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Winner:Redevelopment of a highly contaminated gasworks at Sir John Rogerson's Quay, Dublin

THE 2004 FLEMING AWARD - An open approach allowed development of a hybrid retaining and cut-off wall on a Dublin redevelopment site, with significant savings.

Sir John Rogerson's Quay, on the southern bank of the River Liffey to the east of Dublin city centre, was reclaimed from the Liffey estuary in the mid 18th century. A gasworks operated on the site from the 1830s. This has been decommissioned and demolished over the past 20 years.

The 9ha site was identified as a prime development area by Dublin Docklands Development Authority (DDDA), which acquired the site in 1998 for mixed use, including residential, commercial and retail.

Polluting byproducts of gaswork operations left significant amounts of contaminants in the ground. DDDA commissioned consultant Mouchel Parkman to design and project manage the extensive remediation needed as part of the redevelopment.

Work required a waste management licence issued by the Irish Environmental Protection Agency (EPA) because of the need to remove and treat highly contaminated materials on site.

Ground conditions are highly variable and comprise fill with obstructions, soft alluvium, gravels, boulder clay and limestone.

Groundwater is generally within 2m of site ground level.

The new development includes large areas of underground car parking. A 2km long perimeter retaining wall was required to act as a cantilever wall to allow excavation to a maximum 5m depth for the car parking, to control groundwater and to prevent the migration of any contamination from beyond the site boundaries.

The construction team consisted of Pierse as main contractor with Bachy Soletanche as foundation contractor.

The most significant construction risk associated with the retaining wall was the potential for contaminated groundwater and leachates penetrating the site following remediation. The chosen solution therefore had to provide continuous low permeability and ensure a high degree of construction certainty.

A number of conventional retaining wall options were considered, including diaphragm walling, which was considered to be the 'conforming option' This offered a high degree of watertightness, the flexibility to overcome cobbles in the gravels and minor obstructions in the fill as well as a high bending moment capacity necessary for the cantilever condition during excavations.

The design considered walls between 500mm and 600mm thick, with 5m long panels separated by vertical waterbars to provide a seal at the cold joints. This solution provided the least number of construction joints and therefore minimised the risk associated with contaminated groundwater and leachates penetrating the remediating sites, with an estimated cost of over £3M (E5M).

A variety of interlocking bored pile wall options were also considered but was deemed to be too risky, with low confidence in providing a continuous seal.

Following extensive discussions between the project team members, a hybrid wall solution was chosen, consisting of a 600mm wide slurry trench with CFA piles to provide the structural capacity. Once the bentonite/cement mix had cured, 750mm diameter CFA piles were installed along the front face of the slurry wall.

Piles were positioned at up to 2.25m centres to suit the soil profile and retained heights at each location.

The wall was built in two phases around two sections of the site and is between 8m and 16m deep, keying into the glacial till.

This option also offered the most economic wall solution at E2.5M with cost savings of about 50%.

The slurry and CFA piled hybrid retaining wall solution has provided real benefits for this major development. Significant cost and time savings were generated together with offering a flexible solution that addressed the fundamental requirements of containment and retaining wall stability.

The openness of the project team and willingness to consider all options has been essential to achieve the continued success of this project.

The confidence that the hybrid wall would prevent recontamination of the site after the completion of the remediation works allowed the EPA to surrender the waste management licence; a first in Ireland.

Peter Kingston, Bachy Soletanche Project team Dublin Docklands Development Authority Environmental Protection Agency Mouchel Parkman Bachy Soletanche Pierse Contracting

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