The Science of Soil

Over the last century and a half, Auckland’s land has been subjected to a range of uses that have impacted its soil. A significant part of Piritahi’s role in preparing land for new homes is the remediation of it to make sure it’s safe and ready to build on.

When you’re out for an evening stroll or on your way home from work, you may have noticed an increase in housing construction happening in your neighbourhood. But, have you ever thought about the ground underneath it? Ensuring land is safe and ready to build on is a key part of the process in creating new, healthy homes for Kiwis.

The testing and remediation of land is a significant part of Piritahi’s role throughout the Auckland Housing Programme’s large-scale developments. Land is remediated with one overarching objective - to make land safe and sustainable for construction workers and future generations of residents.

Although Piritahi is working with land that has generally been used for state housing, all of Auckland’s land has been impacted by human use. Building practices and materials commonly used in the past, such as lead paint and asbestos, have affected the soil that many homes were built upon. Our land remediation team tests each of these sites and safely deals with impacted soil before construction begins.

Levels of lead in soil vary from property to property. One of the reasons for such differences can be attributed to the different maintenance levels that have occurred across different properties. Sites with higher levels of lead for example may have been painted with lead paint more frequently, or painters may have been less diligent about removing sanding dust.

Another source of contaminants is building or domestic waste. A common practice in the past was to throw waste material generated from building houses (offcuts) into the trenches that form the home's foundations. Uncontrolled filling and disposal of domestic waste has also occurred in some areas of Auckland.

Piritahi has also found very low levels of asbestos in some soil being tested across sites. Traces of asbestos can stem from rainwater running off asbestos roofs, or from fragments of broken roof tiles, cladding and fencing over time being buried in the soil of these properties.

When levels of soil impaction are above the conservative thresholds Piritahi upholds, anything from 100mm to as much as half a metre of soil is often scraped off the surface and removed from the site, or in some instances can be encapsulated and covered with fresh soil. This topsoil is also often added at the end of the building process in areas where landscaping activities follow. After scraped soil is removed and transported to an approved landfill, the newly revealed ground is tested again and must meet an acceptable level in line with national environmental standards set by the government.

Piritahi and Kāinga Ora set a highly conservative approach to land remediation. Safety is top priority - it’s vital to make sure that workers developing the land, and its future residents, are not exposed to highly impacted soil. Effective land remediation also ensures that rainwater runoff entering the stormwater system doesn’t negatively affect the environment.