Hobart sits on a complex foundation of Permo-Triassic sedimentary rocks overlain by Quaternary alluvial deposits along the Derwent Estuary, with elevations ranging from sea level to over 1,200 m at Mount Wellington. This geological diversity means that undisturbed sampling is critical for projects where soil structure, density and moisture content must be preserved exactly as they exist in situ. Whether you are designing foundations for a commercial building in Sullivans Cove or assessing slope stability in the foothills, obtaining a Shelby tube sample rather than a remoulded one can be the difference between a reliable settlement estimate and an expensive overdesign. Our team has been deploying thin-walled tube samplers across Hobart for years, ensuring the samples we extract meet the strict requirements of AS 1726 for geotechnical site investigations.
A Shelby tube sample preserves the soil's natural structure, density and moisture content — essential for reliable triaxial and consolidation test results.
Methodology and scope
- Careful advance of the sampler at a controlled rate to minimise disturbance
- Sealing and transport in chilled containers to preserve moisture content
- Logging of recovery ratio and any signs of sample disturbance
Local considerations
In Hobart, many geotechnical issues stem from the alluvial terraces along the Derwent, where soft clay and silt layers can be sensitive to disturbance. If a contractor pushes a Shelby tube too fast or fails to maintain a steady advance rate, the sample can lose its integrity, leading to misleading void ratio and strength parameters. We have seen cases where a poorly taken sample indicated double the expected settlement, causing unnecessary remedial works. Our crews are trained to recognise when the ground is too gravelly or dense for a thin-walled tube — in those instances we switch to a split-spoon or piston sampler rather than risk a compromised core. This local experience saves our clients from wrong design assumptions.
Explanatory video
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
Standard Penetration Test (SPT)
When the soil is too dense or gravelly for Shelby tubes, we use SPT to obtain disturbed samples and N-values for bearing capacity estimates. The test follows AS 1726 with automatic trip hammer energy measurement.
Triaxial Compression Testing
Our NATA-accredited lab performs CU and CD triaxial tests on undisturbed samples to determine effective strength parameters. Tests are run in accordance with AS 1289.6.4.1.
Typical parameters
Frequently asked questions
How does undisturbed sampling differ from disturbed sampling in Hobart clay soils?
Undisturbed sampling using a Shelby tube preserves the soil's natural void ratio, fabric and moisture content. In Hobart's soft alluvial clays, a disturbed sample can show 20 to 30 percent lower strength in a triaxial test, leading to conservative and costly foundation designs. The intact core allows our engineers to run reliable consolidation and strength tests that reflect the real ground behaviour.
What is the typical cost range for Shelby tube sampling in the Hobart region?
For a standard 76 mm diameter Shelby tube sample taken from a depth of 5 to 15 m, the cost typically ranges between AU$740 and AU$1,570 per sample point. This includes field extraction, sealing, transport to the lab and a basic sample log. Bulk or multiple samples from the same borehole reduce the per-sample cost.
When should I request undisturbed sampling instead of SPT for my Hobart project?
You should request undisturbed sampling when the design relies on accurate settlement estimates, such as for raft foundations, bridge abutments or road embankments on soft ground. In Hobart's alluvial areas where consolidation parameters govern the design, a Shelby tube sample is essential. For granular soils or where only N-values are needed, SPT remains the faster and more economical option.