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Soft option for tough going


Boggy ground, the remains of an ancient road and flooding made improvement works on a main road in the Republic of Ireland a challenging task. Mike Walter reports.

Part of a key road in Ireland with a long history of settlement has been rebuilt using an innovative soft ground improvement technique.

The 300m long site, on the N4 just east of Carrick-on-Shannon, crosses a bog beside the River Shannon. Over the years the road's varied patterns of settlement has given the carriageway an irregular profile. This section is the final one to be upgraded.

Piling contractor Roger Bullivant undertook the soft ground improvement scheme over the winter, finishing in May 2002. The work was part of an £1.6M contract to rebuild the section, carried out by main contractor Wincove Holdings for Leitrim County Council.

Work involved installation of 1,700 driven precast concrete segmental piles and construction of a load transfer platform to distribute traffic loads evenly on the new carriageway. Designed to carry loads of up to 230kN, the piles were manufactured by Bullivant in Northern Ireland.

A custom-built 19t Komatsu KT5 piling rig with 360° movement and fitted with a 3t hammer was used to install 162 rows of 12 piles across the 7m wide road.

The rig was remotely controlled by a unit strapped to the site foreman's waist so pile installation could be watched closely, ensuring accuracy of positioning and rake.

The piles were driven through soft soil and boulder clay to the underlying rock, typically to depths of between 9m and 13m.

About 20 rows of piles at each end of the site were driven to shallower depths to allow a small degree of settlement.

Bullivant contracts engineer John Stenson explains: 'The road always has and would have continued to settle because of the nature of the soft soil on which it is built.

Piles in the transition zone between the new and old section of the road ranged from 9m long to 2m where it met the original road. Reinforced concrete pile caps were cast over each pile rather than over groups.

Individual caps were used to allow the degree of controlled settlement needed. 'Design engineer Mott MacDonald did not want to create a rigid deck, ' Stenson says.

The load transfer platform comprises two layers of geogrid matting in a herringbone pattern and prevents the road from sagging between the pile caps. A 1,200mm thick layer of aggregate fill was placed on top, overlain by a 150mm capping layer.

Use of a load transfer platform also allowed the road to meet precisely the level of the small piled bridge in the middle of the site. On either side of the bridge, three rows of piles were installed at slightly closer centres to ensure road settlement here was minimal.

During piling Bullivant discovered the remains of an ancient road and a redundant concrete pipe packed with stone.

'The section of buried road could have been centuries old and a great deal of fill had been placed on top of it over the years as the road above continued to settle, ' Stenson says. 'The buried road certainly led to problems installing vertical piles, so we needed to break through what was already in the ground.' Bullivant used its Star Probe, developed in-house, to 'burst or knock' underground obstructions out of the way. The 4m long, 175mm diameter hollow probe is driven into the ground using the piling rig.

Site foreman Mark Forester says he was pleased with the performance of the probe: 'Since we started using the device on site we have not had a single pile breakage.' Before piling began in earnest, a number of test piles made up from 4m sections were installed to three different depths. Testing aimed to measure deflection and determine where the load was being supported along the pile shaft in the boulder clay.

Strain gauges were fitted to each of the test pile sections and static testing was carried out using kentledge blocks, with the load increasing to 580kN over a 36-hour period. Sonic wave testing was also carried out to test dynamic pile capacity.

Earlier this year work was delayed for five weeks when half the site was immersed under 2m of flood water. Up to 16 submersible pumps were brought in to shift water to the nearby River Shannon. Once the site was drained, four pumps remained for the duration of the contract to continue pumping away surface water.

Despite this setback, Bullivant made up for lost time by installing piles at a rate of up to 60 a day. Traffic continued to use the road throughout the piling, with work finished on one half of the carriageway before beginning on the other side.

'We found a number of different ground conditions we were not expecting but it did not delay the contract completion. We were always confident that the soft ground improvement solution would work, ' Stenson says.

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