We set up the sampling rig directly on the sand flats behind Bunbury's port, where the water table sits at 3.2 metres and the topsoil is a tan, low-plasticity loam. For collapsible soil evaluation in Bunbury, our team deploys thin-walled Shelby tubes and a 50-kN hydraulic push system to extract undisturbed samples — the loam tends to lose its apparent cohesion the moment you break the crust. We bag each metre-length in sealed poly tubing to preserve the as-dug moisture content, then run the double-oedometer test in the lab within 48 hours. The procedure follows AS 1289.6.4.1, and we always cross-check collapse potential against the Jennings & Knight criteria before issuing the report.
A loam that looks stable dry can settle 80 mm when the water table rises — the collapse potential is real and rapid.
Methodology and scope
Local considerations
A single-storey warehouse on Sandridge Road was founded on 0.8 m of strip footings, with no collapsible soil evaluation in Bunbury performed beforehand. Two wet winters later, the slab had settled 65 mm at the north-east corner, cracking the tilt panels and jamming the roller door. The cause: a metastable loam with a collapse potential of 5.2 percent that had never been tested for wetting collapse. We were called in to install 12 jacked piers to 4.5 m depth — a fix that cost three times what the original evaluation would have. That job taught us never to assume the surface crust tells the whole story.
Applicable standards
AS 1726:2017 Geotechnical Site Investigations, AS 1289.6.4.1:1999 Determination of Collapse Potential, Jennings & Knight (1975) Collapse Classification, AS 1289.7.1 Standard Test for Collapse Potential of Soils
Associated technical services
Undisturbed Sampling with Thin-Walled Shelby Tubes
Hydraulic push of 76-mm-diameter tubes to 6 m depth, preserving in-situ void ratio and moisture. Samples sealed and transported upright to maintain metastable fabric.
Double-Oedometer Collapse Testing
Two identical specimens loaded to 400 kPa — one at natural moisture, one soaked. The difference in void ratio gives the collapse potential at each stress level.
Dry Density & Moisture Profiling
Nuclear gauge and sand-cone density checks at 0.5 m intervals. Correlated with collapse index Ic to identify the critical collapse zone within the profile.
Foundation Mitigation Report
Recommendations for ground improvement (dynamic compaction, pre-wetting) or deep foundations (driven piles). Includes settlement estimates under design loads.
Typical parameters
Frequently asked questions
How long does a collapsible soil evaluation in Bunbury take from sampling to report?
The field sampling takes one day for a typical site (four boreholes to 6 m). Laboratory double-oedometer and density tests run for five working days. The full report is delivered within 10 business days. Rush service available at an additional charge.
Which Bunbury suburbs are most prone to collapsible soils?
The sand flats in Withers, the alluvial terraces along the Preston River in Carey Park, and the reclaimed areas near the port. These zones have loams and silty sands with dry densities below 1.55 t/m³ and collapse potentials above 3 percent.
What are the typical costs for collapsible soil evaluation in Bunbury?
Costs range from AU$1.360 to AU$3.900 depending on borehole depth, number of test pits, and laboratory scope. A standard residential block with two boreholes to 4 m and four double-oedometer tests is around AU$2.100 inclusive of reporting.
What foundation design changes are recommended for collapsible sites?
Options include removing the metastable layer and replacing with engineered fill, pre-wetting to trigger collapse before construction, or using deep foundations such as driven piles or CFA piles bearing on the dense sand below 5 m. The choice depends on collapse potential and building load.