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Sat September 19 2020

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Go with the flow

26 Oct 15 Margo Cole reports on a Skanska project to ease congestion at the Catthorpe Interchange in Leicestershire.

Aerial view of the site showing the M1 running from south (top right) to north (centre left) and the new interchange under construction
Aerial view of the site showing the M1 running from south (top right) to north (centre left) and the new interchange under construction

Delays at Junction 19 of the M1 are something of a mainstay of the daily traffic reports on local radio and TV. But, as anyone who has driven through the area in the past 18 months will testify, a major project is currently under way to ensure these delays become a thing of the past.

The junction, known as the Catthorpe Interchange, sits close to the Leicestershire/Warwickshire border and is where the M1 meets the southern end of the M6 and the western end of the A14. In its current configuration, there is very limited direct access between the three major roads. Traffic heading north on the M1 can peel off onto a link road that joins the M6 while a viaduct carries a slip road for vehicles doing the opposite manoeuvre, from the end of the M6 onto the M1 southbound. For any other connection, drivers must navigate a “dumbbell” road layout consisting of two roundabouts – one either side of the M1 – and a narrow underpass beneath the motorway that is shared with local traffic.

 “It has always been a problem,” explains Duncan Thompson, project director for Skanska, which has a £191m contract to design and build the new junction. “With everything going through the three-lane underpass, you get a lot of congestion, accidents and inconvenience, as well as a very difficult situation for pedestrians, cyclists and horse riders.”

Highways England (formerly the Highways Agency) first proposed improving the junction more than 10 years ago and appointed Skanska under an early contractor involvement (ECI) arrangement in 2005. The contractor and its designer Jacobs considered various options and by 2010 a design had been finalised and granted planning approval – only to be put on hold when the incoming coalition government announced a review of all major projects.

One element of the project, deemed essential for safety reasons, escaped the review process. This was the replacement of the Catthorpe viaduct which carries the M6/M1 southbound link over the M1. “That was considered important, and was effectively a bridge maintenance project,” explains Highways England assistant project manager David Reed. “The structure was failing and needed to be replaced.”
Skanska started that work in 2010 and finished it in 2012.

A year after the junction improvement project had been halted, the government concluded that the scheme did stack up financially, and would contribute to economic growth. Around 20% of the 157,000 vehicles that pass through the junction every day are HGVs, carrying freight to and from the east coast ports via the A14, and accessing the many freight distribution centres that are situated in this part of the Midlands.

But the government insisted that cost savings had to be made so the client, together with Skanska and Jacobs, set about redesigning the scheme and going back through the planning process with a new design. Skanska was finally able to move onto the site in November 2013 and started construction two months later.

The key to the new layout is that it provides free-flow connections for all of the main traffic movements. In addition to the existing links between the M1 and M6, there will also be direct links from the A14 to the M1 northbound and from the M1 southbound to the A14, as well as between the M6 and the A14 in both directions. Skanska is also building new local roads to serve the villages of Catthorpe and Swinford. “The big improvement is the free-flow link between the M6 and the A14,” explains Thompson. Once the scheme is complete, in autumn 2016, drivers heading south on the M6 will simply continue straight on if they want to travel east on the A14; and likewise for traffic going in the other direction, with slip roads peeling off to join the M1.

The new interchange will have three levels: the A14/M6 running in a new cutting beneath the M1; the M1 remaining at its current level; and the new slip roads on viaducts at a higher level. As a result, six major new structures have to be built, all without interrupting the flow of traffic on the major roads. “A lot of our programme is dictated by accommodating these 157,000 vehicles a day. We have a 50mph speed limit, and must maintain the existing number of lanes,” explains Thompson, who says the biggest challenge on the project is making sure traffic continues to flow each day. “Everything we do is about minimising impact on road users.” To do this, the contractor has devised 22 main traffic management phases, each of which sees existing carriageways moved or remodelled to allow the new structures to be constructed. Thompson likens it to one of those sliding tile puzzles, where the end result can only be achieved by moving the various tiles into different positions in the correct order.

Under the first of these traffic management phases, a section of the northbound M1 was widened and traffic moved onto the widened section to allow a bridge pier to be built in the existing central reservation to support the new viaduct carrying the A14/M1 northbound link.

The second phase involved moving the M6 southbound onto temporary roads in order to build the south abutment for the new viaduct carrying the M6/M1 southbound link over the M6/A14.

Subsequent phases have opened up the central area of the site so that Skanska can get on with building three main structures on the project: the bridge carrying the M1 over the M6/A14 and the huge curving viaducts carrying the A14/M1 northbound and M6/M1 southbound slip roads. “2014 was about installing temporary roads and moving the roads out to allow us to get into the middle of the site,” explains Thompson.

The two new slip road viaducts both carry two running lanes and a hard shoulder and are of similar construction. The A14/M1 northbound link flyover is the longer of the two at 280m while the M6/M1 southbound link over the M6/A14 is 248m long. Both have bored piled foundations, consisting of 750mm-diameter concrete piles installed to a depth of up to 21m and in-situ concrete “torch” columns.

Preparing the deck of the bridge carrying the M6/A14 over the local Swinford Road for a concrete pour
Preparing the deck of the bridge carrying the M6/A14 over the local Swinford Road for a concrete pour

The deck beams are made of weathered steel, with a total of around 1,860 tonnes of structural steel going into the M6/M1 viaduct and 1,482 tonnes into the M6/A14 structure. When The Construction Index visited the site at the end of May, the concrete deck on the M6/M1 southbound viaduct was being poured. Construction was also well under way on the A14/M1 northbound viaduct, with pairs of steel deck beams being lifted into position by
a 600t-capacity Hitachi Sumitomo
6000SLX crawler crane supplied by heavy-lift contractor Sarens. The biggest lift is expected to be 180 tonnes.

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The steel beams for these two viaducts and two other structures on the project have been supplied by Mabey Bridge, sadly the last order to go through the company’s struggling Chepstow factory before it closes with the loss of 150 jobs. The other two steel bridges are a 35m structure carrying the M1/M6 northbound link over a local road, and the 42m span that will carry the full width (three lanes and hard shoulder in each direction) of the M1 over the A14/M6.

This bridge is being built using top-down construction. The abutments were formed by installing 90 contiguous bored piles, 60 of them 1,500mm in diameter and 30 of 1,800mm diameter, to a depth of around 20m into the underlying lias clay. Diaphragm beams were cast on top of the piles at ground level and the steel deck beams placed on top before the ground beneath was excavated. The bridge has been constructed in two halves, with the motorway traffic running in contraflow to enable each half to be built without interrupting traffic.

The material excavated during construction of this bridge contributes to the 500,000 onnes of earthworks being moved on site, predominantly by muckshifting contractor Walters. “There is a cut/fill balance, so everything stays on site,” explains Thompson. “We have worked hard to minimise materials by reusing everything that’s here already.” This includes material recovered from re- routing existing roads and from demolishing an old bridge on the M6. “The demolition material is being used in the temporary works, for the crane and piling platforms, and then ultimately will go in as capping layer,” says Thompson.

There are currently around 300 people working on site, and Thompson is aware of the importance of maintaining the project’s high demand for both labour and materials as work progresses. “A real challenge for the job is that the civil engineering market is starting to boom and is drawing on resources; so we have to use people and materials wisely,” he says. “It has been a progressively increasing challenge for all of us [in the industry]: how to train more resource to get more competent people in; and also to meet the demand for more quality materials and products.”

Reed says the creation of Highways England back in April should help address this problem – in the roads sector at least – because it can offer better financial certainty than its predecessor. “This is part of the principal behind Highways England. We will have a pot of money for five or six years, so we can go to the major contractors and tell them what we’re planning.”

A 600t-capacity crane is used for the main bridge beam lifts
A 600t-capacity crane is used for the main bridge beam lifts

Working With BIM

Building information modelling (BIM) has been an important factor in the success of the Catthorpe Interchange project so far, according to Skanska project director Duncan Thompson: “We have made extensive use of BIM... The design was done in BIM, and we took all our information from the model.”

Skanska design and engineering manager Steve Hamer adds: “BIM wasn’t an obligation on this project, but we took a leap of faith and decided to do it. [Our designer] Jacobs was on a big learning curve, but they produced a very robust 3D model that all the drawings are produced from.”

The contractor has issued 30 iPad minis to staff on site linked to the BIM model. “They can pull up the relevant drawing and interrogate it on site rather than having to come back to the office,” explains Hamer, who adds that the iPads are also loaded with the project’s standard checklists and health & safety information. “They can also use it to brief the workforce on the work that’s planned for that day.”

The BIM model has proved very useful as a communication tool, helping staff and subcontractors to visualise how the elements of the complex junction fit together. But it is also far more than that, being used both for clash detection and for guiding the earthmoving machines.

“The groundworks contractor, Walters, is geared up to receive all the electronic information from the model,” explains Hamer. “We have utilised a lot of their experience, and worked with them in the early days to find out what information is important to them. It’s working well; there are no profile boards out there.”

This article first appeared in the September 2015 issue of The Construction Index magazine. To read the full magazine online, click here.

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