Our growing urban population increasingly lives on sections that are radically shrinking in size. Green space is often lost at the expense of built living space in both new subdivisions and infill housing developments. Read more on how this affects urban soil in this article by Simon Upton, Parliamentary Commissioner for the Environment
In my 2023 report on urban green space, I argued that green spaces are a type of city infrastructure providing essential services like stormwater management, heat mitigation and air filtration. Underlying green space, and its capacity to provide such services, is healthy urban soil.
While researching urban green spaces, I identified widespread residential land development practices that are putting urban soil, and the services it supports, under pressure. I have just released a follow-up enquiry, Urban ground truths, that investigates the fate of urban soil during new subdivision and infill housing developments. It considers the drivers behind the extensive and sometimes excessive soil disruption and removal that occurs and recommends greater protection for this important asset.
Why urban soil matters
Nutrient-rich, freely draining soil of an adequate depth is essential to grow vegetation that provides shade, cooling, water retention and biodiversity habitats in our cities. This soil also slows the movement of stormwater and filters contaminants like heavy metals.
Healthy soil will be increasingly valuable as we face the impacts of climate change – it buffers intense and frequent rainfall events and supports trees that provide shade and cooling. The depth and extent of whatever soil remains after development is completed matters.
Healthy soil consists of a dark, carbon-rich layer of topsoil and underlying layers of subsoil. Deep topsoil of up to 300mm combined with deep, freely draining subsoil of up to 900mm provides a productive medium in which a wide range of trees and plants can thrive.
If soil volume is limited or subsoil is compacted, the range of trees that can be successfully grown is restricted – in some cases so severely that only shrubs will succeed. Trees can reduce air temperatures by between 0.4°C and 4.5°C, but this depends very much on the size of the tree.
The surface area of green space in new residential developments is shrinking as hard impervious surfaces increase.
Larger impermeable surfaces increase the potential for surface water run-off and the need for costly engineered solutions. Increasing the impervious coverage of an area from 55% to 65% potentially increases run-off by 25%. Reducing soil depths further increases the risk of run-off.
Landform shaping and commercial incentives
The final landform of a development is influenced by a range of factors. Geotechnical considerations, soil contamination and regulations all play their part, but so do commercial decisions on how to efficiently deliver the project.
My report identifies efficiency as a key driver behind the practice of soil stripping, where all topsoil and some subsoil is removed from a site to provide a flat, build-ready platform for buyers. The definition of a build-ready site is commonly understood to mean homes built on concrete slab-on-ground foundations. Concrete slab construction is the market’s clear preference.
In Porirua, a city we studied, up to 90% of new builds use this foundation. A certain path dependency has developed. The building workforce is currently well skilled to deliver this construction formula, and it is seen as cheaper and faster to deliver. Bulk stripping to prepare sites for concrete slab foundations is then considered the most efficient way to take advantage of current building techniques.
Alternative foundation choices like screw pile systems that do not require the same extent of soil removal are relatively new to the market, being only certified by CodeMark last year. This lack of familiarity is one reason this option can be overlooked during the design stage.
Regulatory factors in soil stripping
Various regulations relating to roading gradients, soil contamination and concrete slab foundations require a degree of soil removal from site. As these regulations may require extensive excavation, some developers consider it more efficient to bulk strip the site rather than deal with a patchwork of excavated areas.
How these individual regulations are interpreted influences the extent of soil removal. Developers consulted while preparing the report said that some councils were overly conservative in their application of soil contamination standards. This led them to dig and dump lightly contaminated soil that could be kept for lower-risk land uses. Similarly, rules around maximum roading gradients and layouts were seen to be inadvertently incentivising developers to level hilly sites.
Perhaps the biggest regulatory driver is the New Zealand Building Code. Concrete slab foundations must be built on ‘good ground’, which at a minimum requires the removal of topsoil. Changes currently under way to NZS 3604 Timber-framed buildings will make it an Acceptable Solution to construct timber-framed buildings of up to three storeys on concrete slab foundations. As this change will mean lower compliance costs, it could further incentivise concrete slab foundations in new urban developments.
Landscaping
Landscape design determines what soil ends up in new city subdivisions. Research undertaken for this report found that specifications for landscaped soil depths varied greatly across local authorities (see Table 1). This variety impacts the extent of environmental services offered.
For instance, lawn supported by 200mm of soil can soak up double the volume of rainfall that 100mm can. In terms of soil for trees, the volume available is proportionate to the canopy provided. As seen in Figure 1, greater depth allows for larger trees that support more cooling and shading.
To address this, I recommend that greater guidance is provided to councils on soil specifications. I suggest that local authorities consider incentives for developers to encourage soil preservation. An approach could be to draw on London’s Urban Greening Factor programme, which offers credit to developments for providing soil that supports tree canopies of a certain scale.
Regulatory protections needed
My investigation also found that current regulations do not adequately protect urban soil. Local authorities have some valuable tools, but their ability to implement these controls is tempered by development pressures, a lack of clarity and guidance and the sequencing of consenting processes.
To address this, I have recommended that national guidance is provided on urban soil that includes clarification for councils on their role in soil conservation. I also suggest that NZS 4404:2010 Land development and subdivision infrastructure is updated to make more explicit reference to soil. The standard should guide professionals to identify areas of deep, freely drained topsoil and subsoil and encourage efforts to conserve this soil on site where possible.
A further recommendation is that all territorial authorities use tools such as integrated catchment management plans to coordinate stormwater management for urban growth in a way that takes better account of the role of soil in the urban water cycle.
The fate of urban soil is shaped both by regulations and the choices made by developers throughout the residential development process. By minimising soil disturbance where possible and providing good quantities of healthy soil in landscaping, the building industry can help safeguard this important resource and the future resilience of urban developments.
Original article by Simon Upton, Parliamentary Commissioner for the Environment. The article was printed in Issue 201 of BRANZ Build magazine at www.buildmagazine.co.nz. The full report is available at pce.parliament/publications/urban-ground-truths