As part of the development of a masterplan for Blanchardstown Town Centre, Clifton Scannell Emerson Associates (CSEA) identified to need to upgrade the existing N3-Mulhuddart Interchange to improve traffic flows in and out of the Town Centre, whilst improving north-south linkages across the N3 corridor. The finished scheme also provides improved pedestrian and cyclist facilities at the busy interchange.
The key pieces of work associated with the interchange upgrade were the construction of a new bridge parallel to the existing to provide dedicated northbound lanes.
The new bridge consisted of a 4-span integral bridge consisting of precast prestressed concrete beams with an in-situ concrete deck slab. The use of high strength concrete and an efficient prestressing strand layout enabled a shallow beam to be utilised. This shallow depth of beam allowed the structural depth of the new bridge to mimic that of the existing bridge to avoid clearance issues below whilst not imposing onerous horizontal alignment conditions on the new road on the new bridge.
Innovative solutions such as precast concrete cross-heads on the piers and parapet edge beams cast in the factory with the bridge beams simplified erection operations for the new bridge over the existing operational N3 National Route - a road with an AADT of 20,000.
The alterations to the existing bridge deck, a 700mm deep reinforced concrete slab with tapered edge sections, consisted of the rearrangement of lane widths and locations. This required an assessment and re-analysis of the existing bridge to ensure the loadings from the new lane arrangements could be carried on the existing bridge. The assessment, carried in accordance with BS5400 and DMRB, showed there was no requirement for structural strengthening of the existing bridge. The works to the existing bridge hence consisted of re-waterproofing of the deck and installation of new parapet.
CSEA maintained constant contact with the National Roads Authority (NRA) during the design and construction process.
CSEA completed statutory approval and contract documentation for the project and provided construction supervision, including provision of Resident Engineering staff, through to handover. CSEA were the Employers Representative for the scheme.