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The Cross Border Challenge

Northern Ireland's DBFO Package 2 promises to significantlyimprove road links in the province. NCE reports on a complex project which is being delivered in three stages.

Ireland'sprivately financed cross border road link betweenBelfast and Dublin is in full swing.

Design, Build, Finance and Operate Package2 (DBFO2), as it is known, is a 30-year, £250M Private Finance Initiativescheme aimed at reducing congestion, travel times and the number of accidentblack spots between the two cities.

DBFO2 is beingdelivered by Amey Lagan Roads, a consortium of Amey and Lagan Construction.Lagan Ferrovial, a joint venture between Lagan Construction and FerrovialAgroman, is responsible for the construction package, which is divided into three main schemes (see box).

When constructionis complete in 2010, Amey Lagan Roads will be responsible for maintaining 125kmof existing and new road for 30 years.

The scale andcomplexity of the project is immense.Scheme 1 alone will have 20 new structures including a109m long viaduct – while Scheme 2 includes two over-bridges and twounderpasses.

Scheme 3 incorporates a further 22 new structures, including a 50mspan composite bridge.

As subcontractor toLagan Ferrovial, FK Lowry Piling is delivering a foundation package over threephases. Two phases, together worth £120,000, involve installing continuousflight auger (CFA) piling for the retaining wall of an underpass atLoughbrickland. The third phase involves installing precast piling for bridgeabutments, in a contract valued at £250,000.

FK Lowry operationsdirector Douglas Cook says: "We recommended a conventional CFA solution usingpiles at 900mm centres to form a contiguous retaining wall. In case of lowpenetration rates or rig refusals the alternative method was to install rotarybored piles.

"Piles wereconstructed with a minimum 500mm socket into competent rock. A trial bore was carried out next to a known borehole location, allowing geotechnical andconsulting engineers to analyse the specified rock socket.

"About 2m to 3m ofthe overlying stiff clays above bedrock contained more cobbles of greywackethan clay. This was penetrated and a rock socket constructed using a hightorque CFA rig."

Following integritytesting, and with design depths and specified rock sockets achieved, FK Lowryadopted CFA piling throughout the first phase, saving time and costs.

The firm installedthe piles over three weeks in June using rock digging tools equipped withtungsten teeth to achieve depths of 12m to 13m.

The piles wereinstalled using a Junttan 18-30 rig, in CFA mode, mounted on a two-tierplatform.

Because of the heatgenerated by coring the rock at depth, concrete moisture content was low. Avibro-hammer was therefore required to install the reinforcing cages.

Protecting theenvironment is high on the agenda. The route of the proposed road runs from theNewry Basin through river valleys, pastures and small woodlands before crossingfields criss-crossed with hedgerows, streams and culverts as well as lowlandvalleys, upland hills, narrow sea inlets and the Blackwater River.

At one location an existing culvert was discovered beneath thecontiguous piled wall at a depth of about 8m. Here, pile installation depth wasreduced to 6m so the piles would act as a retaining section only. Cook says: "Piling to a reduced depthmeant the ground above the culvert would not be disturbed during constructionand would not be subject to loading on completion of the underpass, therebyprotecting the integrity of the existing structure."

Discovery of theculvert also meant that precautions had to be taken to prevent concrete wastefrom entering the water. A washout area was built to return any excess concreteto the batching plant.

CFA piling completed,FK Lowry commenced the precast phase. Following testing to determine the pilelengths, installation of working piles got under way in August to create thefoundations for the piers and abutments for three bridges.

The contractor isinstalling 297 355mm x 355 mm square precast concrete piles, driving throughsandy, gravelly clays and founding on the granite rock. The piles at all threestructures are designed to carry loads of up to 1.5MN and are being installedusing a Junttan PM20 rig equipped with a 5t accelerated drop hammer.

Most of the pilesare reinforced with four, H20 bars with H6 helical at 150mm centres. A smallnumber are strengthened to provide additional tensile strength at certainbridge piers.

"Some piles arebeing constructed with heavy shear reinforcement to allow for impact loadingbecause of the possibility of future traffic accidents," Cook says.

The precast works have been completed and FK Lowry willreturn for the second phase of CFA piling in early 2009.

Precautions had to be taken to prevent concrete waste fromentering the water. A washout area was built to return any excess concrete tothe batching plant

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