The new bypass on the A31 loops around the town of Magherafelt in Northern Ireland, and is expected to ease congestion in the town by as many as 50,000 vehicles a week, reducing journey times on a major route. Ramboll contributed highway, structures and geotechnical design, environmental co-ordination (including landscape design and assessment of alternative designs) and site supervision.
A much needed scheme
The town of Magherafelt lies on the A31 route which connects the south west of the province (Tyrone, Fermanagh) to the north east (Coleraine, Ballymena etc) by carrying traffic from the south towards the main roads for Derry and Belfast.
The route was used heavily, with up to 25,000 vehicles per day. As there was no realistic alternative route, all traffic passed through the town centre and had to negotiate several urban junctions, coming into conflict with local town centre road users.
This resulted in long traffic queues and associated delays, particularly at peak times. Also, trade in the County Londonderry town suffered because the traffic made Magherafelt an un-attractive shopping option.
The original plan for improving the road scheme was published in 1976 and updated in 2005. Costs of £9.4m publicised in 2005 had risen considerably to £35-45m by Feb 2014. These costs are partly due to construction inflation but mainly due to the increasingly ambitious nature of the design which saw the road taken further and further out from the urban area and consequently becoming longer and longer.
The bypass comprises 5.9 kilometres of single and 2+1 carriageway to the east of Magherafelt town. It extends from the A31 Moneymore Road at its junction with Coolshinny Road in the south, to the A6 Castledawson Roundabout in the north. New roundabouts have been provided at key junctions while three existing minor roads have been bridged over.
The short stretches of 2+1 carriageway (two lanes in one direction, one in the other) are designed as 'differential acceleration’ lanes to allow faster vehicles an opportunity to overtake slower moving vehicles, for example when leaving a roundabout.
The scheme also features tan coloured high friction surface coating, on the approaches to roundabouts to reduce the chances of skidding by braking vehicles.
The main works began on site in June 2015 and the dry weather in October 2015 was particularly beneficial to the earth moving operations. The route passes through boggy land, which required deep foundations.
Through the 2015/2016 winter months the construction focus shifted towards a number of structures across the site. Good progress resulted in early completion and the road was opened on 6 October 2016.
The old A31 road through Magherafelt was renamed B40 when the bypass opened.
Ramboll was part of the delivery team working on two separate road improvement schemes for Transport Northern Ireland on the A31 and A26.
Employed as contractor’s designer by BAM McCann JV, the scope of Ramboll’s involvement included highway, structures and geotechnical design, environmental co-ordination (including landscape design and assessment of alternative designs) and site supervision.
Ramboll design teams developed advanced parametric tools that greatly reduced the time and cost of designing structures of different geometry but same structural form.
The structures were designed by the Bridges Team in Southampton and Ramboll Engineering Centre in Delhi working closely together. Ramboll teams also collaborated closely with the main contractor and his sub-contractor to optimise buildability and incorporate preferred structural details into the works.
Overall there are 26 structures on the scheme, which is an unusually large quantity for a scheme of this length. They use large amounts of precast construction allowing elements to be fabricated off-site then rapidly constructed on site.
The structures include road underbridges, watercourse crossings, farm accommodation overbridge, flood alleviation structures and retaining walls:
1 x Accommodation overbridge
1 x River bridge
3 x Road underbridges
7 x Accommodation underpasses
10 x Watercourse box or piped culverts
3 x Flood alleviation culverts
1 x Retaining wall
The overbridge, river bridge and one of the underbridges comprise precast beam decks either integral with reinforced concrete abutments or on reinforced earth substructures.
The remaining road underbridges, underpasses and culverts are either in-situ or precast concrete boxes or portal frames.
Structure designs incorporate Environmental mitigation measures including mammal ledges and pipes and bat and bird boxes.
Innovation was brought to the detailed design by optimising the vertical alignment, side slopes and earthworks balance in order to reduce earthmoving and increase sustainability.
Additional geotechnical testing resulted in justification of parameters to steepen side slopes from 1 in 2.5 to 1 in 2 in several sections.
Additionally several culverts were stitched together in places allowing, following confirmatory flood modelling, the alignment to be dropped in these areas, reducing the embankment heights
Both measures gave scope for optimising the vertical alignment, improving balance and reducing the overall earthworks.
Mid way though the design period, Transport NI proposed changing the strategy for and locations of overtaking lanes - the resulting design change was incorporated with no resulting delay to the construction programme.
Geotechnical challenges on the scheme included widespread soft ground and peat, high groundwater and numerous watercourses. It was therefore crucial that Designer and Contractor developed the earthwork strategy during tender. A strategy was agreed which enabled any changes resulting from changed site conditions to be readily incorporated into the design.
The principal environmental issues included cultural heritage (an ancient mill pond and mill race) and ecology where provision was made for badgers, bats and otters.
Near Killyfaddy Road a local river ran along the line of the Bypass for a couple of hundred metres and had to be shifted to the south. Great care has gone into making the new watercourse as natural as possible - with boulders along the banks, as well as a small, deep channel for normal flow and with a wide, shallow channel for dealing with floods in a way that mimics natural watercourses. The banks will be seeded but otherwise nature will be left to take its course.
Several mammal underpasses were built at various points along embankments, in an effort to reduce the road kill toll of animals including badgers and foxes.