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Onekawa Park investigations

A summary of geotechnical and contamination investigations undertaken at Onekawa Park in 2020.

Onekawa Park Geotechnical and Contamination Reports

Napier City Council is reconsidering locations for a potential new aquatic facility that was originally proposed in the long Term Plan 2018-28. In 2020, Napier City Councillors requested further information on the cost, risks and timeframes of constructing a new aquatic centre on five potential locations at Onekawa Park.

A survey of the site was undertaken by the engineering consultancy Tonkin & Taylor. They conducted geotechnical and contamination investigations across these five locations in Onekawa Park that were identified as potential locations for a new aquatic centre.

The investigations were necessary due to the nature of the facility that would potentially be constructed there. An aquatic facility requires deep excavations for the building foundations and pool tanks. This type of investigation is not necessary nor relevant for residential construction purposes.

Testing

Pattle Delamore Partners had undertaken similar investigations of Onekawa Park in 1997 and 2011. The recent testing was carried out in parts of Onekawa Park that were not previously investigated. The work involved digging 43 holes or small scale test pits, similar to the earlier investigations. After the assessment, all holes and excavations were refilled, and sealed with a cap of clean soil over the top.  

Summary of results

The results of this recent investigation confirmed to us that conditions underground at Onekawa Park in the five test locations were similar to those in the other parts of the park that were investigated in 1997 and 2011.  Across the site there is consistency with the geotechnical conditions, but some variability with landfill debris.

 1. In-situ ground conditions

Underneath the fill layer, and any uncontrolled fill from the landfill are soft compressible silt and layers of liquefiable sand. 

 2. Uncontrolled fill

There is uncontrolled fill (refuse material) across much of the site.  The quantity of uncontrolled fill varies across the site, with generally greater quantities along the site from the north-west corner (by the tennis courts) to the south-east corner (close to Maadi Road). 

 3. Contamination

 Heavy metals (lead, copper and zinc) found, as well as positive tests for asbestos. Contamination results were variable across the site, with patterns generally aligned to the quantities of uncontrolled fill. Landfill gas was observed in some test pits.

 4. Groundwater levels

Groundwater levels across the site are very shallow and range from 1m – 3m. 

 5. Liquefaction

Overall there is a minor to moderate liquefaction risk at the site, for all building locations.

 6. Existing infrastructure

There is a lot of existing underground infrastructure across the site that will require removal or bridging during construction.

It is important to note that these do not pose any risk to residents or park users as they are capped with a layer of top soil and lie undisturbed.

We don’t yet have the full picture on risks, costs or timeframes. However, these results mean that to construct a new aquatic facility at Onekawa Park, major ground improvements and specialised building foundations are required to ensure building stability, along with the safe excavation and disposal of ground contaminants.

Next steps

We are using information from the reports to understand the costs, risks and timeframes of constructing an aquatic facility on one or two of the recently tested locations at Onekawa Park. This information will be ready to present to councillors in February 2022.

Councillors will use this information to help them decide which potential new aquatic facility options will eventually be consulted on with the community. The consultation will likely include one or two options based at Onekawa Park as well as the Prebensen Drive location proposed in the Long Term Plan 2018-28. No decisions have been made on site locations or when consultation will take place. Budget for construction of a new aquatic facility is not in Council’s current Long Term Plan.

Onekawa Park Geotechnical Assessment

Onekawa Contamination Final Report

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