Irish Rail continue to upgrade Dublin’s suburban network

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Ireland’s recent financial difficulties have resulted in the axing of many capital projects but the ongoing programme by Iarnród Éireann (Irish Rail) to upgrade Dublin’s existing suburban network has thankfully survived. This is known as DASH – an acronym for DART and Suburban Enhancement Project. In turn, DART stands for Dublin Area Rapid Transit, the electrified section of the network. Currently underway is a resignalling project, designed to extract more capacity by safely reducing the headway between trains.

North of the capital, Enterprise expresses to Belfast have to share two tracks with the DART service to Malahide, resulting in some conflicts. Last year’s opening of a station in the newly-built suburb of Clongriffin, five miles north of Dublin, was welcomed by local commuters but created another bottleneck for intercity operators, already struggling to compete with quicker car journeys. So it was decided to construct a loop line immediately west of Clongriffin Station, allowing express trains to bypass DART services.

Retaining walls

Coffey Construction is a civil engineering and building contractor with operations in the UK and Ireland. Although better known for heavy civils works, it also specialises in building works, slope stabilisation, trenchless techniques and excavator-mounted piling. The company has been a principal contractor for Irish Rail for over 25 years and, in August 2010, it was appointed to deliver the civil works for the Clongriffin loop. This essentially involved widening the existing embankment on either side of the station over a distance of 700 metres. To avoid land acquisition costs and planning issues that could have delayed the project, Irish Rail elected to carry out this work within its existing property boundaries, requiring a retaining wall solution instead of a traditional side slope.

The majority of this wall is constructed as an L-shaped in-situ concrete section. However there was insufficient land available at the northern taper to continue with this approach and a cantilevered contiguous pile wall system was proposed for the remaining 100 metres.

Piling close to a 90mph railway would mean night-time possessions and associated isolations of the overhead line equipment. But on this busy line such blockages are limited to a 3½-hour window between 01:00-04:30. Even then, they can be interrupted by goods traffic.

The installation of contiguous piles would also require the construction of a piling platform. In effect this would amount to the temporary widening of the embankment by a further 10 metres over a distance of 100 metres, demanding importation, placement and compaction – as well as subsequent excavation and disposal – of 3000m3 of material.

Both Irish Rail and Coffey Construction were keen to explore alternative safe methods of working whereby the wall could be built during the day without line blockages. Irish Rail’s costs would be reduced as the provision of lookout staff was its responsibility under the contract whilst the contractor’s wage bill would also be cut if full shifts could be worked in daylight hours. Clearly this would be a win-win situation for client and contractor, with the added bonus of shortening the programme.

The key factor determining the need for possessions and isolations was the depth of the required piles – almost 10 metres – and therefore the height of the piling rig. A conventional rig is typically 15 metres high and would present a hazard to the railway as there is a risk of overturning.

CoffeyConstructiontrack
Track laying gets underway on the loop line

Excavator-mounted auger

A design based on shallower piles would allow the use of Coffey Construction’s telescopic auger – the Movax TAD-30. This versatile device can be limited to a height of 5.5 metres. Because it is relatively light and mounted on a 30-tonne excavator, it can operate on any firm surface and does not need an engineered piling platform, saving import and disposal of structural fill. Coffey proposed that the auger be set up at ground level at the foot of the existing embankment. The piles could then be installed with a cut-off level with the embankment’s toe. This would present no hazard to the railway, allowing work to continue in daylight without causing disruption.

Coffey Construction’s engineers worked closely with Irish Rail’s designers to develop a solution based on the low-rise piling rig. This involved shorter 5.5m piles terminating at ground level. A pile cap was then cast and a reinforced concrete retaining wall built above it.

Another advantage was that instead of a contiguous piled wall requiring around 200 piles, this layout demanded only 51 at 1.8-metre centres. The new piles were to be 600mm diameter – twice that originally envisaged for the contiguous piled wall – but this larger size was better suited to the Dublin boulder clay. Fewer piles meant fewer setups and a faster programme. Casings were generally not required in the stiff glacial till as the relatively shallow piles were not subject to high ground water pressures and the bore was mostly dry.

All 51 piles were bored by the TAD-30 and a reinforcement cage lowered into the bore. With the cage positioned, concrete was placed using a 200mm diameter Trémie pipe. A ground beam 1m high was then cast on the top of the piles to support the 300mm thick reinforced concrete retaining wall. This was then backfilled and topped with ballast to support the new track. Ancillary works included the installation of drainage and service ducting as well as the casting of reinforced concrete foundations and holding-down bolts for the overhead line stanchions.

Less disruption

Clongriffen’s retaining wall works are not particularly remarkable in themselves. What is worthy of note is that an operational railway embankment was widened over a distance of 700m, with a 500m long retaining wall built alongside it, during the day without any possessions being taken. At the northern taper where the loop line diverges from the existing tracks, the piled foundations for a 100m section of retaining wall were installed safely while trains passed just a few metres from the piles without a speed restriction being enforced. All this was made possible through use of low-rise excavator-mounted piling plant.

Although a number of night-time possessions were needed during the project, these were far fewer than initially anticipated. Those that were taken were generally to facilitate the backfilling of the new retaining wall, to construct the OLE foundations and to lay new track.

By employing value engineering combined with innovative methodology, Coffey Construction helped Irish Rail to deliver the project ahead of schedule and under budget.

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