Australian Standard AS 1726 (Geotechnical Site Investigations) sets clear requirements for sub-surface characterization, and in Mackay those requirements become critical. The city sits on a deep alluvial plain with a high water table and intense seasonal rainfall, creating soils that spend much of the year in an unsaturated state. Our unsaturated soil analysis focuses on matric suction, soil-water characteristic curves, and the volume-change behavior that drives foundation movements. Before we start field work we always cross-check with a density test using the sand cone method to correlate in-situ moisture conditions with lab results. This integrated approach means the client gets a realistic picture of how the ground will behave under drying and wetting cycles.
For the Mackay black earth, ignoring unsaturated behavior means designing for a soil that doesn't exist nine months of the year.
Methodology and scope
Mackay receives over 1,600 mm of rain annually, with a distinct wet season from December to March. The dominant soil is the Mackay black earth — a high-plasticity clay that shrinks and swells dramatically as its moisture content changes. In unsaturated soil analysis we measure four key parameters: total suction via the filter paper method (AS 1289.2.2.1), matric suction using tensiometers or pressure plates, the soil-water characteristic curve (SWCC) over a range of suctions, and the unsaturated shear strength through modified triaxial tests. These parameters feed directly into foundation design for residential slabs, road pavements, and retaining structures. Combining this with a classification of soils via Atterberg limits gives a complete picture of the shrink-swell potential that affects every new development in the Pioneer Valley.
Technical reference image — Mackay
Local considerations
We bring a pressure plate extractor and a dew-point hygrometer into the Mackay lab to run the unsaturated tests. These are not standard field tools — they require careful calibration and temperature control. The biggest risk we see locally is the assumption that saturated parameters apply year-round. In Mackay, the soil profile dries out significantly between April and November, and that drying generates suctions that increase soil strength temporarily. If a foundation is designed based on wet-season saturated tests, it can settle or crack when the dry season arrives and the soil shrinks. We've seen this cause differential movement in slab-on-ground homes across Glenella and Beaconsfield. That's why we insist on running unsaturated soil analysis at multiple suctions to capture the full range.
AS 1289.2.1.1 – linear shrinkage + shrinkage index
Associated technical services
01
Shrink-Swell Index Assessment
Determination of the shrink-swell index (Iss) per AS 1289.7.1.1, using both undisturbed and remoulded samples. We correlate the index with the soil-water characteristic curve to predict movement under expected climate cycles for residential and light commercial slabs.
02
Unsaturated Triaxial Testing
Consolidated drained triaxial with controlled suction using the axis-translation technique. We measure the angle of internal friction and cohesion intercept at suctions of 50 kPa, 100 kPa, and 200 kPa to model the actual shear strength profile during dry periods.
03
Field Suction Monitoring
Installation of in-situ tensiometers and thermal conductivity suction sensors at depths of 0.5 m, 1.0 m, and 1.5 m. Data logged over a full wet-dry cycle to capture the real seasonal suction variation at the project site.
04
Volume Change Prediction Report
A geotechnical report that uses the measured SWCC, shrinkage index, and suction data to calculate the expected vertical movement (heave and shrinkage) for a given foundation type. Includes recommendations for moisture management and sub-grade preparation.
Applicable standards
AS 1726:2017 – Geotechnical site investigations, AS 1289.2.2.1 – Standard test method for measurement of soil potential (suction) using filter paper, AS 1289 – Standard test methods for determination of the soil water characteristic curve for desorption using a hanging column, pressure extractor, or dew-point hygrometer
Frequently asked questions
Why is unsaturated soil analysis important for Mackay homes?
The Mackay black earth is a highly reactive clay that can move more than 50 mm between wet and dry seasons. Unsaturated analysis captures the soil's behavior at intermediate moisture levels, not just at saturation. This allows engineers to design slabs and footings that accommodate the actual movement range, reducing the risk of cracking and structural damage.
What is the typical cost range for unsaturated soil testing in Mackay?
For a standard residential block with three test pits and full unsaturated laboratory work, the cost ranges between AU$1,710 and AU$5,260. The variation depends on the number of suction points tested, the need for field monitoring, and the turnaround time requested.
How long does the testing process take from field to report?
Field sampling takes one day. The laboratory phase — including SWCC generation, triaxial tests with suction control, and shrinkage index — takes between 14 and 21 business days. The final report with movement predictions is issued within 5 days after lab completion.
Can unsaturated soil analysis help with pavement design in Mackay?
The reference range for this service in Mackay is AU$1.710 - AU$5.260. The final price depends on the project scope and volume.
