Bunbury sits on the Swan Coastal Plain, where the superficial geology is dominated by Tamala Limestone and Guildford Formation clays. The water table fluctuates between 5 and 12 m depth, leaving a thick vadose zone where soils remain partially saturated year-round. That unsaturated condition governs how foundations behave, especially under seasonal rainfall. We measure soil suction directly in the field with thermal conductivity sensors and then run SWCC tests in the lab to predict volume change. Before designing a slab footing, our team often complements the unsaturated soil analysis with a plate load test to confirm bearing capacity at the design moisture content. This dual approach catches problems that isolated tests miss.
A five-metre drop in the water table can double the suction in a clay profile, triggering differential movement that cracks a slab.
Methodology and scope
Local considerations
The Mediterranean climate of Bunbury brings a dry summer followed by intense winter rains. That cycle rewrites the moisture regime of the ground every year. In our experience, the most common failure is heave of a lightly loaded slab after a wet winter, caused by swelling of unsaturated clay that had dried during summer. Adding to the risk, the Tamala Limestone contains karstic cavities that can collapse when the overlying unsaturated sand loses apparent cohesion upon wetting. A proper unsaturated soil analysis identifies these triggers before concrete is poured.
Applicable standards
AS 1726 – Geotechnical site investigations, AS 2870 – Residential slabs and footings (reactive sites), AS 1289.2.2.1 – Standard test method for soil suction
Associated technical services
Field suction profiling and SWCC determination
Install thermal conductivity suction sensors at three depths, log readings monthly for one wet-dry cycle, then retrieve cores for laboratory SWCC testing. The result is a site-specific soil water characteristic curve used to predict volume change under any rainfall scenario.
Collapse potential assessment for fill and natural sands
Run double-oedometer tests on undisturbed samples taken from each metre of the profile. We quantify the collapse index at the in-situ moisture content and at full saturation. The report includes a wetting-risk map for the property.
Typical parameters
Frequently asked questions
How is soil suction measured in the field?
We use thermal conductivity sensors buried at the target depth. They heat a porous ceramic tip and measure the cooling rate, which correlates with soil suction. Readings are logged automatically every hour for months to capture seasonal changes.
What is a typical cost for unsaturated soil analysis in Bunbury?
The cost ranges between AU$1,760 and AU$4,200 depending on the number of suction sensors, laboratory SWCC tests, and the depth of profiling. A quote is prepared after reviewing the site area and foundation type.
Do I need unsaturated analysis for a slab on Bassendean Sand?
The reference range for this service in Bunbury is AU$1.760 - AU$4.200. The final price depends on the project scope and volume.
Which standard governs the testing procedure?
We follow AS 1726 for site investigation and AS 1289.2.2.1 for suction measurement. The SWCC is fitted using the van Genuchten model, which is widely accepted in Australian practice for reactive soils.