A380 South Devon Highway

The South Devon Link Road Project was one of the biggest construction projects and one of the most complex highways schemes in the South West. Involving many partners and engineering challenges, the project has received awards for considerate practices that show respect to the community and the environment.

After more than 50 years in the planning, and a huge effort and financial commitment from Devon County Council (DCC) and Torbay Council, the long awaited improvements to a strategic road of upmost regional importance were widely celebrated when the road was officially opened in February 2016. The improved road scheme includes 5.5km of dual carriageway costing £110m. It is expected to bring lasting economic benefits, including the creation of nearly 8,000 jobs in South Devon, with around 3,500 of these in Torbay. It will provide a bypass for the village of Kingskerswell, which is predicted to remove 95% of traffic, restoring and revitalising the village’s tranquillity.

Scheme highlights

Working as designer for contractors Galliford Try/SIAC, Ramboll work began on the A380 South Devon Link Road in the autumn of 2012. From the road-building point of view, design highlights for the improved access to Torbay and South Devon include multiple rail bridges (with tight possession dates), road structures (squeezed between houses, railway and river), environmental/water issues (badger tunnel, refuge for reptiles, fish culvert), reuse of site materials, and the many underground utilities at Penn Inn roundabout.

Solutions to a congested site

The new carriageway crosses Penn Inn roundabout, running in line with the existing A380 for the first 2km before turning west over the Torbay branch line and bypassing Kingskerswell along the Aller Brook watercourse. It will provide a safer, less congested, and faster route for through traffic, with the existing road remaining to provide a quieter route for local traffic, buses, cyclists and pedestrians.

The northern online section squeezes between the railway, the housing at St Luke’s Road/Aller Park Road and Aller Brook. The steep side long ground here required major retaining walls – up to 10m in height.

Two way traffic has been maintained on the A380 at all times, a major influence on the design of the walls, and wherever possible temporary and permanent works have been combined. In one location the site was so narrow that a combination of soil nails and reinforcement was used. The nails provide temporary support to the excavation and were then connected to geogrid reinforcement in the wall, significantly reducing the overall width. Ramboll’s meticulous modelling enabled wall heights to be reduced, and some walls were deleted altogether.

Working over the railway

In total the scheme requires three major road structures over the Torquay branch line. The longest is the Aller Tunnel, which is a 21m span, 270m long structure carrying the highly skewed A380. The preferred solution comprises U shaped, pre-cast beams placed on high level bank-seats, in turn supported by reinforced earth abutment walls. Extensive ground improvement works were achieved with an innovative soil technique using cement bound site-won fills. Sophisticated numerical analysis of the ground stabilisation process and the detailing of ground strength targets ensures that excavations were kept to a minimum, and settlement kept within prescribed limits. Ramboll worked with the South Devon Link Road partnership (DCC, Torbay Council and Galliford Try) to reassure Network Rail of the integrity of the solutions. The works were carried out successfully with real time monitoring of rail movements.

There are also four culverts installed by cut and cover methods through the rail embankment as part of flood relief measures. Keyberry Culvert in particular posed a significant challenge to the design team when possession was organised at short notice in collaboration with Network Rail, DCC and GT, to take advantage of the Dawlish storm damage rail closure in January. The entire design, including checking and certification, was carried out successfully in less than one week.

Highway structures

A total of 36 structures including nine major road bridges, the flood alleviation culverts, and retaining walls were developed by Ramboll in liaison with DCC. The centrepiece of the scheme is Penn Inn flyover- an elegant three span 154m steel composited bridge, with an open aspect for pedestrians crossing the roundabout. There are many underground utilities and, by adjusting the span arrangements to minimise diversions, substantial cost savings were made.

At Aller Junction, Ramboll provided expert advice on the changes that needed to be made to the Published Side Road Orders regarding a modification to remove a skew bridge beneath the new road so that DCC were able to accept the revised layout.

Another design challenge was the protection of a high pressure trunk gas main crossing the route 8m below ground. Ramboll’s solution was a structure designed to limit ground movements and also enable future access to the main. This was achieved with a single span portal structure utilising precast reinforced concrete beams, with a large diameter secant piled wall. The abutment wall was designed as propped embedded retaining walls, with an exposed height of 13.0m at maximum excavation.

Environmental design challenges - safe crossing and natural habitats

The scheme specified several mammal crossings. One particular badger route was now in a cutting and required a tunnel with steep slopes and flooding protection. The unusual solution included sumps in the underpass to collect water and pipe it away, and roughened concrete to allow badgers to climb inside the tunnel.

The scheme also includes a culvert designed to promote the passage of fish upstream. This is over 100m long and has a series of areas where fish can rest. To make the channel more naturalistic, stones in a range of sizes were laid on the bed to provide variable flow conditions.

Site ecologists also developed a reptile refuge, creating a habitat that will benefit a range of species, and which features re-used material from the site.

Sustainability - recycling, lorries and CO2

A key sustainability objective was to re-cycle as much material from the site as possible, reducing lorry movements. The weathered Breccia contained a high proportion of fine material which initially appeared to be unsuitable for specialist fills and pavement formations. However appropriate testing demonstrated that it could be useful in a wide range of applications without concern over long term performance or durability. 

The team exceeded normal industry measures and went above and beyond industry best practice:

• 1.5million cubic metres of material and excavated rock were re-used to create stone facing on bridges and retaining walls
• 17,800 fewer lorry trips off site and a significant reduction in the amount of materials imported to the project
• This reduced the number of lorry journeys by some 30,000 HGV miles and the avoidance of approximately 43.9 tonnes of C02 emissions

Awards

• Considerate Constructors Awards gold status, April 2016 for showing respect to the community and the environment. This is the third consecutive year that the project has been recognised at the awards. 
• Chartered Institution of Highways and Transportation (CIHT) South West award winner for Large Transportation Project
• Green Apple 2015 gold award for UK Environmental Best Practice in the Waste Management category. More >
• Considerate Constructors Awards silver status 2015
• Considerate Constructors Awards silver status 2014