Snugborough Interchange Upgrade Scheme

N3 Snugborough Junction Upgrade

Clifton Scannell Emerson Associates were employed by Fingal County Council to design and deliver the Snugborough Interchange Upgrade Scheme. It was a complex undertaking with the goal of introducing significant improvements to the area at this confluence of pedestrians, cyclists, public transport, traffic and utilities as they all cross the N3 National Road and Tolka River between Blanchardstown Village, Blanchardstown Shopping Centre and the Dublin Enterprise Zone.

The main objectives of the Snugborough Interchange Upgrade project were to:

• Improve pedestrian and cyclist measures and increase junction safety;
• Improve public transport priority measures through the existing interchange;
• Provide for existing and future traffic demands;
• Improve traffic capacity of interchange from all approaches, particularly the N3 off-ramp, to the greatest extent possible; and
• Facilitate possible future upgrade of existing N3 to 3no. lanes in each direction.

This Project involved the design and construction of 4 main structures and numerous secondary structures such as substantial retaining walls, stairs and utility chambers. The main structures involved twining of the old bridge over the N3 at Snugborough, with a new 2 span bridge; provision of a new 70m long 2 span pedestrian and cycle bridge adjoining the road embankment; as well as a precast concrete portal frame underpass and a precast concrete arched underpass. The site extents are extremely constrained and there was very little opportunity for repetition of design solutions from location to location. We often note that this project has, in a civil and structural engineering sense, a little bit of everything and not a lot of anything.

The project was essentially a multi-storey civil engineering project. There are 3 discrete levels on the site each with their own particular complexities. They are the N3 Level, Tolka Valley and Underpass Level and the Local Road Network (bridge deck) Level.

Through on-going detailed and extensive consultations with the client, Fingal County Council, it was established that the end product had to do a lot of different things for different people.

1. At N3 Level the scheme had to minimise queuing on the busy off-ramp to ensure vehicles queuing at the interchange traffic signals do not tail back onto the N3 as there are obvious safety implications of having stationary vehicles next to those about to enter a 100km/h speed limit zone. In addition, the new bridge structure had to allow for future widening of the N3 to 3 lanes in each direction while narrowly avoiding underground utilities. The 1.2m diameter 9C sewer had to be diverted to allow for the construction of the 2 new bridges and because of its invert level there were very limited options available where it could be diverted to and still get over the triple culverts that carry the Tolka River under the Snugborough Road embankment.
2. At Tolka Valley and Underpass Level the scheme provides a high quality greenway standard shared pedestrian and cycle route that will form part of the proposed Tolka Valley Greenway. Short, open and airy underpasses replace a pre-existing much longer narrow underpass, thereby making the route attractive for all users. There is also the surprise high quality artwork on the underside of the arched underpass.
3. At Local Road Network (Bridge Deck) Level the scheme provides high quality pedestrian and cycle facilities at the busy interchange while also providing increased traffic capacity and bus priority for BusConnects.
   The project delivers significant improvements for both motorised and non-motorised users of the interchange. Some of the highlight figures are shown below.

Key Project Particulars:

• 775 linear metres of New Bus Lanes;
• 1,317 linear metres of Grade Separated Cycle Facilities;
• 480 linear metres of new on road cycle lanes;
• 429 linear metres of new & upgraded shared paths in parkland areas;
• 20,000m2 of finished road surface (new & resurfaced);
• Over 1,300m3 of new attenuation storage and infiltration areas for surface water;
• New Bridge crossing N3 – 56m twin span with 1,165 m2 bridge deck area;
• New Pedestrian & Cycle Bridge over Tolka River Valley – 72m twin span with 412 m2 bridge deck area;
• 44 no. Precast Bridge Beams & 30 no. Piles; and
• 750 linear metres of new retaining wall.