We recently worked on a subdivision extension in Shepparton's southern fringe where the top 6 meters of the profile were consistently below 80% saturation. The site sat on Shepparton Formation clays and silts, and the client needed reliable parameters for footing design without saturating the samples artificially. That is where unsaturated soil analysis becomes essential. We combined suction measurements with consolidated triaxial tests under controlled matric suction, following the axis-translation technique. Before mobilizing the rig, we carried out a program of undisturbed sampling to preserve the in-situ structure and moisture regime. The difference in interpreted bearing capacity between saturated and unsaturated conditions was close to 35% on that job.
Unsaturated shear strength can be 30-50% higher than saturated values in Shepparton's clay crust, directly affecting foundation costs.
Method and coverage
Regional considerations
The dominant soil in the Shepparton region is Quaternary alluvium and lacustrine clay, often with a desiccated crust extending 2-4 meters deep. Below that, the profile can remain damp but not saturated, creating a transitional zone where engineers either overestimate strength by assuming full saturation or underestimate it by ignoring suction. The risk is not just conservative design — it is differential movement when seasonal wetting collapses the structure of a partially saturated soil. We have seen slab heave in one sector and settlement in another on the same subdivision because the unsaturated analysis was omitted. Following AS 1726-2017 requirements for site classification, we now flag any soil with degree of saturation below 85% as a candidate for unsaturated testing.
Standards that apply
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, AS 2870-2011 Residential slabs and footings (classification of reactive sites)
Complementary services
Unsaturated Triaxial Testing
Consolidated drained and undrained triaxial tests with controlled matric suction using the axis-translation technique. We measure phi_b and cohesion intercept at net confining stresses up to 600 kPa. Suitable for embankment stability and shallow foundation design in Shepparton's clay crust.
Soil-Water Characteristic Curve Determination
Full SWCC from near-saturation to 1,500 kPa using pressure plate and dewpoint hygrometer. Output includes van Genuchten and Fredlund-Xing parameters for use in seepage and slope stability models. We calibrate the curve against local Shepparton clay data from previous projects.
Typical parameters
Top questions
When is unsaturated soil analysis necessary for a Shepparton site?
Any site where the water table is deeper than 3 meters and the soil profile shows a desiccated clay crust — typical of Shepparton's elevated terraces. We recommend it for residential slabs on reactive clay (Class M, H, or E per AS 2870) and for any embankment or cut slope where seasonal moisture change exceeds 5% volumetric water content.
How much does an unsaturated soil analysis cost in Shepparton?
The cost typically ranges between AU$1.680 and AU$5.270 depending on the number of suction-controlled triaxial stages and whether a full SWCC is required. A basic package with two unsaturated triaxial tests and SWCC for one soil type usually falls in the lower half of that range.
What is the difference between saturated and unsaturated shear strength in local clays?
In Shepparton's clay crust, the unsaturated friction angle (phi_b) is typically 10° to 18° lower than the saturated phi, but the apparent cohesion from suction can add 20-50 kPa. That net increase often means a footing width can be reduced by 15-25% compared to a fully saturated assumption. The effect diminishes below the active zone depth.