What is claimed to be the world’s longest – and the UK’s first ever – box bridge slide over a motorway ran like clockwork. Three months on from a record breaking bridge slide for High Speed 2 (HS2) across the M42 in Warwickshire, the team involved has revealed the years of planning behind its successful delivery. In December, HS2 Ltd celebrated a world first as a team of around 450 people, led by its construction partner in the Midlands Balfour Beatty Vinci (BBV), slid the 12,600t bridge 165m across the M42 motorway near junction 9. The impressive footage of the bridge sliding into position across the motorway was the BBC’s most watched video at one point over the Christmas break. Its success was an engineering triumph that could lead to wider use of the method in the UK. It was good news for motorists too. The project team worked closely with National Highways to eliminate two years of reduced lane widths, speed restrictions and overnight closures. And the operation ended a 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 BBV construction director Sasan Ghavami who oversaw the project. “This complex and challenging puzzle was solved thanks to lots of different teams coming together.” Box slides are not uncommon in the UK but are usually over or under railway lines. This one, which took place between Christmas Eve and Boxing Day, was the first slide of its kind across a motorway in the UK. Civil and structural engineering specialist Freyssinet, which designed the slide, believes it to have been the world’s longest. The slide involved coming up with an alternative design for the bridge and meant that BBV’s designer Systra – as part of the Mott MacDonald Systra Design Joint Venture – had to rethink the structure. Systra worked closely with Freyssinet on the interfaces between the temporary and permanent works to ensure the bridge meets its 120 year design life.
Originally the bridge was designed as a pergola-type structure. The box slide alternative came out of a wide-ranging value engineering exercise that took place in 2018 after BBV was awarded the £2.5bn Lots N1 and N2 package on Phase 1 of HS2. The new bridge design consists of a two cell concrete box, skewed at an angle of 61° to the motorway with each cell accommodating one of the motorway’s carriageways. The monolithic structure incorporates the bridge deck and has a wide, flat base to reduce bearing pressure during and after the sliding operation. Between the two cells the central dividing wall forms the bridge’s central support pier while the outer walls form the outer support piers. “We were looking at ideas, construction methods and innovation – how we could do things better than the way assumed at the hybrid bill [to enable HS2 to go ahead] stage,” recalls Systra UK highways lead Andy Baines. It was a big step for National Highways to agree to this construction method as the roads operator had never used it before and a failure could have led to the M42 being closed for a long time. There were also some compelling reasons why it looked like a good alternative, according to Baines. First, this was far safer for workers and drivers, as much less work took 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, 10 day road closures.
Like all the best engineering solutions, Freyssinet’s patented sliding system is simple in its concept. The box structure was constructed on a concrete guide raft, a huge temporary slab that stretched from where the box structure was cast to the edge of the motorway. The slab had three groups of three grooves cast into it. These grooves guided the box structure as it was jacked across the motorway. Grease and polythene sheeting separated the top surface of the slab from the lower surface of the box, while bentonite was added as lubrication. Jacks were deployed to push the structure out across the motorway. For Marston Box there were nine of them, each with a 1,000t capacity. Each jack was attached to a cable running in the groove beneath the base of the box structure and anchored to the motorway end of the structure. The raft was prevented from slipping by several 1.7m deep spades – downstand beams – cast into the ground at intervals along its length. Ahead of the slide the motorway surface had been torn up to reveal an area of improved ground prepared for the slide in a previous motorway closure. Once the box reached the end of the guide raft, it slid onto the compacted ground and across the motorway alignment. The advance ground improvement was needed as the existing ground was too unstable for the slide. Excavation of material to 3m depth was necessary. More stable replacement material was then brought in and compacted. Understanding the composition of the ground, which is weak mudstone overlain with fluvial deposits, was vital to the long term design. One of the early challenges for Systra was to create a geotechnical model with limited borehole information – and with no way to insert bores through the live motorway. The lessons 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 when the M42 was shut at Christmas.
First For All
Cooperation and coordination between all the stakeholders were required to execute the slide, with the operation taking 34.5 hours at a speed of 4.64m/hour. “There were many meetings and workshops to explain every aspect of the design to National Highways, HS2 Ltd and BBV,” says Systra design manager Paul 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.” Systra HS2 engineering manager Hani Benkhellat adds: “This has been a huge team effort, with daily challenges.” This will not be the last time, however. HS2 has two more box slides planned: one across the railway line between Coventry and Leamington Spa this summer and another over the A46 in Kenilworth.