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Pressing engagement in Dublin

CONTRACTS

LANDPAC GROUND Engineering's high energy impact compaction technique (HEIC) has been used on part of the Dublin Port Tunnel project.

The North Point Junction is a 7ha site that will form an eightlane toll-gate plaza for the tunnel.

It is underlain by up to 6m of construction fill placed over the last 60 years, comprising sand, gravel and clay as well as timber, glass and concrete.

Below this there is sand and gravel to 13m, with silt and then clay beneath. Site investigations found groundwater at 1.8m. Fill was found to be dry and loose down to the water table.

The main ground treatment operation will involve placing 2m to 3m of surcharge to induce settlement in the top 6m of the ground. Before this took place, Landpac was asked to make a proof rolling investigation of part of the site using Continuous Impact Response (CIR), a system linked to HEIC (GE May 2002).

Plate load tests were carried out at depth to determine the fill properties. Up to 1m of fill was spread over the site and graded to give a level surface to work from.

Proof rolling was carried out on a 600m long wedge-shape area, 60m wide at its west end and 200m wide at the east. Exclusion zones were placed around buried services that cross the site to minimise the risk of damage.

'As a rule of thumb the limit of treatment is two and a half times the invert depth away, ' said Dermot Kelly, managing director of Landpac Ground Engineering.

HEIC is a dynamic compaction technique that uses a pair of linked three- or five-sided drums pulled by a tracked machine. At Dublin, a three-sided 25kJ machine was used.

The first coverage, to level the site, was done without any data monitoring. CIR was used on the second coverage and results analysed in the deceleration range 5g to 8g (discarding the remaining data), which Kelly said pinpoints good stiffness and exaggerates the size of areas where potentially very weak soils are.

The Dublin site was split into four areas, A, B, C and D. Area A responded well to compaction in the north east but there were some potentially weak or highly voided sections towards the south and south west.

Area B had very low strength fill below most of it and C had pockets of weak or loose fill. Area D had a large dense area but Kelly said a large percentage of the area still had its original pavements which rebounded during compaction - 'implying there are voids or potentially weak ground below'. Large indentations were seen after proof rolling.

'Proof rolling confirmed the fill was highly variable and needed compacting, ' Kelly said. The poor and variable nature of much of the fill means that ground treatment will be needed before surcharging, he said.

The Dublin Port Tunnel project is jointly funded by the Irish government and the EU. It is being built by joint venture contractor Nishimatsu-Mowlem-Irishenco for client the Dublin Corporation.

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