Most people spend a lot of time choosing tiles, colours and floorplans, but almost no time thinking about the soil their home or pool will be built on. Soil is not exactly a glamorous topic, but the reality is that the ground beneath a building has a huge influence on structural performance, project cost and long-term durability.
This guide explains what geotechnical work involves, how soils form, why conditions can vary so much from one site to the next and when a geotechnical investigation can protect you from serious and unexpected problems. Think of it as a simple and practical look at what is happening beneath your feet.
What is geotechnical work?
Geotechnical engineering is the link between geology and structural design. Our role is to understand the ground beneath a site so that the building on top of it can be designed safely and economically.
To do this, we draw from a range of earth sciences including:
• Geomorphology – how landscapes have developed
• Hydrology – groundwater behaviour
• Geography – the shape and features of the terrain
• Mineralogy – the minerals and rocks present
• Climatology – long-term climate influences on the ground
• Biology – vegetation that hints at soil conditions
• Sedimentology – what soil is made from
Using a combination of site inspections, laboratory testing, historic records, and specialist drilling equipment, we assemble a clear picture of your site and determine the safest and most economical design for your project, literally from the ground up. The governing Australian standard for geotechnical investigations is AS 1726-2017 Geotechnical Site Investigations.
For those in Western Australia, the WA Soil Characteristic Map is a useful reference to get a broad sense of the soil types in your region before any investigation begins.
How soils form and why it matters for your project
Soil is constantly being created, shifted and reshaped over time. Rocks break down through weather, water, chemical reactions, temperature changes and even plant roots. Over millions of years, this produces everything from coarse sand to fine silts, clays and organic materials such as peat.
These variations matter because each material behaves differently under load, in wet conditions, and through seasonal change. Some soils support buildings well. Others shift, drain poorly, or compress unpredictably. Understanding what sits beneath your site directly affects your design, your cost, and the long-term performance of everything built on top of it.
Why would I need a geotechnical report?
Clays, silts, and peats are often the soils of greatest concern for foundations and site works costs. They are called reactive soils because they swell when wet and shrink when dry. This movement happens slowly through the seasons, but it can cause significant structural damage if the footing system is not designed to handle it.
The record ground movement recorded in one Western Australian laboratory was 157mm per 10 linear metres of wall. That is enough movement to seriously damage or even split a structure if footings are not calculated to absorb it.
A geotechnical report helps you avoid problems like this by identifying:
- The soil layers beneath your site
- Whether reactive soils or weak materials are present
- The expected amount of ground movement across seasons
- The foundation design required to manage that movement
Even two blocks on the same street can behave very differently, so assumptions based on nearby builds can be misleading. With modern technology, conditions can at least be predicted and calculated so that measures can be put in place to reduce risk. uDrew OnSite can give you an early read on site conditions before you commit to an investigation or start quoting builders.
The complication of human-made fill
Many properties, especially older ones, contain buried material from past land use. This is known as uncontrolled fill, and it can be one of the most costly surprises in any construction project. Common examples include:
• Old household rubbish pits
• Decommissioned pools or septic tanks
• Construction spoil or low-quality imported fill
• Contaminated material including asbestos or chemicals
Uncontrolled fill compresses unpredictably and may contain contaminants that affect both structural performance and the health of occupants. In serious cases, the entire site may need to be scraped back and remediated before construction can begin, costing upwards of $30,000 for a typical residential block.
When would I need a geotechnical report?
A geotechnical investigation is strongly recommended for any structure that will be lived in or occupied, or for any project where safety is a priority. It is particularly important if you are in an area known for clay, peat, or rock, if there is any chance the site contains old fill, or if you simply want certainty before investing in foundations and site works.
Typical costs range from around $1,500 to $5,000, depending on:
- The complexity of your design
- Local council requirements, especially around drainage and water table depth
- The local geology, for example whether you are on clean coastal sand or in an area with volcanic or seismic features
- Whether the site is near known clay, peat, sinkholes, or fault lines
- Proximity to potential contamination sources such as older industrial sites or properties known to contain asbestos or uncontrolled fill
It is often said that every $1 spent on a geotechnical investigation can save up to $50 in design adjustments and long-term maintenance. From personal experience, I can confirm that this ratio holds up when corners are cut.
When a full investigation is not required
In many residential areas, particularly sandy regions, a simple Site Classification may be enough. This involves a driller logging the soil profile, identifying any reactive materials and collecting basic samples and recording observations where needed.
Typical (Metro) pricing is:
• $400 to $800 dollars for sandy, non-reactive soils
• $1,000 to $1,700 dollars when laboratory testing is required to assess clay reactivity
Additional costs can apply for remote locations, difficult access, or specialist equipment. It is worth getting a quote from a local geotechnical professional early in your planning process, well before site works begin.
Understanding the site classification system
Following a report or site classification, your building site is assigned to one of the following classes under Australian Standard AS 2870. Class A is the most straightforward; Class P is the most complex and costly to build on:
Class A: Stable, non-reactive. Very little or no ground movement from moisture change. Typical of sand, gravel, and rock sites.
Class S: Slight ground movement possible. Common on gravel, sand, clay, and silt mixed sites.
Class M: Moderate ground movement possible. Typical of clay or silt mixed with some sand.
Class H: High ground movement possible. Clay, organic silt (peaty), and inorganic silt sites with limited sand or gravel content.
Class E: Extreme ground movement possible. Sites approaching pure clay or silts.
Class P: Problem sites. Ground movement may be very severe, unknown due to uncontrolled fill, or the site may be contaminated. Consultation with a geotechnical engineer and structural engineer is required before construction. In many cases, the top 600mm to 2,000mm of material must be removed and replaced with controlled fill. This can become very expensive.
Builders can often spot early warning signs, but a formal classification or investigation is the safest way to confirm conditions before construction begins. Foundation and site remediation issues are among the costliest to fix once building is underway.
Simple ways to get a sense of your soil before calling anyone
While a professional assessment is always the right call before committing to a build, you can make some basic observations on your own block that can help you ask the right questions early.
- Dig a small hole and wet the soil. If it can be rolled into a sausage shape, clay is present.
- Rub a small amount of soil in your palm. A stain in your palm print suggests silt; a gritty feel suggests sand.
- Look at your garden. Patchy or uneven growth can indicate shallow rock, clay pockets, or drainage inconsistencies.
- Watch how water drains after rain. Fast drainage usually means sand; slow drainage or pooling suggests finer soils or compaction.
- Observe the natural slope of the block. Coarse soils support steeper gradients; loose sands form gentler ones.
The jar settle test
A rough but useful home test for soil composition. While the timing is approximate and based on field experience rather than laboratory precision, it gives a visual idea of what your soil contains:
- Fill a jar about halfway with soil from your site.
- Add half a teaspoon of laundry or dishwashing detergent.
- Fill to about 90% with water and shake for 30 seconds.
- Mark the settled layers at the following intervals: 5 seconds (gravel and coarse sand), 20 seconds (fine sand), 2 to 8 hours (inorganic silt), and 24 hours or more (clay). If the water clears faster than expected, silt and clay content is lower.
Here is a helpful video demonstration: Jar settle test on YouTube. Once the sediment has fully settled, you will have a clear visual of your soil composition and can see the layering for yourself.
The bottom line
Every building site has a unique geological story. Understanding it is one of the most reliable ways to avoid unnecessary cost and ensure long-term structural performance. A relatively small investment in testing before you build can prevent expensive surprises once construction has started.
If you are unsure where to start, speak with your builder, contact your local council, or go straight to the source and engage a qualified geotechnical professional. For an early read on your site’s conditions, overlays, and planning constraints before any of that, uDrew OnSite gives you instant access to site intelligence that would typically take days to compile.
Curious what the data says about your block? Check your address with uDrew OnSite.
More in the How to Build in Australia series:
