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- Client
- Ōtautahi Community Housing Trust
- Location
- Christchurch
- Services
- EnvironmentalGeotechnical engineering
- Status
- Completed in 2022
The Brief
We were engaged by Ōtautahi Community Housing Trust to complete geotechnical and environmental assessments ahead of the proposed redevelopment of a community housing project in Spreydon, Christchurch. The existing housing had served the city for 88 years. Built in 1940, the buildings had reached the end of their service life and needed to be replaced to support the next generation of social housing.
Challenges Identified
Given the age and construction type of the buildings, there was potential for lead-based paints to have been used on the weatherboard exteriors. Over time, this may accidentally result in lead entering the soil, creating a potential soil contamination scenario that would require assessment.
What We Did
We carried out a Detailed Site Investigation (DSI), which identified ground contamination associated with lead-containing paint present on buildings at the site. Arsenic and lead were detected at concentrations above national soil contamination standards for residential land use.
Isolated fragments of asbestos-containing material (ACM) were also identified on the ground surface in the southeast corner of the site, associated with former shed locations, and in one location along the eastern fence line.
We produced a Remediation Action Plan (RAP) in July 2022 and submitted it to Christchurch City Council with a consent application to complete the required remedial earthworks, in accordance with the Resource Management (National Environmental Standard for Assessing and Managing Contaminants in Soil to Protect Human Health) Regulations 2011 (NESCS).
Remedial works were completed on 8 September 2022 and subsequently validated by us, allowing sitewide earthworks to commence.
Our geotechnical engineering team completed investigations that identified a soil profile of bedded sand and silt with interspersed gravel units. Groundwater was measured at between 1.7 and 2.0 metres below ground level.
Seismic settlement estimates ranged from MBIE TC2/TC3 to TC3, with the site considered most consistent with an intermediate TC2/TC3 category. A hybrid TC2/TC3 foundation with a geogrid-reinforced gravel raft (minimum 0.6 m thick) and a TC2 Option 2 or 4 was deemed suitable.
Based on the findings of our geotechnical investigation, Quoin Engineers designed the building foundations. We then assumed responsibility for construction monitoring of the gravel rafts in November 2022, marking the early stages of construction.
Over the course of this early construction phase, a total of seven inspections were conducted, which also involved supplementary testing aimed at satisfying the requirements stipulated by the Council. The geotechnical phase of this project was successfully concluded in December 2022, culminating in the issuance of a Subgrade Bearing Capacity Producer Statement (PS) 4 for each of the seven blocks involved in the project.
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