Snugborough Interchange Upgrade

General Description

Clifton Scannell Emerson Associates were employed by Fingal County Council to design and deliver the Snugborough Interchange Upgrade Scheme with BAM Civil appointed as main contractor for the construction of the works. 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 crossed 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.
• Facilitate possible future upgrade of existing N3 to 3 no. lanes in each direction.

This Project involved the design and construction of 4 main structures and numerous secondary structures including substantial retaining walls, stairs and utility chambers. The main structures involved constructing a new 2 span bridge adjacent to the old bridge over the N3, the 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 were extremely constrained and there was very little opportunity for repetition of design solutions from location to location.

The project was essentially a multi-storey civil engineering project. There were 3 discrete levels on the site each with their own particular complexities:

1.      At N3 Level the scheme had to minimise queuing on the busy off-ramp to ensure vehicles queuing at the interchange traffic signals would not tail back onto the N3 causing safety issues. 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. A 1.2mdiameter trunk foul sewer had to be diverted to allow for the construction of the2 new bridges and because of its existing gradient there were limited options available in terms of diversion route.

2.      At Tolka Valley and Underpass Level thescheme now provides a high-quality greenway standard shared pedestrian and cycleroute that will form part of the overall proposed Tolka Valley Greenway. Short,open and airy underpasses now replace much longer narrow underpasses, thereby makingthe route more attractive and safer for all users.

3.     AtLocal Road Network (Bridge Deck) Level the scheme provides high qualitypedestrian and cycle facilities at the busy interchange while also providing increasedtraffic capacity and bus priority for BusConnects.

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The appearance of the finished project was always a constant consideration throughout the design and construction stages. Items such as the parapet barrier system were chosen both for their technical performance but also their aesthetic features. The project delivers significant improvements for both traffic and pedestrians/cyclists that now use the interchange as part of their daily commute in terms of safety for vulnerable users and reduction in journey times.

Some of the highlight figures for the scheme are as follows:

Key Project Features:

• 775m of New Bus Lanes.

• 1,317m of Grade Separated Cycle Facilities.

• 480m of new on road cycle lanes.

• 429m 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 - a 56m long two span bridge with 1,165m2 deck area.

• New Pedestrian & Cycle Bridge over Tolka River Valley – 72m long two span bridge with 412m2 deck area.

• 44 no. Precast Bridge Beams & 30 no. Piles.

• 750 linear meters of new retaining wall.

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For all the heavy civils works that have been completed on the project, the combination of landscaping, natural stone finishes to structures, bright and open underpasses, and the beautiful artwork in the underpass areas bring respite from what is a busy interchange. The lower levels encourage pedestrians and cyclists to dwell and enjoy the oasis of nature and calm next to the river that is both literally and metaphorically on another level from the busy traffic routes only a stone’s throw away.

Most of the materials used for structural elements are tried and trusted. Reinforced concrete, both precast and in-situ, have been used extensively in the structures to satisfy the TII’s Technical Acceptance Standards for design life of 120 years for bridge structures. However, where possible, alternative material finishes and detailing have been used to improve the aesthetics of the works and help the road transition into the neighbouring Tolka Valley.

Challenges

The success of a project of this type that involves merging new works within the overall existing infrastructure will always be judged on how well the existing users and stakeholders can continue to use the junction during the course of construction with as little disruption as possible. The junction is a principal thoroughfare on the N3 which provides connectivity to the BSC, Ballycoolin Industrial zone, Connolly Hospital as well as residential developments, schools and bus routes.

The key challenge of this project was to develop diversionary routes for traffic and the public that maintained continuity of flow and provided safe pathways for pedestrians and cyclists. No particular road or path could be closed without first providing an alternative. In many cases the offline widening works were carried out first in order to provide the space to divert users. This then allowed the existing road sections to be reconstructed. In many instances, works had to be carried out at night to prevent disruption. For example, when installing the beams to the new structure spanning the N3 dual carriageway.

Another significant challenge in the design and construction of the scheme was to accurately identify and map the existing underground services that traverse the site at each of the different levels. With the concentration of business and population in the area and being a main throughfare the junction contained multiple utility providers in the form of telecommunications, electricity, water and drainage. These services were critical supplies to business and local stakeholders and had to be diverted as part of the works. Complex work arounds had to be developed and protected which also married with the road and path routes at any given point during the construction works to prevent as little downtime as possible to the end users.

Some of the utilities in the area were installed prior to the growth in the area over the last few decades and their locations relative to surface features have long since been lost due to multiple upgrades to the road network, including when the Snugborough Road Bridge became an interchange on the N3 around the turn of the century. Extensive investigations were undertaken to redraw the records of existing utilities to a degree of accuracy unimaginable when originally drawn. This process involved desktop studies, the commissioning of CCTV surveys of drainage networks, Ground Penetration Radar (GPR), slit trenching and even metal detection to establish by cross reference the location of previously unmapped manholes, bends and correctly position pipelines in 3D. As extensive as those investigations were, unforeseen utility conflicts did occur and had to be effectively managed during the course of the construction.

Of significant note is that through the collaborative efforts of all involved in the design, planning and construction phases, which involved assessment of all the risks, no accidents occurred to the public during the construction. The team are particularly proud of this outcome and are convinced that the project is of huge benefit to the local area in terms of active travel and reducing journey times.

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The Team

Main Contractor - BAM Civil

Client – Fingal County Council

Design Team  - Clifton Scannell Emerson Associates

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