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M1-M6 junction's complex £191M remodeling

Catthorpe aerial shot

Major civil engineering surgery to eliminate one of the worst bottlenecks on the UK’s highway network is heading towards its long-awaited completion. 

Junction 19 of the M1 is undergoing a £191M makeover that should see traffic flowing freely again by the end of 2016. 

When the A1-M1 link road opened in 1994, it was the final section of the new A14, which provided a direct route between East Anglian ports and the conurbations of the Midlands and the North. By 2015, 45,000 vehicles a day were using the link, 20% of them HGVs. All of them had to pass through the infamous Catthorpe Interchange.

Originally this was simply Junction 19, where the M6 diverges from the M1, and where the M1 crossed above a local road that linked the nearby villages of Catthorpe and Swinford. Connecting the new A14 to the two existing motorways was a major challenge with the potential to cause massive disruption to traffic flows.

Catthorpe aerial shot

Catthorpe aerial shot

Dumbbell junction takes the A14 under the M1

Presumably to minimise such disruption the decision was taken to construct a dumbbell roundabout system below the existing M1 motorway, with all A14 traffic from the East squeezing through the original underpass. Coming the other way was traffic heading to the A14 from the M6. Deep, obtrusive haunches on the underpass restricted the width, so only three lanes could be accommodated, with traffic from the M6 restricted to a single lane. And the junction still had to be used by local traffic.

Congestion was inevitable, and by the turn of the century it was virtually intolerable, especially as the interchange became part of the Trans-European Transport Network in 2006. Accidents on the approaches to the interchange were increasing: in 2008 there were six fatal collisions in six months. 

Nor could abnormal loads be routed through the underpass. Palliative measures were ruled out, radical surgery was the only realistic option.

Catthorpe looking south towards A14 to M1 bridge

Catthorpe looking south towards A14 to M1 bridge

The interchange between the M1, M6 and A14 is one of the busiest in the country

In 2005, Skanska and its design partner Jacobs won the Early Contractor Involvement (ECI) contract to develop proposals for a much revised junction. Highways England project manager Ivan Marriott says the first proposal was a complex four level design that maximised connectivity between all three major roads.

“But this included an 800m long viaduct, and would have been very expensive. It would also have a major environmental impact on the area,” he says.

By 2009, a simpler three-level design had been adopted. This lacked direct links from the A14 to the M1 southbound and between the M6 and M1 north, but otherwise ticked all the boxes. High priority was given to building good relationships with local residents as they would have no direct links to the junction, says Skanska project director Duncan Thompson.

Catthorpe A14 to M1 northbound

Catthorpe A14 to M1 northbound

Bridge carrying the A14 to the M1 northbound

In exchange, there would be a dedicated connection to the trunk road network via the nearby A5. Local roads and cycleways would be improved and rights of way re-established.

Thompson adds: “One key decision was to keep all construction traffic off local roads, something that was really appreciated.”

Naturally, top priority was to minimise disruption to the main traffic flows. Then in 2010, following the economic crisis, the project was suspended.

 We were concerned about excessive movement at the hinge joints

Ivan Marriott, Skanska

However, Skanska was given the green light to replace the original and ageing Catthorpe Viaduct which carries the southbound M6 over the M1 a short distance away to the south. “We were concerned about excessive movement at the hinge joints,” Marriott explains.

“We also took the opportunity to extract lots of concrete samples that were sent off to the Transport Research Laboratory for analysis.”

In late 2011 the project, officially dubbed the M1 Junction 19 Improvement Scheme, was relaunched, with the tough target of starting on site before 2015. “We had to back engineer the statutory processes from the desired starting date,” says Marriott.

18 weeks ahead of public enquiry

“This gave us just 18 weeks in advance of the Public Enquiry – which was enough.”

Detail design began in March 2013. There were a number of challenges apart from traffic management. The site sits in a hollow adjacent to the infant River Avon, and initially it was thought the interchange would need its own pumping station to handle run-off drainage. Underlying the site is weathered Blue Lias, rich in ammonite fossils but with a high sulphate content.

All concrete in contact with the ground had to be sulphate resistant. This was achieved by adding pulverised fuel ash (PFA) to the mix. Weathering steel was chosen for all exposed structural steelwork.

Finding room to build the six new bridge structures was the biggest challenge. More than 20 traffic management phases over two and a half years were devised after much burning of the midnight oil. The final plan turned out to be so effective it has required virtually no modification to date.

Network of temporary roads

“The key to the project’s success is the network of temporary roads,” Thompson says. “In all, there are eight individual roads totalling more than 3km, all built to motorway standards, at a cost of £4M.”

He adds that the challenge of co-ordinating all the diversions and contraflows, along with the associated 4,000 traffic cones and 600t of VarioGuard temporary barriers was akin to “a child’s brain teaser”.

In the longer term, as many as possible of the temporary roads will be converted into emergency access roads into the interchange. Any unwanted roads will be recycled, part of an overriding policy to keep all excavated material and demolition waste on site.

Catthorpe demolition

At 11pm on a Saturday night in late March 2015, under a strict media blackout, 140kg of explosives were detonated below the redundant M6 southbound to M1 southbound bridge. Almost 3,000t of concrete crashed down onto the M6-A14 link road below. Fifteen hours later, traffic was flowing along the link road again, and Skanska had a clear field to continue construction of the new dual carriageway M6-A14 link.

“Leaving the old bridge until later would really have complicated our work on the new structure,” says Highways England project manager Ivan Marriott. “But normal methods of demolition were not really practical.

Catthorpe deck demolition

Catthorpe deck demolition

Excavators demolish Catthorpe bridge

“The bridge deck was really high. Building up a working platform high enough for machines to reach the deck wasn’t a realistic option in the short 18 hour closure window we would have to work with.”

There were worries about the potential stability problems on such a high working platform as well. Dropping the viaduct down to ground level where it could be broken up safely and quickly seemed a very attractive alternative.

Originally built in 1971 to a Sir Owen Williams & Partners design, the bridge featured 28 concrete columns and an insitu concrete deck. The articulation of the deck was somewhat unusual, raising questions as to the safety of conventional progressive fragmentation demolition methods.

So explosive demolition was chosen. International demolition expert Dick Green was called in, and complex safety procedures devised by Highways England traffic officers and the police.

A total of 560 holes had to be drilled into the columns to hold the explosive.

Blast curtains were hung from the deck to help contain flying debris. A 1,000t layer of crushed concrete formed a protective mat on the M6-A14 link road below.

This link and the adjoining M6-M1 link roads were closed for the actual demolition. On the A14 and the M1 “co-ordinated rolling road blocks” were in operation, with police cars slowing down traffic to create a gap that lasted for 10 minutes on both roads in both directions on the approaches to the junction.

Watched only by the site staff and a handful of local residents the explosives did their job exactly as planned. When the dust had settled and the protective mat had been removed a slight depression in the link road surface was discovered, but no significant damage occurred.

All the concrete from the viaduct will be crushed and recycled on site. Marriot says that dealing with the parallel bridge that originally carried the northbound M1-M6 link was a much more straightforward affair.

“By last October the A14-M6 traffic had moved to a new alignment and no longer passed beneath the bridge. So we could construct a safe working platform and use normal progressive fragmentation methods without any need for road closures.”

One area turned out to be a former borrow pit used to win material for the original construction of the A14. “Basically it was backfilled with uncompacted site arisings, which isn’t helpful,” says Skanska construction manager Mark Sutton. He says the crushed concrete from the various demolitions has been used as sub-base over much of the site, a positive boon.

We brought in Active Tunnelling to install 200m of 750mm diameter drains using a combination of pipe jacking and a microtunnel boring machine.

Mark Sutton, Skanska

A better solution to the drainage challenge has also been adopted. Sutton explains: “To avoid having to install a pumping station, with all the costs that would entail, we went for gravity drainage.

“But because of the ground profiles this could have meant trenches up to 11m deep. So we brought in Active Tunnelling to install 200m of 750mm diameter drains using a combination of pipe jacking and a microtunnel boring machine.”

Run-off is mitigated by a series of attenuation ponds, one equivalent to the size of 12 Olympic swimming pools. The ponds are ‘future-proofed” against climate change, being capable of dealing with a 20% increase in current rainfall levels.

Different logistical challenges

Each of the six main structures posed a different logistical challenge. Top down construction was selected for the bridge that carries the M1 over the M6-A14 link, the rest were more conventional. Towards the end of 2015, the overall plan was really coming together, with main road traffic starting to switch to new, permanent alignments.

In December the link between the A14 westbound and the M1 northbound opened, four months ahead of schedule. Work is well advanced on the two remaining bridge structures that will eventually carry the M1 southbound to A14 eastbound and the M1-M6 westbound over a new local link road.

This will be entirely free of main road traffic, a blessed relief for long-suffering local residents when it finally opens this autumn. By then the new interchange will be fully operational and at last worthy of its presence on the Trans-European Transport Network. It will also be able to handle abnormal loads.

”Ignore satnav”

Given the scale of the transformation and the complexity of the traffic management, the fact that so few road closures and contraflows were involved is a tribute to the project team. Motorists now approaching the junction are greeted by large signs advising them to “Ignore Satnav’’, so fundamental are the changes to the original road layout.

Marriott has no doubts about where the credit for the project’s success ultimately lies. “We’re on budget and on programme. This is down to having a very good team that has really shown the benefits of ECI. And we’ve taken full advantage of the new capabilities offered by building information modelling techniques.” 

 

Environmental mitigation

Gone are the days when major construction projects went ahead with little or no regard for their impact on the local environment – times when there was not much consideration of the long-term effects on the lives of local residents.

Thankfully, attitudes have changed over the last 50 years or so. The M1 Junction 19 Improvement project had environmental concerns at its heart from its inception, as well as an awareness of the needs of the residents of nearby Catthorpe and Swinford villages.

Environmental surveys back in 2003/4 revealed the presence of Great Crested Newts, badgers, bats and many species of birds. Most of these could have been expected, the newts in particular seeming to turn up on almost every roads project, and there are well-proven techniques for mitigating environmental impact on them. But two much more unusual finds posed very different challenges.

“We’re working close to the River Avon, so there was always the possibility of otters being present,” says Skanska senior environmental advisor Duncan Healey. “This was confirmed by surveys undertaken in 2008. We now know we have a breeding pair in the area.”

Less well-known and much rarer was the large flightless predatory Necklace Ground Beetle, currently in severe decline in the UK. Topsoil, vegetation and leaf litter had to removed before actual construction began in areas where the beetle was known to be present,and carefully stored to avoid damaging larvae or eggs. Later it will be spread out again in agreed areas of new planting.

Back when the M1 was built horses, as well as local traffic, used the underpass. A long established bridleway was diverted through it. But once the A14 opened, the sheer volume of traffic effectively acted as a barrier. As part of the project a new 1.8km bridleway route around the interchange has been established, alongside the River Avon for much of the way.

Nine sections of the riverbank have been regraded to a more otter-friendly profile, and several artificial otter holts have been created from recycled materials. Beneath the A14, the river was improved as an otter crossing point by installing a ledge above the traditional flood level.

Two new bridges have also been installed. Where the 3m wide bridleway runs through the floodplain it has geotextile and 300mm of Type 1 sub-base below a topsoil surface to ensure good drainage.

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