Services

Bloomfield Interchange Culvert Techspan

Winner in the Specialist Category of the Construction Excellence Awards 1999

The Bloomfield Interchange is part of the Cork South Ring Road. As part of the interchange, a bridge had to be constructed over the Douglas river to safeguard the integrity of a 1000mm carbon steel water main and to protect it from loading and consequent settlement. This bridge would also allow access to the water main for possible future maintenance or duplication. It was also intended that this bridge would accommodate intertidal movement to an area within the interchange that was designated as a Special Protection Area.

The structure chosen was a 20.5m span precast Techspan arch. Techspan was developed by the Reinforced Earth Co. and uses finite element design programmes with funicular curve theory to define the optimum curvature for an arch based on the project-specific span length, soil coverage and construction sequence. The resulting three-pinned design optimises the arch shape which in turn minimises tensile stress and thus reduces costs. Although the arch at Bloomfield was over 20m span it was only 400mm thick.

Because the concrete arch and surrounding backfill act together during backfilling and under permanent load, the soil structure interaction is vital to the design and is evaluated by a specially developed finite element programme for each backfilling sequence. This programme is non-linear and takes into account all the characteristics of the structure, the soil and the foundation.

Construction

The 58 units forming a 35m length of arch were constructed by a team of 6 men in 4 days. The ends of the units were placed in a slot formed in a 5m wide foundation cast on well-compacted rockfill material by the main contractor, Ascon. The pinned top joint was formed by semi-circular steel male/female plates cast into the units. These are simply placed against each other in a ball and socket type arrangement to form a true pin joint. The untis were levelled using shims in the base slot and on completion this slot was grouted up. Backfilling was then carred out by the main contractor and the Reinforced Earth headwalls and wingwalls were constructed as part of the backfilling operation.

This Techspan arch is remarkable mainly because of its size. At 20m span, it encloses a very large volume as can be seen from the photographs. Because of the use of Finite Element design and the soil-structure interaction model, it was possible to use a very thin section (400mm thick) which led to a very economical and elegant shape. The economics of this structure were also enhanced by the extremely quick construction time and the use of Reinforced Earth in the headwalls and wingwalls.

General

The Bloomfield Interchange forms the junction of the South Ring Road (N25) and the Slighe Carrig Donn (N28). It is one of the final parts of the road network system envisaged under the Cork Land Use and Transportation Plan. The South Ring Road provides a complete by-pass of Cork city on its southern perimeter. It consists of a 4 lane dual carriageway connecting the N8 (Dublin) and the N25/E30 (Waterford/Rosslare) routes east of the city with the N71 (West Cork) and ultimately the N22 (Killarney) routes to the west of the city, as well as the N28 (Carrigaline/Ringaskiddy) and the N27 (Airport/Kinsale) routes to the south of the city.

The South Ring Road connects to the city centre by way of the South City Link Road. The road was funded by the National Roads Authority and substantially grant aided from the European Union Structural and Cohesion Funds. Work has been ongoing since the mid 1980s. The intersection of Bloomfield Interchange, Bloomfield Bridge and Douglas Estuary Bridge links the southern and western section of the South Ring Road and the Jack Lynch tunnel.

Excavation started in 1998 and required excavation to a depth of up to 6 m and the removal of up to 100,000m 3 of material (soft aluvial, mainly silt) under tidal conditions. A bridge, 10m wide, was constructed where the slip road crossed the city and harbour water main. The Bloomfield Interchange is located on the south bank of the Douglas River and it is estimated it will carry up to 30,000 vehicles per day. The interchange slip roads are constructed on embankments which are a maximum height of 11.0m above the mud flats. The total cost of the works is IR£15.0m. The Douglas Estuary Bridge carries a 4 lane dual carriageway over the Douglas River Estuary. Careful consideration was given to the design of this bridge due to its environmentally sensitive location and the desire to conserve this wildlife sanctuary.

On the southern side of the estuary is an area known as the Bloomfield marsh, which is inhabited by various birds and other wildlife including foxes. Again the Corporation treated this wildlife area with great care and attention during the construction of both the South Ring Road and the Bloomfield Intersection/Douglas Estuary Bridge.

An Environmental Impact Assessment was undertaken in 1996/97 by consultants on behalf of Cork Corporation. The assessment addressed such issues as ecology, human environment, landscape, aquatic environment, ground, conditions, archaeology, air and noise. All these issues were taken into account in the planning and construction of the works. As part of the design process Cork Corporation examined possible alternative designs for the Bloomfield Interchange. On the basis of standard design criteria the projected traffic flows require a grade-separated junction.

Project Team

Client:                                                                  Cork City Council
Design and Construction Supervision:                 Reinforced Earth Co. (Irl.) Ltd.
Consulting Engineer:                                           Fehily Timony & Co.
Main Contractor:                                                  Ascon Ltd.
Arch Precaster:                                                    John A. Wood