Field Vane Shear Test (VST) in Sydney – Geotechnical Investiga…

Sydney's coastal geology is dominated by Quaternary alluvial and estuarine deposits, particularly the soft clays of the Botany Sands and the Hawkesbury Sandstone weathering profile. Groundwater levels sit shallow across many suburbs, from the Parramatta River flats to the Cooks River corridor. In these conditions, the field vane shear test (VST) becomes indispensable for measuring undrained shear strength in saturated cohesive soils without disturbing the natural fabric. The test follows AS 1726:2017 and uses a four-bladed vane pushed into the base of a borehole, rotated at a standard rate until failure. It gives engineers a direct reading of peak and remoulded strength, which feeds directly into foundation design and stability calculations. Before mobilising the vane equipment, we often run a complementary placa de carga to correlate surface stiffness with deeper vane results, ensuring the profile is consistent from the ground down.

Illustrative image of Field vane shear test (VST) in Sydney

The field vane shear test (VST) directly measures intact undrained strength in soft clays without sample disturbance, providing critical data for foundation design in Sydney's alluvial deposits.

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Scope of work

A typical job along the Eastern Suburbs railway corridor involved a 14-storey residential tower founded on stiff clay over sandstone. The geotechnical model required undrained parameters at 2 m intervals down to 18 m. The field vane shear test (VST) proved ideal because it captures the intact strength of the clay without the sample disturbance that plagues thin-walled tube sampling in these soils. We deployed a 65 mm x 130 mm vane with a calibrated torque head, recording peak torque at each depth. The key parameters we report include:

Peak undrained shear strength (su) in kPa
Remoulded shear strength for sensitivity ratio
Torque head calibration factor (N·m per division)

For this project we also integrated a presurometro at three depths to cross-check modulus values, and used cimentaciones superficiales analysis to confirm allowable bearing pressures for the raft foundation. The VST data aligned within 12% of the pressuremeter results, which gave the structural team confidence in the design.

Technical reference — Sydney

Area-specific notes

The coastal humidity and shallow water table across much of Sydney create conditions where soft clay behaviour can shift rapidly with seasonal rainfall. In the Cooks River floodplain, the field vane shear test (VST) must be performed within 24 hours of borehole advancement to minimise pore pressure dissipation and soil relaxation. If the vane is left idle in the hole too long, the measured torque may underestimate true undrained strength by up to 30%. Equipment calibration drifts under the same moisture, so torque heads are checked against a dead-weight system before each shift. The biggest operational risk comes from gravel layers or cemented nodules within the alluvium, which can damage the vane blades and invalidate the test. Pre-drilling a pilot hole with a continuous flight auger, then verifying the absence of obstructions with a georradar-gpr scan, reduces this risk substantially.

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Standards used

AS 1726:2017 Geotechnical site investigations, AS 1289.6.2.1 Standard test method for field vane shear test in saturated fine-grained soils, BS 1377-9:1990 Methods for test for soils for civil engineering purposes – In-situ tests

Technical parameters

Parameter	Typical value
Vane diameter	50–75 mm
Vane height-to-diameter ratio	2:1 (standard)
Rotation rate	6–12°/min
Torque measurement range	0–200 N·m
Peak shear strength range (soft clays)	10–150 kPa
Sensitivity ratio (St)	2–8 (typical Sydney clays)

Quick answers

How much does a field vane shear test (VST) cost in Sydney?

The typical cost for a field vane shear test (VST) in Sydney ranges from AU$880 to AU$2,150 per test, depending on depth, number of readings, and site access conditions. For a standard 15-metre profile with readings every 2 metres, the total service cost usually falls between AU$4,500 and AU$8,000. Volume discounts apply for projects requiring more than 10 test locations.

At what depths is the vane shear test most reliable in Sydney soils?

The field vane shear test (VST) is most reliable between 2 m and 20 m depth in Sydney's soft alluvial clays. Below 20 m, the clay often transitions into stiff residual soil or weathered sandstone, where the vane's torque capacity may be insufficient to cause failure. In the Botany Sands area, the best results come from the upper 10 m where the water table is at 1–3 m and the clay is fully saturated.

How does the VST compare to the unconfined compression test for soft clays?

The field vane shear test (VST) measures undrained strength in situ, preserving the natural structure, fabric, and in-situ stress state. The unconfined compression test (UCT) is run on a trimmed sample in the lab, which inevitably introduces disturbance from sampling, transport, and trimming. For soft clays with sensitivity above 4, the VST typically returns 20–40% higher strength values than UCT because it avoids that disturbance. AS 1726:2017 recommends the VST as the reference test for soft clays in Sydney.

Location and service area

We serve projects across Sydney and its metropolitan area.

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