Soil classification in Australia

In the grand scheme of building a home, soil type probably isn’t included in your list of top priorities. But it’s a vital piece of information that helps determine the very foundation of any house.

Before building anything, a soil test is necessary to determine how easy or complicated it is to build a home on your block of land. A soil test helps to identify processes that solidify the foundation of your house and can save you additional costs down the track.

Let’s dive deeper into the importance of soil types, how they’re classified, and what the soil classes mean. It’s handy to know these for when you receive those soil test results!

Why is soil type important when building?

Finding out your soil type before building a new home is important because soil can affect the stability and safety of a structure. Different soil types have different properties that can affect how they support the weight of a building, how they respond to changes in moisture or temperature, and how they behave during seismic activity.

Some soil types are typically well-suited for building because they provide good drainage and are relatively stable. Other soil types can be problematic because they may expand and contract with changes in moisture, which can lead to foundation movement and damage to the structure.

For stability and safety reasons, it's essential to analyse the soil type and other geotechnical conditions at the building site before construction begins. This analysis can help determine the appropriate foundation design and other construction techniques to provide a stable and safe structure over the long term.

To ensure your dream home is built on the correct foundation, qualified professionals will drill holes in different sections of your block to perform a soil test. The result will classify your soil and advise your builder of your home’s slab and footing design requirements.

How soil types are classified

Soil types are generally classified according to their reactivity — the likelihood of soil movement as it gets wet and dries out. The more reactive soil is, the more expensive it becomes to build a house on it, so your soil type will indicate any extra engineering costs needed. And the earlier you find out how much it will cost, the better informed your decision will be!

There are other factors related to reactivity that can affect the slab and footing design of your home. Being proactive (and not reactive!) about this can help mitigate certain risks involved with the type of soil on your block.

Erosion

The higher the soil reactivity, the more likely it absorbs water and changes in volume. As water can sometimes cause erosion, engineers will outline measures for building on highly reactive soil. Some types of soil are more susceptible to erosion than others — such as sand. Even clay soil with a sand component can experience erosion.

Bearing capacity

The bearing capacity of soil refers to its ability to support the loads applied to the ground. It’s possible for certain soil types to have low soil reactivity but also have a low bearing capacity. For example, sand is very soft and would require proper compaction to take a heavy load.

Seasonal swelling and shrinkage

It’s crucial to account for prolonged wet and dry seasons. When soil absorbs moisture, it can swell and create an upward force on the footings of a building. When soil dries out, it can take away from the support needed by the footing to maintain equilibrium.

While this phenomenon may only be noticed if it continues abnormally for weeks or months, it’s good practice to factor this in early. If the area you live in is prone to flooding or dry spells, the engineer may recommend ways to prevent swelling or shrinkage, whether it’s **[installing a good drainage system or creating concrete paths to keep the soil moist](https://build.com.au/building-reactive-soil-sites)**.

What factors influence soil classification in Australia?

Soil classification for building is determined by various factors, including the soil's composition, density, and load-bearing capacity. Here are some key factors that are taken into consideration when classifying soil for building.

Particle size and distribution

The size and distribution of soil particles can significantly impact their strength and ability to support weight. Soils with smaller, well-graded particles tend to be stronger and more stable.

Plasticity

Plasticity refers to the ability of the soil to change shape or flow under pressure. Soils with high plasticity are typically weaker and more susceptible to settling.

Organic content

Soils with high levels of organic matter tend to be less stable and less suitable for building.

Moisture content

The amount of moisture in the soil can affect its strength and stability. Soils that are too dry or too wet may be less suitable for building.

Load-bearing capacity

Load-bearing capacity measures how much weight the soil can support without settling or shifting. It depends on the soil's composition and density.

Soil profile

The layers of soil beneath the surface can also affect its classification for building. For example, if there is a layer of weak or unstable soil beneath a layer of stronger soil, this could affect the overall stability of the ground.

These factors are taken into consideration by geotechnical engineers and soil scientists when classifying soil for building. The classification of soil can then be used to determine the appropriate foundation type and building design.

Soil classifications

Soil types in Australia are classified according to the Australian Standard AS2870-2011 "Residential slabs and footings - Construction". This standard provides guidelines for designing the foundations of residential buildings based on the soil conditions.

Your soil report will return one of six letters (A, S, M, H, E, P). Each stands for a soil type. Most A-S classes will only require a basic slab supported on the ground with footings. Soils with high reactivity (H to P) require further evaluation by an engineer to ensure the stability and integrity of the slab and footings. Some may need stiffening while others need more intricate solutions such as closely spaced ribs.

IMAGE: Example of slab on ground (source: Build Right)

The Residential Slab and Footing Code (AS 2870) outlines the following soil classifications:

Class	Stands for	Foundation	Surface Movement
A	Acceptable	Little to no ground movement from moisture changes (not reactive); most sand and rock sites	0mm
S	Satisfactory	Slightly reactive with only slight ground movement from moisture changes; clay sites	0-20mm
M	Moderately Reactive	Moderately reactive with moderate ground movement from moisture changes; clay or silt sites	20-40mm
H	Highly Reactive	Highly reactive with high ground movement from moisture changes; clay sites	40-60mm
E	Extremely Reactive	Extremely reactive with extreme ground movement from moisture changes; clay sites	60-75mm
P	Problem	Includes soft soils, such as soft clay or silt or loose sands; landslip; mine subsidence; collapsing soils; soils subject to erosion; reactive sites subject to abnormal moisture conditions or sites which cannot be classified otherwise.	N/A

Getting soil tested before building your house

A soil test report and site inspection will follow upon confirmation of your build to identify different soil classifications and any implications for the building process.

If you’re ready to make your new home vision come true, book an appointment at your nearest display centre today.