Unearthing Australia’s Soil Classification: A Comprehensive Guide for Landowners and Builders

It’s easy to overlook a site’s soil type when constructing a structure. However, the soil’s make-up plays a crucial role in the ability of the building to stand the test of time since it significantly impacts the solidity of the structure.

As all the regions in Australia have varying climates and geology, the factors influencing soil classification can be intricate. The classification system accounts for the wide variance in earth types throughout the country.

This guide provides everything you need to know about soil classification in Australia by revealing its significance, factors influencing classification, classification standards, and soil characteristics appropriate for building a home (or any structure).

What is the importance of soil classification?

Classification is essential to construction because it explains the results of a soil analysis on a block of land.

A soil report or geotechnical report will define its type, chemical make-up, and physical characteristics, which affects the ability of the soil to withstand temperature variations and seismic activity.

The report also defines its sensitivity to humidity and load-bearing capacity, determining the foundation type and structure’s design.

Here are a few reasons builders should care about soil classification:

• Stability and safety: Accurate classification aids construction, ensuring the structure is stable and safe and protects it from failure due to soil erosion, subsidence, or settlement.
• Site selection: Builders can use the classification system to determine the most suitable site for a building project. A high-rise building, for instance, may not be appropriate on soil susceptible to liquefaction or ground shifting.
• Foundation design: A building’s foundation is essential for structural integrity. Soil classifications allow us to identify soil strength, defects, compressibility, and holding ability, which is crucial in determining the understructure type.

What factors influence soil classification in Australia?

Here is a look at some of the significant factors influencing soil classification in Australia: 

1. Particle size and distribution

Particle size and distribution of soil can significantly affect its strength and load-bearing capacity.

The ratio of each particle size in soil determines its structure and affects its physical and chemical qualities, such as flow and water retention. Smaller, well-graded soil particles are associated with strength and stability.

2. Plasticity

Plasticity is the soil’s ability to move or change shape without cracking or breaking when subjected to pressure, influenced by attributes such as clay content, organic matter, and dampness levels.

High-plasticity soils tend to shrink and swell more, which may lead to structural issues in building underpinning.

3. Organic content

Organic matter aids the soil’s structure and fertility. But if it breaks down or rots, it can make the ground unstable. High-organic-content soils are typically unsuitable for construction.

4. Moisture content

Liquid content affects soil strength, stoutness, and workability.

Sites subject to abnormal dampness may experience issues. For instance, highly damp earth tends to be less stable and more prone to erosion, while low-moisture-content dirt will be more challenging to work with and compact.

5. Load-bearing capacity

It refers to how much weight a soil will support before deformation or failure. Several variables affect it, such as the soil’s composition, structure, and dampness level.

6. Soil profile

Vertical sections of the ground showing varying horizons and layers from the surface to its parent material. It can provide information on the soil’s chemical, biological, and physical features and its formation history.

Soil Classification in Australia

The Australian Soil Classification System (ASC) is the standard and most widely acknowledged language for standardising and classifying soil. International classification systems are not ideal since they are not representative of the soil characteristics found in Australia.

Before the ASC came into play, different soil classifications, such as The Great Soil Groups and The Factual Key, preceded it.

How is soil class determined & classified?

Here’s how soils are classified according to the ASC:

• Anthroposols: Undergone significant changes from human activities like urbanisation, burials, or agriculture.
• Arenosols: Mostly made of loose sands, at least 1,000 mm deep, usually found in arid or semi-arid areas. 
• Calcarosols: High concentration of calcium carbonate (calcareous), often found in arid and hot climate regions. Primarily (80%) consists of sand, clayey sand, or loamy sand in the upper 1,000 mm.
• Chromosols: Often brightly coloured with an evident change in texture at the B2 horizon. Have a pH of 5.5 (water) or higher in the upper B2 horizon.
• Dermosols: Shallow profiles, clay skins, and little to no B-horizon development, typically found in areas with a high water table. 
• Ferrosols: Iron and aluminium-rich soils, usually found in wet tropical and subtropical areas. 
• Hydrosols: Long-term water saturation and mostly found near water sources or in places with significant rainfall. 
• Kandosols: Have a subsurface layer of clay accumulation.
• Kurosols: Contain significant amounts of clay. Common in areas with a cold and humid climate. 
• Organosols: Rich in organic matter, typically found in wetland environments.
• Podosols: Refers to low-PH soils primarily found in areas with high rainfall.
• Rudosols: Shallow with a high concentration of rock fragments and gravel (rudimentary soil development).
• Sodosols: High concentrations of sodium, generally found in arid and semi-arid regions.
• Tenosols: Extremely young soils that have yet to develop distinct soil horizons.
• Vertosols: Rich in shrink-swell clay (more than 35% clay) and can fracture and fissure when dry. Typically have cracks at least 5 mm wide (unless too moist) extending upward to the surface; Found in regions experiencing alternating wet and dry seasons.

What is a geotechnical report or soil report?

A geotechnical classification report is a comprehensive soil and rock analysis of a building site. Architects, engineers, and designers utilize it when deciding how to lay a building’s base.

A geotechnical conditions report can include the following information:

• Soil classifications
• Groundwater conditions
• Soil testing
• Geologic conditions
• Footing design recommendations for appropriate foundation design
• Borehole plan identifying the location of samples taken

Engineers and builders in Australia must take into consideration the following general site classifications based on soil reactivity, as specified in Standard AS 2870-2011:

• Class A: Usually composed of sands and rocks sites with little to no movement caused by changes in moisture.
• Class S: Slightly reactive soils (soft clay or silt sites) prone to slight ground movement due to moisture changes.
• Class M: Moderately reactive clay soils susceptible to moderate ground movement caused by moisture changes and soil conditions.
• Class H1: Highly reactive soil (clayey) prone to high ground movement caused by moisture changes and soil conditions. 
• Class H2: Highly reactive clayey soils that can experience significant high ground movement because of moisture changes.
• Class E: Extremely reactive sites subject to abnormal moisture conditions or extreme ground movement due to moisture changes.
• Class P: Problematic sites characterised by fill material, which can vary significantly across the site. It includes soft soils, mine subsidence, collapsing soils, landslips, erodible soils, reactive sites with abnormal moisture conditions, and sites that cannot be classified otherwise.

Different types of footing systems & slabs

Slabs have become an increasingly common option for the foundations of new homes in Australia. The AS 2870 requirements for concrete slabs and footings will help your engineer choose which type of slab is ideal for your home’s base.  

• Mat slabs: Fit for structures with a large footprint and in areas with weak soil. The building’s large surface area transfers the load-bearing capacity to the earth. 
• Strip footing: The most popular type of footing in homes in Australia. It includes a strip of reinforced concrete around the perimeter of the building’s base. Strip footing is appropriate for low- and medium-rise homes.
• Raft slabs: Aim to distribute the structure’s weight over a broad area. Ideal for projects in weak ground with high soil movement risks. 
• Pile footings: Involves driving piles into the soil to an extensive depth—a good option for sites with subsidence risk or weak ground.
• Suspended slab footing: Does not rest on a continuous footing system but rather on beams or walls. Suitable for building projects with unlevel ground or elevated structures.
• Waffle slab: Although these use more concrete than most other slabs, waffle slabs have become increasingly popular because they halve labour costs and combine simple construction methods and excavation all in one depth. Typically used in large spans, such as auditoriums, as it is lighter and stiffer than an equivalent flat slab. You generally won’t find this in a soil report, so you must request its addition.

Does soil type determine the type of slab required?

Different soil types affect building foundations differently due to their individual attributes. Soil types containing more rock and compacted sand or gravel tend to be more stable.

These are general features of foundation soils best for any type of base:

• Have enough strength to support the structure’s weight and any additional pressures without settling.
• Solid structure and physical qualities that stabilise the building process.
• Stable during wet and dry seasonal weather that causes erosion.
• Low shrink-swell potential reduces the likelihood of differential settlement caused by variations in liquid content.
• Good drainage properties prevent waterlogging and saturation, weakening the ground and leading to subsidence.

What’s Next

Understanding soil classification is essential in Australia before beginning any building project. The soil test results can be used to make informed decisions regarding the structure’s design and foundation. 

At SQM Architects, we have trained professionals that provide site analysis and selection services. Our team of experts will be there for you every step of the way to ensure the completion of your construction project to the highest standards. If you plan to build a dream home or office block in Victoria, don’t hesitate to contact us for reliable architectural services during the building process.