The Cumberland Gap is a 9km break in the motorway north of Carlisle. Once plugged, it will allow drivers to use a continuous 640km of motorway, from Scotland to Dover. Ed Owen reports.
Those unfamiliar with the drive to Scotland along the M6 in England could be easily forgiven for thinking they had taken a wrong turn when passing Carlisle.
The M6 stops, and drivers have to take the A74 dual carriageway for 9km, picking up the A74(M) over the border at Guards Mill, near Gretna Green.
Capita Symonds’ Carlisle office director David Knight: “The M6 was built through Cumbria in the early 1970s to Carlisle, and then the A74 went to Glasgow.
“In the 1990s, with Scotland gaining quasi-independence, it used its money to improve the A74 to the A74(M) south from Glasgow to the border. This 9km section was left in the middle."
Highways Agency project manager David Brindle says completing this last link is essential: "This route carries 90% of heavy goods traffic between England and Scotland. Part of the job included replacing a key crossing of the West Coast Main Line (WCML), which is past its sell-by date.
"The bridge is still functioning, but has a history of corrosion of post-stressed tendons. We needed to replace it to keep the whole scheme going long into the future. One-third [of the scheme] is replacement, two-thirds is new activity, making best use of the existing A74," he says.
The Highways Agency awarded the upgrade to contractor Carillion and consultant Capita Symonds back in 2003, under early contractor involvement (ECI) – it was only the third such contract it had let.
ECI has allowed important design changes to be made very early on, which has been fruitful as Carillion and Capita Symonds share in a pain/gain relationship. “We get a share of the profit, so it is worth spending to get the right alignment,” says Knight.
Running from the south, the first 3km section is a relatively straightforward widening job, adding a new hard shoulder on either side. But thereafter the problems start. Capita Symonds project manager Warren Rocca says: “For 6.6km we had to add an extra lane and a hard shoulder. But the land is boggy and wet. We needed to put in a lot of drainage.”
ECI was an important influence on the route design. One consideration was where to place a Vehicle and Operator Services Agency (VOSA) test site, which is used to assess trucks’ roadworthiness.
"The VOSA site had to have gradients of less than 1:100 because random testing requires trucks to leave their brakes off. It was quite difficult to get the drainage right, but we did," says Knight. Surfaces at the site are either concrete or blacktop. Concrete had to be used where trucks are turning: "Wheels turning tear the blacktop up, so we used that on the straights."
Changes such as this were made to the design prior to the public inquiry, which took just two days. "I think people could see that there was no alternative to this route," says Knight.
The team also had to build a new 7km all purpose road adjacent to the motorway to maintain access for the farms and communities that will lose access to the old A74 once it is upgraded to a motorway.
The first change of alignment comes in the area at Todhills (see map). "Here the road had to be extended to the south so it would not encroach on the houses there, and then to the north at a landfill site beyond,” Rocca says. This gives the road a gentle chicane as it approaches the new bridge that had to be built over the River Esk to take extra lanes and complements the existing structure.
At the Esk, the crossing is located at the place where Thomas Telford’s Metal Bridge once stood.
However, the existing bridge, built in the 1970s, remains and is now supplemented by a second, with abutments built into those of the older bridge. "The older bridge was found to be in better shape than expected but was re-surfaced and waterproofed," says Knight. The old bridge takes northbound traffic and the new carries southbound traffic.
The latter was constructed with three piers – one fewer than the existing bridge – to reduce costs. "We basically built artificial islands in the Esk, and dug down 6m to the rock for the foundations," says Carillion project director Chris Hayton.
Steelwork contractor Fairfield Mabey built the 180m long, 18m wide, 1200t steelwork on the north embankment and jacked it along at 17m/hour.
"The bridge climbed up a cable, pushing the beam over. The nose drops around 500mm to 600mm, between piers, catching the nose and then riding up again," says Hayton.
But building the new Mossband viaduct over the WCML was the most expensive and complex task. The viaduct has embankment approaches leading up to a concrete box which takes the railway under the road.
Piling the abutments and embankments was a mammoth task. "Mossband is a bog, with a layer of topsoil, then clays and peat, followed by usable clay and finally rock," says Hayton.
The abutments were piled directly to the rock some 6m below with 144, 1.5m diameter bored piles at depths between 17m and 25m. Moving away from the abutments, are 1050, 500mm diameter continuous flight auger (CFA) piles, 4,650 vibro concrete columns (VCCs), each 7m to 9m long and installed by Pennine, and finally 21,000, 7m to 9m long band drains.
The embankment was pre-loaded to encourage settlement and ground sensors installed to monitor the movement. "The CFA piles settled 20mm, the VCCs 150mm and the band drains allowed water to be squeezed out, giving up to 1.5m settlement," Hayton says.
Then the reinforced concrete walls of the railway box could be built. Network Rail prohibited any work over the line other than during possessions leaving a very tight space to work in. To overcome these problems, the team devised a moving formwork system, with a Faraday Cage, which protects workers from electrical discharges.
The formwork is mounted on a rail and produces 12m sections at a time. Once the concrete is poured, it is left to cure, the formwork is struck and moved on to repeat the process. This technique safely contained the work and meant the job did not need any additional possessions, which are extremely expensive.
Seventeen existing possessions overnight on Saturdays were then used to hoist 132, 35t beams and two, 54t pre-stressed concrete Y-beams to complete the box. Rebar from the ends of each beam ties into rebar jutting from the top of the walls and the final casting is done in-situ.
Value engineering also came into play when building the embankments and walls. Network Rail had insisted that special concrete beams be set into the embankment to secure cranes, in case they toppled onto the line. The three 1.1m deep beams are between 70m and 185m long.
Hayton said the risk of plant toppling over was extremely low, but the team complied and kept the slabs in the embankment, incorporating them into the design. "Rather than eliminate [the beams], they became part of the earthworks, giving the structure increased strength," says Knight.