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Dublin's double act

Foundations : Dublin Wall

A 'world first' underground wall design is acting as both retaining structure and groundwater cut-off during excavation of a contaminated site in Dublin, reports David Hayward.

The Dublin Wall is this month being 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, innovatively designed to fulfil two totally separate modern day roles.

So named by its designer, geotechnical contractor Bachy Soletanche, the concrete wall is claimed as a first - not just in Ireland, but worldwide. The hybrid, dual function underground structure 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 2km run of wall - which on completion later this month will enclose 9ha of contaminated land on Dublin's quayside - is based on conventional ground engineering techniques.

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.

Low risk, flexible construction programming, three times faster to install than a diaphragm wall alternative and, not least, little over half the cost - are the main advantages claimed for this innovative hybrid.

'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, ' recalls 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, now derelict, site on reclaimed land alongside Dublin's River Liffey, a few hundred metres seaward of the city centre, housed the imposing gasworks, which supplied most of the city.

As the gasworks and associated coal tar factory were demolished over the past 20 years, they left behind a site rich in heavy metals, polyaromatic hydrocarbons, underground tar tanks plus a sprinkling of cyanide, arsenic and mercury. And, with a near surface water table, areas of the surrounding ground are now also contaminated.

Before client Dublin Docklands Development Authority can sell on this prestigious location for a mix of commercial, residential and retail use, it must not only remove all the nasties, but also ensure the site's future protection against recontamination from polluted land around it.

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

The £2.2M worth of perimeter wall is due to be complete by Christmas but is already fulfilling its initial role as a cut-off against surrounding groundwater. This allows the site to be dewatered, so easing current excavation work. 'We ruled out sheetpiling and, in requesting a concrete structure, expected a secant pile or diaphragm wall solution, ' recalls John Skeet, resident engineer for the client's consultant Parkman.

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

The chosen solution consists of an unreinforced slurry wall, dug in 600mm wide panels up to 14m deep 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. For this function, 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 to act compositely.

'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.

Desktop analysis of the £2.2M solution with a diaphragm wall shows that, to complete the latter in the available 16 months contract, would have cost around £4M. Secant piling, in any of its design configurations of hard and soft piles, worked out at around £3M, but carried much higher risks in obtaining verticality and impermeability in the boulder-strewn ground conditions.

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

But, deeper down, the high torque 21t/m rig struggled in places with a maze of very hard boulders, the largest of which is vividly described by site supervisor John Bevington as 'the size of a Ford Escort'.

He adds: 'All six tungsten carbide tipped teeth on the auger cutting head could be wiped off in a single pile bore. 'And welders remain on constant call to repeatedly strengthen auger flights.'

Yet, with current wall completion rates of around 30m run per 12h 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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