Kingskerswell has waited more than 60 years for its bypass but it is now being fast-tracked and there are lots of ground engineering solutions to ensure congestion relief is delivered next year.
Ground conditions are both a bane and benefit for the new South Devon Link Road that is currently being built to divert traffic around the village of Kingskerswell and reduce delays into Torquay. On
the one hand, cut and fill earthworks have been balanced and rock deposits are being put to good use, but elsewhere the geology is calling for piling, soil nailing, surcharging and ground improvement.
The project is a £110M scheme to build a new 5.5km dual carriageway to relieve traffic on the A380, a key route into Torquay, and also reduce rat run traffic on surrounding roads. Combining online and offline construction, the scheme presents main contractor Galliford Try and its ground engineering contractor Keller with a number of challenges, not least keeping traffic flowing while work is undertaken.
Work on the scheme started in October 2012 and the route is due to be opened in December 2015.
The scheme benefited from government backing through funding of shovel-ready schemes and got the financial go ahead in May 2012. The Department for Transport is meeting £76.9M of the cost and Teignbridge District Council has contributed £500,000, but the remainder of the funding is equally shared between the two clients for the scheme – Devon County Council and Torbay Council.
The concept has been in development for more than 60 years so it was more than just shovel ready – local residents and holiday makers who flock to the area were also more than ready for relief from the daily traffic problems.
“Around 34,000 vehicles use the existing road each day,” says Devon County Council chief engineer – highways Robert Richards. “But there is a big rat run problem as people try to avoid the congestion so there are another 4,500 vehicles using Marychurch Lane which contributes to the 10,000 vehicles using the side roads.
“Traffic volumes haven’t grown significantly on the main road – all the growth has been on the side roads and tends to be tidal at peak times. It is creating restrictions on investment in the area, which extends well beyond Kingskerswell itself.”
The scheme involves a mix of online widening and a new offline route – and none of it is straightforward. The online section involves squeezing six lanes between houses and a supermarket, while the offline route has a busy rail line, rivers and undulating topography to cross.
The online section uses compulsory purchased gardens and retaining walls on one side and soil nailing on the other to steepen the existing embankment. On the offline section, Galliford Try is working on constructing a number of culverts, a 270m long rail tunnel over the rail line and balanced cut and fill with contiguous piled retaining walls to deal with the topography.
The rail work on the scheme has also resulted in two Network Rail employees being seconded to the scheme to help with liaison. One of the first phases of the scheme to combine both the water culvert and rail working is already well progressed following a track possession over the Christmas period. Despite the poor weather, the site team managed to position new culverts below the rail line during the 54-hour track possession.
Preparation for the operation included improving drainage and pumps were installed around the site, which meant the 1m of flood water that covered the area less than 48 hours before the start of the track possession subsided in time for the operation to go ahead as planned. The only compromise was rescheduling of crane operations around a period of forecast high wind.
The next big challenge for the site team is constructing the 270m long Bebo arch tunnel over the railway line in the centre of the scheme, which will be covered by as little as 300mm of fill.
“We originally planned to span the railway with a piled solution but the ground investigation reached 24m before reaching competent ground,” says Galliford Try technical manager Ian Yelf. “We’re undertaking the scheme under an NEC Option A lump sum contract with a value engineering clause, so an alternative design for crossing the railway presented the potential to reduce both costs and risk. We will be removing some of the alluvial clays from the formation level and replacing them with engineered fill before constructing the arch.”
According to Richards, work on the Bebo arch structure is expected to start in May and will be carried out between 11pm and 4am during nightly track possessions.
At the northern end of the site, as well as squeezing the additional lanes into the online upgrade section, the team will also have to construct a flyover above the Penn Roundabout to allow traffic to bypass the roundabout and flow straight onto the existing A380 dual carriageway.
“The triple span flyover will have reinforced earth access ramps and will be supported in the centre of the roundabout by CFA piles,” says Yelf.
Dealing with live traffic and preventing further congestion is a key challenge on the scheme, but so is
diversion of services, which account for around £4M of the work and this has had a big influence on the Penn Roundabout solution.
“A 25 bar water main passes through the site and is mounted on piled thrust blocks below the Torquay exit of the roundabout and the planned location of the flyover piers also clashed with BT’s services, so we have lengthened the centre span of the viaduct to eliminate the need for diversions,” says Yelf.
At the southern end of the site, there are still utility issues despite the greenfi eld location as there is a 70 bar gas main 6m below surface. Diversion could have delayed the whole project so Galliford Try has worked with Keller to construct a £2.6M piled bridge to carry the road over the pipeline and maintain access for maintenance.
Yelf describes the ground conditions as variable across the site with a combination of clay
Bovey formation gravel, limestone and Breccia. Several different ground improvement techniques are required, including ground replacement, piling, soil nailing, vertical stone columns, settlement periods and cement stabilisation. While the variability may create challenges for some parts of the scheme, in others it has been a benefit with over a million tonnes of material being reused on site.
“Around 700,000m3 material has to be cut for the earthworks but will be reused on site and a further 321,000m3 of rock is being crushed and screen for reuse or dressed for use as facing stone,” says Yelf.
“Ground conditions in some areas have proved to be worse than expected, though,” he adds. One area affected was by the supermarket on the online upgrade. Keller completed soil nailing work to stabilise the existing slope before moving on to installing vibro stone columns to support the new reinforced earth embankment using straps tied into the soil nails that will enable the road to be widened and moved closer to the rail line.
On the other side of the online upgrade where the ground rises up, work has been completed on the 600mm diameter contiguous piled wall, the work hidden by stone sourced from within the project and the land handed back to residents.
While the work achieved on site in less than 18 months is impressive, the site team will need to maintain this level of focus to overcome the remaining challenges to complete the new link by the end of next year.