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Dublin site yields buried treasure

An innovative underground wall design is performing a dual role during excavation of a contaminated site beside the River Liffey. David Hayward reports from Dublin.

A 2km wall has just been unearthed from deep beneath a contaminated riverside site in the Irish capital. But far from being an historic relic, the wall is a brand new structure designed to fulfil two separate roles.

Designed by geotechnical contractor Bachy Soletanche, it acts both as deep cut-off against contaminated groundwater ingress and as a perimeter retaining wall when the site it surrounds is later excavated.

As with many a successful invention, the wall - now enclosing 9ha of contaminated quayside land - is based on conventional ground engineering techniques used unconventionally. A standard bentonite/cement slurry wall, keyed into stiff cohesive glacial till, provides the impermeable barrier, while a row of continuous flight auger piles, drilled along its inside face, offers the structural rigidity to convert it into a cantilevered retaining wall.

Bachy claims the system offers low risk and flexible construction programming and is three times faster to install than a diaphragm wall at a little over half the cost.

'We looked at a range of design options including several forms of secant piling, plus diaphragm or slurry walling with sheet piles placed within the panels, ' says Bachy Soletanche senior design engineer Peter Kingston. 'The optimum solution emerged after we separated the wall's two functions and considered risk control as key to providing best value.'

For nearly two centuries the vast area of reclaimed land alongside the River Liffey housed a local landmark, the Sir John Rogerson's Quay gasworks which provided most of Dublin's 'town gas' Over the past 20 years the gasworks and associated coal tar factory have been demolished but they left the site extensively contaminated with heavy metals, polyaromatic hydrocarbons and underground tar tanks as well as cyanide, arsenic and mercury.

The area's position in the heart of the docklands and only a few hundred metres seaward of the city centre meant it was ripe for redevelopment. But before client the Dublin Docklands Development Authority can sell it on, for a mix of commercial, residential and retail use, it must remove all the contaminants and ensure the site is protected against recontamination from polluted land around it.

Main contractor for the IR£26M (£21M) site clean-up is local company Pierse Contracting, in joint venture with Belgian decontamination specialist DEC.The team is now three quarters through excavating and meticulously testing contaminated ground up to 5m deep. So far, more than 35% of material being removed - which will total 734,000t - has proved sufficiently contaminated to be shipped from the quayside for treatment or disposal in Belgium or Holland.The remainder is clean enough to be retained as backfill (GE June 2001).

Last month Bachy completed the £2.2M worth of perimeter wall which is now fulfilling its initial role as a cut-off against groundwater. This allows the site to be dewatered, easing excavation work.

'We ruled out sheetpiling and, in requesting a concrete structure, expected a secant pile or diaphragm wall solution, ' says John Skeet, resident engineer for the client's consultant Parkman, explaining the brief for Bachy's design and build subcontract which started in August 2000.

Kingston confirms that his company regarded a diaphragm wall as the obvious solution but, after a risk and cost benefit analysis, came up with the hybrid alternative.

The 600mm wide, up to 14m deep, slurry wall panels were excavated by a Volvo 460 long reach excavator. The slurry mix, of bentonite, cement and fine-grained blast furnace slag, is designed to offer near total impermeability, plus a relatively strong 1N/mm 2strength at 56 days.

But, as the structure is exposed during 5m deep excavation of the contaminated site - and then partially backfilled to provide a basement level for future underground car parks - it must also act as a cantilevered retaining wall. Structural capacity is provided by heavily reinforced 750mm diameter, average 11m deep, CFA piles, drilled partially into the front face of the hardened slurry panels wall and spaced at up to 2.25m centres.

'The slurry wall acts like a structural arch in compression, transferring pressures from the retained soil and groundwater across to the piles, ' explains Bachy Soletanche project manager Jeff Lewis.

A desktop comparison showed that to complete a diaphragm wall within the 16-month contract would have cost about £4M against the £2.2M of the chosen solution. Secant piling, in any of its design configurations of hard and soft piles, worked out at about £3M, but would have meant it was much more difficult to obtain verticality and impermeability in the boulder-strewn ground.

In practice, both the cost and time advantage have been proved despite the alluvial and boulder clay ground containing a significant scattering of hard obstructions. The upper 6m of ground around the wall was pre-dug to loosen large cobbles and the remains of buried concrete tanks, while the exact line of the slurry trench was also pre-drilled with the Casagrande C50 CFA rig.

But, deeper down, the high torque 21tm rig struggled in places with a maze of very hard boulders. Site supervisor John Bevington describes the biggest as being 'the size of a Ford Escort' 'All six tungsten carbide tipped teeth on the auger cutting head could be wiped off in a single pile bore, ' he claims. 'And welders remained on constant call to repeatedly strengthen auger flights.'

Yet, with wall completion rates of about 30m per 12 hour shift well in excess of the programmed targets, engineers on both sides think the Dublin Wall is proving its worth.

'It is a clever design that works well in practice, ' says resident engineer Skeet.'It appears to be doing all it is required to, and technically I cannot fault it.'

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