---
title: HS2 slides into the record books
url: "https://www.systra.com/uk/news/hs2-slides-into-the-record-books/"
type: post
date_published: 2023-05-27
date_modified: 2023-09-14
schema:
  @type: Article
language: en-GB
word_count: 1349
reading_time: 7 min
canonical: "https://www.systra.com/uk/news/hs2-slides-into-the-record-books/"
featured_image: "https://www.systra.com/uk/wp-content/uploads/sites/11/2023/06/hs2-vl-31659-m42mar_2-m42-marston-box_web.jpg"
categories:
  - Non classé
---

# HS2 slides into the record books

![HS2 slides into the record books](https://www.systra.com/uk/wp-content/uploads/sites/11/2023/06/hs2-vl-31659-m42mar_2-m42-marston-box_web.jpg)

**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.  

“We had to look at all the positive and negative tolerances and understand how
they would work in multiple scenarios. As a project team, we worked closely with
HS2, BBV and Freyssinet to understand their methodology and with our
geotechnical engineers to understand the ground conditions. And we had to build
some flexibility into the design.”

\~ Paul Mills, Design Manager, SYSTRA Ltd

### 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.

![Drone images of the Marston Box bridge after it was successfully installed over the M42 motorway in Warwickshire](https://www.systra.com/uk/wp-content/uploads/sites/11/2023/06/marston-box-construction-hs2-vl-50987-dji_0110_web-edited.jpg)

_Drone images of the Marston Box bridge after it was successfully installed over the M42 motorway in Warwickshire._

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.”

**Categories:** Non classé