The structure will be constructed at Hardengreen roundabout which is near to Eskbank, south of the Edinburgh bypass.
Work on the two-span bridge, which will span 71.5 metres in total, will be carried out over two weekends; from the 14 to 17 February and the 28 February to 3 March 2014.
As a result of the works, the Hardengreen roundabout will be required to close, as well as the A7 and local roads from 10pm each Friday to 6am each Monday for the specialist engineering task to be carried out.
The ambitious project will see a 1,200-tonne crane used to hoist the bridge's four beams into place, with each weighing 107 tonnes. Borders Railway has said a 30-strong construction team will work throughout the road closures to install the bridge beams and create the new bridge deck.
Local diversions will also be in place during the closures and motorists are advised of the following:
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• Motorists coming to and from the Edinburgh City By-Pass, A720 are advised to use the A68 instead of the A7.
• HGVs travelling north and south should use the A68 rather than the A7.
• Midlothian motorists travelling north on the A7 should follow diversion signs for B704 along Cockpen Road.
• Motorists travelling towards Edinburgh from Newtongrange will be diverted along the B6482.
• Motorists using the B6392 (Dalhousie Rd) and B703 ( Newbattle Rd) are asked to follow the local diversion via A6094 (Eskbank Road) or B6392 (Melville Rd).
• Only local access will be permitted on A7, South of Hardengreen Roundabout to Dalhousie Mains cottages and local businesses.
Parking restrictions will also be in place to allow for the moving of materials on the A6094 across Bonnyrigg Road, Eskbank Road and Lothian Street and along the B704 across Dundas Street and Cockpen Road.
The preparation works at Hardengreen Roundabout commenced in September 2013 and saw the project's main contractors, BAM, carry out extensive piling works and construction of bridge abutments.
Commenting on the scheme, Hugh Wark, Project Director, Network Rail, said: "The installation of this new bridge structure is a major construction milestone for the team. Not only will it transform the visual impact of the landscape and connect the railway from north to south but it is the longest bridge to be built from scratch along the new route.
"We have managed to keep the roundabout open while we construct the bridge supports, however the installation of the bridge deck is a major undertaking which will require two weekend closures.
"During the first weekend we'll be craning in the concrete bridge deck. During the second weekend we'll be completing the installation."
(JP/CD)
