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IT'S A MINI ADVENTURE

Difficult ground and restricted access have complicated a contract on the A55 in North Wales. Damon Sch³nmann reports.

Mini piling has its pros and cons and a project on the A55 between the Conwy and Llandudno junctions highlights both.

Piling Solutions won the £38,000 contract from main contractor Dawnus Construction and is using concrete injected sectional auger piles to get around site access problems.

Problems plural, because the rig crew could not enter the field that runs close to the edge of the road and there is also a railway line running along the other side of the road.

The job, to build two variable message signs (VMSs) over the road, sees Piling Solutions installing 6, 600mm bored piles to support each sign. Contracts manager David Wandless says: 'It seems a lot of piles but the weight of the sign isn't really the big load. This comes from designing for an impact if a truck hits it.' While assessing the safety of work alongside the road is relatively straightforward, next to the railway is a different matter, Wandless says.

'Network Rail takes a far stronger view - if there's any possibility that the worst could happen we've got to assume it will.' 'Network Rail can't have anything endangering infrastructure, trains and lives so it's a lot more restricted working next to the railway than the road, ' Wandless says.

Because the strip of land being worked on is only a few metres wide, the company is using a 6m high rig that is more stable than a taller CFA machine.

It also has a small collapse radius that complies with working restrictions on the narrow site. Even so, traffi c on the dual carriageways is reduced to a single lane during work.

But the fl p side of using mini piling for this job is that pile reinforcement had to be changed.

Although the cages were originally intended to run the full length of the pile, the decision to use the low headroom rig meant a redesign - there was a risk of the concrete going off during the slower sectional auger drilling process.

Wandless says: 'We should really always be thinking about using shorter cages next to roads.

We're smarter these days and we should be using our knowledge but in fairness to Owen Williams [the designer] they didn't need a lot of prompting.' The piles are 14m long for one VMS and 17.5m for the other.

Although the pile cages were shortened to 9m for both, design includes a central full length 40mm threaded Gewi bar that is able to lance into the hardening concrete.

Ground conditions comprise alluvium and peat layers down to 7m, with gravel beneath, presenting a further challenge to the rig crews.

Piling Solutions foreman Paul Osment says there is a soft silty layer at one of the VMS bases.

As the auger went down for the second pile it was 'dragging in' the wet concrete from the fi rst even though they were more than 2m apart.

To prevent this, the subcontractor has been installing in a sequence that makes consecutive piles as far apart as possible.

Piling began in mid-January and was due to fi nish within two weeks.

Dawnus Construction started work on 6 January and should have completed at the beginning of this month.

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