When preparing a section for building or landscaping, earthmovers use two fundamental techniques: cut and fill. Simply put, cutting removes soil from high areas, while filling adds material to raise low areas. Understanding how these methods work — and how they interact — will help you make better decisions about your section and avoid costly surprises.
What Is Cut Earthworks?
Cutting (sometimes called excavation or cut) involves removing soil, rock, or other material from an elevated part of your section to lower the ground level. A digger or bulldozer scrapes, scoops, or blasts away material until the desired level is reached.
Common reasons for cutting include:
- Levelling a sloped section to create a flat building platform
- Excavating for foundations, basements, or retaining walls
- Creating driveways or access tracks on hillside sections
- Removing unstable topsoil before construction
The material removed during cutting is called spoil or cut material. This spoil can either be reused on-site (as fill elsewhere on the section), carted away, or disposed of at an approved facility.
What Is Fill Earthworks?
Filling involves bringing in or placing material to raise the ground level in a low-lying or uneven area. Fill material can come from the cut done elsewhere on the same section (known as cut-to-fill), or it can be imported from an external source.
Common reasons for filling include:
- Raising a low-lying section above flood levels
- Creating a level building platform on sloped ground
- Filling in gullies or depressions before landscaping
- Building up ground around a new structure
Not all fill material is equal. Engineered fill (compacted in controlled layers, often tested by a geotechnical engineer) is required under and around buildings. Using unsuitable material — like topsoil, organic matter, or uncompacted rubble — can lead to settlement, subsidence, and serious structural problems down the track.
Cut and Fill Working Together
On many NZ sections, especially sloped ones, cut and fill are used together in a balanced approach. The earthmover cuts from the high side of the slope and uses that material to fill the low side, creating a level platform. This is called a balanced cut-and-fill operation and is cost-effective because it minimises the need to import or export material.
However, achieving a true balance is rarely straightforward. Soil changes volume when disturbed — it bulks up when excavated and compacts when placed as fill — so careful calculation is needed. Your earthmover or engineer will account for these factors.
Retaining Walls: The Essential Partner
Whenever you cut into a slope or build up fill, you often need retaining walls to hold the changed ground levels in place. In New Zealand, retaining walls over a certain height typically require a building consent — the threshold depends on ground conditions and other factors, so check with your local council from your local council. Always check with your council before starting work — requirements vary by region and site conditions.
For guidance on the Building Code requirements that apply to earthworks and retaining structures, MBIE Building Performance is a reliable starting point.
NZ Building Code and Consent Considerations
Earthworks in New Zealand are subject to the NZ Building Code, particularly clauses relating to site stability and drainage. Significant earthworks — especially those affecting neighbouring properties, waterways, or requiring retaining walls — will almost certainly need a building consent and may also need a resource consent under the Resource Management Act (RMA).
Schedule 1 of the Building Act lists some exemptions for minor work, but these have limits. Always confirm with your local council whether your planned earthworks require consent before any machinery arrives on site. Getting this wrong can be expensive to fix.
BRANZ provides technical guidance on ground preparation and site stability that is widely used by NZ building professionals.
Geotechnical Considerations
NZ's varied geology — from Auckland's volcanic soils to Wellington's greywacke and Christchurch's liquefaction-prone ground — means that a geotechnical engineer's report is often required or strongly recommended before significant earthworks. This report assesses:
- Soil bearing capacity
- Slope stability
- Drainage and groundwater
- Suitability of on-site material for use as fill
Skipping this step on a challenging site is a false economy.
Environmental and Drainage Impacts
Cutting and filling changes how water moves across your section. Poorly planned earthworks can cause:
- Erosion and sediment runoff into stormwater systems (which may breach your council's rules)
- Flooding of neighbouring properties
- Damage to existing drainage infrastructure
Sediment controls such as silt fences and sediment traps are standard practice on earthworks sites. WorkSafe NZ also sets requirements for safe earthworks sites, including trenching and excavation safety.
When to Call a Professional
For anything beyond minor garden landscaping, you should engage a qualified earthmoving contractor and, on sloped or complex sites, a geotechnical engineer. A professional earthmover will:
- Assess your site and plan the cut-and-fill volumes
- Operate machinery safely and efficiently
- Manage spoil disposal and imported fill
- Work alongside your engineer and council requirements
If your project involves building foundations, retaining walls over 1.5 m, or any work near boundaries or waterways, professional involvement isn't just advisable — it may be legally required. You can find qualified local earthmoving contractors on tradedirectory.co.nz.
For questions about licensing and building work, Licensed Building Practitioners is a useful resource.
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