The field vane shear test (VST) is our go-to method when we encounter soft, saturated clays in Rockhampton. We use a standard four-bladed vane, typically 50 mm in diameter and 100 mm high, mounted on a drill rod and pushed to the desired depth. Torque is applied at a constant rate of 0.1° per second until failure, and we record both peak and remoulded shear strength. This gives us a direct measurement of undrained shear strength (Su) in situ, without the disturbance that sampling often introduces. In Rockhampton’s alluvial deposits along the Fitzroy River, this test is essential for foundation design, slope stability, and embankment construction. We follow AS 1726-2017 for testing procedures and calibrate our equipment against certified torque wrenches before every field session.
VST gives us undrained shear strength in situ, avoiding sample disturbance that can halve the true value in soft clays.
Scope of work
Area-specific notes
Rockhampton sits on the floodplain of the Fitzroy River, and the area experienced major flooding in 2011 and 2017 that saturated the soft clay layers to depths exceeding 10 m. In such conditions, undrained shear strength can drop by 30% or more compared to dry-season values. Without a field vane shear test, you risk overestimating bearing capacity or underestimating lateral pressures on retaining walls. We have seen cases where a contractor relied on laboratory triaxial data from disturbed samples that showed Su values twice as high as the in-situ vane results. The consequence was a shallow foundation that settled 80 mm in the first year. The VST prevents that mismatch by measuring the soil in its natural, water-saturated state.
Standards used
AS 1726-2017 Geotechnical site investigations, AS 1289.6.2.1 Standard test method for field vane shear test, Bjerrum (1972) correction factor for plasticity index
Linked services
Standard VST (6 tests per day)
Six vane shear tests at different depths in a single borehole. Includes peak and remoulded readings, plasticity correction, and a summary report. Ideal for small projects or preliminary investigations.
Detailed VST profile (12 tests per day)
Twelve tests at 1.5 m intervals to build a continuous strength profile. Includes sensitivity analysis and comparison with laboratory index tests. Suitable for embankment design and foundation design.
VST for tailings and mine waste
Specialised vane testing for soft tailings deposits, with larger vanes (65 mm) and extended torque range. We follow ANCOLD guidelines for tailings dam investigations. Used in Rockhampton's mining sector.
VST with piezocone (CPTu) integration
Combine VST with CPTu soundings to correlate undrained shear strength with cone resistance. This reduces uncertainty and provides a calibrated site-specific correlation. Best for large infrastructure projects.
Typical parameters
FAQ
What is the difference between a field vane shear test and a lab triaxial test?
The VST measures undrained shear strength in situ, avoiding sample disturbance that can reduce Su by 20 to 50% in soft clays. A triaxial test uses an intact sample extracted from the ground, which inevitably undergoes stress relief and handling damage. The VST is faster and cheaper, but the triaxial test can measure effective stress parameters (c', phi'). We recommend both for critical projects: VST for a quick profile, triaxial for advanced design.
How much does a field vane shear test cost in Rockhampton?
Our typical price for a standard VST program (6 tests per day) ranges from AU$1,030 to AU$2,560, depending on access conditions, depth, and number of tests. Detailed profiles with 12+ tests cost at the upper end. We also offer a mobilisation fee of AU$250 per trip within the Rockhampton region.
When should I use a VST instead of a CPTu or SPT?
Use a VST when you need direct undrained shear strength in soft clays, silts, or organic soils where a CPTu might struggle to differentiate between soil types. The VST is also preferred for sensitivity analysis (peak vs. remoulded strength). In stiff clays or sands, the SPT or CPTu is more appropriate. We often combine VST with CPTu to calibrate site-specific correlations.
How deep can the vane shear test be performed?
With standard drilling equipment we can reach depths up to 30 m in soft soils. In Rockhampton's alluvial deposits, the soft clay layers rarely exceed 15 m, so a 20 m range covers most projects. For deeper testing, we use a wireline vane system that can reach 50 m, but this requires specialised rigs and increases cost.