Practical Environmental Solutions for a Residential Redevelopment

Challenge

Kāinga Ora proposed to redevelop a series of social housing properties on reclaimed land in Ōtepoti Dunedin to create new healthy homes. The new development encompassed the demolition of the existing dwellings, which required a series of extensive environmental and engineering assessments. Due to the historic reclamation of the land, the subsurface conditions were unknown and required a robust environmental investigation.

Solution

SLR was engaged to complete a Preliminary and Detailed Site Investigation of the properties. During the investigation, SLR identified the presence of anthropogenic fill material associated with a nearby gas works which operated from approx. 1889 to 1950. The fill material was covered with a layer of clean sand providing an effective barrier for the contamination for residents in its original layout, but which was unsuitable to manage risk to human health as part of the proposed development. Additional soil vapour monitoring undertaken by SLR identified a low risk to site users from inhalation of vapour associated with the gas works waste.

The initial design recommended a rib raft foundation requiring excavation of a significant (and undefined) volume of contaminated soil. The final volume to be excavated was dependent on site conditions and therefore was unable to be fully defined during the design stage, however this could exceed two metres deep for some of the building foundations. Using the Conceptual Site Model developed from site investigation work, SLR communicated the project risks associated with this foundation type to the client including:

• Increased costs for soil disposal including additional soil sampling and analysis, cartage, and landfill gate fees;
• Uncertainties in project timeframe;
• Significant disruption to neighbours including multiple nearby schools and childcares;
• Potential to intercept groundwater requiring dewatering;
• Potential for additional pockets of contaminated material to be identified at depth due to land reclamation; and
• Disturbance of the gas works waste resulting in increased risk to site workers, neighbouring residents and nearby schools/childcare facilities as well as the surrounding environment.

SLR worked closely with the client and consultant engineers to recommend an alternative foundation solution involving piling. This option minimised the volume of contaminated material to be excavated from the site and transported to landfill while ensuring the risk to human health for future site users remains low. SLR also provided environmental compliance support to ensure the risk to the environment was managed throughout the works and in accordance with resource consents.

Impact

SLR was able to assist Kāinga Ora to minimize the cost and environmental impact of the project, encouraging coordination with the wider project team. Minimising the volume of soil to be excavated helped keep trucks off the road, reduced the disruption to local schools and avoided unnecessary use of valuable landfill space.

Early and effective collaboration between SLR, the client and the wider project team in relation to historic contamination resulted in positive environmental outcomes for Kāinga Ora, the community and other stakeholders.

When I took over Albertson sites, my first task was to review the soil analysis from the Geotechnical Engineer as well as the proposed foundations for the four Albertson sites. I queried the viability financially and the risk associated with the proposed foundations and set up a meeting with SLR to discuss in detail the proposed foundations. We agreed that the piling foundation option is the most appropriate foundation option to minimize the risk as well as reducing the cost overall. SLR’s input professionally for this foundation option is highly regarded as witnessed by me from day one to completion. The sites were managed professionally by SLR’s Team. Well done!

Lito Magadia Senior Programme Manager Kāinga Ora