Bunbury's growth as a regional port and commercial hub has pushed development into areas underlain by the Swan Coastal Plain's deep sand sequences and occasional clay lenses. These deposits, shaped by Quaternary coastal processes, often require undisturbed sampling to capture their true in-situ density and fabric. When we work on projects near the Koombana Bay foreshore or the inner-city precincts, we deploy Shelby tube sampling to retrieve intact cores that standard split-spoon methods would disturb. Before specifying foundation solutions, we often pair this with a calicata exploratory pit to log stratigraphy at the same location, ensuring the tube sample is representative of the critical soil layer.
A Shelby tube sample preserves the soil's true void ratio and structure, turning a field extraction into a laboratory truth.
Methodology and scope
Local considerations
A common mistake we see in Bunbury is contractors relying solely on disturbed SPT samples for settlement analysis in the soft clay layers beneath the city's older fill zones. Those clays have a sensitivity that, once disturbed, can show half the true undrained shear strength. Using undisturbed sampling for those horizons avoids overestimating bearing capacity or underestimating consolidation settlements. Without this care, a foundation pad designed from SPT data alone might experience 30% more settlement than predicted, leading to cracking in the structure above.
Applicable standards
AS 1726:2017 – Geotechnical site investigations, AS 1289.6.0 – Standard Practice for Thin-Walled Tube Sampling, AS 4678-2002 – Earth retaining structures
Associated technical services
Thin-Walled Tube Sampling
Hydraulic push extraction of 75 mm intact cores in clays, silts, and dense sands. Sealed on site and transported to our NATA-accredited lab within 24 hours.
Triaxial Testing (UU, CIU, CD)
Shear strength parameters on undisturbed samples. We run consolidated undrained tests with pore pressure measurement for Bunbury's soft estuarine deposits.
Oedometer Consolidation Testing
One-dimensional consolidation tests on Shelby tube specimens to determine pre-consolidation pressure and compression index for settlement analysis.
Moisture Content & Density Profiles
High-resolution water content and dry density profiles along the tube length, critical for assessing fill compaction and natural soil variability.
Typical parameters
Frequently asked questions
What size Shelby tube do you use in Bunbury?
We use a 75 mm diameter thin-walled smooth steel tube. This is the standard for AS 1726 undisturbed sampling and fits most hydraulic drill rigs operating in Bunbury's sand and clay profiles.
How does undisturbed sampling differ from SPT in Bunbury's soils?
SPT provides a disturbed index of resistance and a bulk sample. Undisturbed sampling preserves the soil's true void ratio, fabric, and cementation. For Bunbury's sensitive estuarine clays, only a Shelby tube gives reliable consolidation and strength parameters.
What is the typical cost for Shelby tube sampling in Bunbury?
The cost ranges between AU$590 and AU$1,680 per tube depending on depth, soil conditions, and whether laboratory testing is included. A quote can be tailored to your specific borehole program.
Can you sample in the dune sands around Bunbury with a Shelby tube?
Yes, but it requires careful technique. Dune sands have low cohesion and can drain during push. We use a fixed-piston sampler and slow hydraulic advance to minimise disturbance. Recovery rates are typically lower than in clay but still usable for density calibration.
How long does it take to get results from undisturbed sampling?
Field extraction takes one hour per tube. Once in the lab, routine moisture and density results are available in 3 days. Triaxial or consolidation tests take 2 to 4 weeks depending on the consolidation stage required.