SYSTRA Design Joint Venture – to rethink the structure, working closely with Freyssinet on the interfaces between temporary and permanent works to ensure the bridge’s 120-year design life. And then there was the plethora of tolerances to contend with, how the structure would settle and the impact on headroom for the motorway and the high-speed rail line above it, since the box sits straight on top of compacted fill with no piles.  

slimmed down

The design for the original M42 Marston Box Bridge, which crosses the M42 near junction 9 in Warwickshire, consisted of a pergola-type structure, sitting perpendicular to the carriageway. The box slide alternative came out of an extensive and wide-ranging value engineering exercise that took place in 2018 after BBV was appointed on lots N1 and N2 phase 1 of the HS2 project.

“We were looking at ideas, construction methods, innovation, how we could do things better than the way assumed at the hybrid bill stage,” recalls SYSTRA’s UK highways lead Andy Baines.

It was a big step for National Highways to agree to this method, but there were some compelling reasons why it looked like a good alternative, says Baines. First, this would be far safer for workers and drivers, with much less work taking place alongside live motorway lanes. Second, there would be far less disruption to road users: rather than two years of reduced lane widths and speed limits and three months of overnight closures, this method required just two road closures.

The new bridge design consists of a two-cell box, skewed at an angle of 61 degrees. It has a large flat base, to reduce the bearing pressure in the temporary and permanent states, three piers which span its width, and a top deck.

There were carbon saving benefits to this solution. Since this structure had to be pushed into position, it was vital to limit its weight. The result is a structure that uses far less concrete, with a smaller plan area than its predecessor design and no concrete piles, leading to a significant reduction in embodied carbon.

The slide

Like all the best engineering solutions, Freyssinet’s patented sliding system is simple in its concept. The box structure is constructed on top of a concrete guide raft, with grease and polythene sheeting between the two concrete surfaces, bentonite is added to lubricate and then jacks push the structure out across the motorway.

The clever bit is how the jacks are deployed. For Marston Box there were nine of them, each with a 1,000-tonne capacity. Each jack creates its thrust by pulling itself along a cable which runs beneath the base of the box structure in a groove cast into the guide raft, anchored into the far end of the raft. The raft is prevented from slipping itself by several 1.7m-deep spades, cast into the ground at intervals along its length.

Once the box reaches the end of the guide raft, it slides onto compacted ground. For Marston Box, the existing ground wasn’t good enough which meant that 3m had to be excavated out, once the carriageway had been removed, and replacement material brought in and compacted. A dewatering system was installed across the area to lower the ground water before the guide raft could be constructed.

“A massive ground replacement exercise took place during the first seven-day closure over Christmas 2021,” says SYSTRA’s geotechnical expert, Tim Ngai. “During that time BBV also carried out a motorway drainage diversion ready for the final bridge position.”

Understanding the ground, which is extremely weak mudstone overlain with fluvial deposits, was vital for the long-term design. One of the early challenges for SYSTRA was creating the geotechnical model with limited borehole information – and no way to do more bores through a live motorway.

Replacing the carriageway was not straightforward either since it is a concrete pavement. Reinstating a concrete pavement after each closure would not have been possible since it would not have reached sufficient strength in the timeframes. Instead SYSTRA, with input from Mott MacDonald, designed an asphalt pavement which required the creation of a special joint detail where it met the concrete pavement at either end.

The lessons learnt from the first closure enabled improvements to be made for the second one. Systra’s highways team, which had been modelling traffic flows and supporting BBV to plan road closures with Staffordshire and Warwickshire County Councils and National Highways, was able to reduce diversions and closures to free up more traffic the second time.

first time for everyone

The video demonstrates BBV’s success in sequencing and logistics, showing the carefully orchestrated preparatory works completed in phases as the huge box structure advances at an average rate of 4.64m per hour. But what it can’t demonstrate is the vast amounts of cooperation and coordination that took place between all the stakeholders involved in this feat.

“There were many meetings and workshops to explain every aspect of the design to National Highways, HS2 and BBV,” says Mills. “We had to make sure everything was signed off and assured and the right consents were in place. And we were all doing it for the first time.”

This won’t be the last time, however. HS2 has two more box slides planned: one across the rail line between Coventry and Leamington Spa this summer and another over the A46 in Kenilworth.

“This has been a huge team effort, with daily challenges,” says SYSTRA’s HS2 engineering manager, Hani Benkhellat. “We are proud to have delivered a ground-breaking design for BBV, HS2 and UK plc.”