If you're working with soft to medium-stiff clays and need a reliable, fast measure of undrained shear strength in the field, the vane shear test is one of the most practical tools available. It's quick to set up, requires no heavy rig, and delivers results you can use directly in foundation design and slope stability assessments.
This guide covers everything you need to know — how the test works, when to use it, equipment selection, and how to interpret your results.
What Is the Field Vane Shear Test?
The field vane shear test (FVT) is an in-situ geotechnical test used to measure the undrained shear strength (Su) of saturated cohesive soils — particularly soft and sensitive clays. It is one of the most widely used and well-established methods in geotechnical practice, valued for its speed, simplicity, and minimal soil disturbance.
The test works by inserting a four-bladed vane into the soil and applying a measured torque until the soil fails along a cylindrical shear surface. The maximum torque at failure is used to calculate the undrained shear strength in kPa.
The field vane shear test is governed by ASTM D2573 in the United States, which specifies equipment dimensions, insertion procedures, rotation rates, and result interpretation. In the UK, the equivalent standard is BS 1377: Part 9.
When Should You Use a Field Vane Shear Test?
The field vane shear test is the right tool when:
- You are working with soft to firm cohesive soils (clays, silts, or sensitive clays)
- You need quantitative undrained shear strength data — not just an index reading
- You want to profile shear strength variation with depth
- Foundation design, embankment stability, or retaining wall analysis requires reliable Su values
- You need both peak and remoulded shear strength to assess soil sensitivity
- You are working in a borehole or trial pit where direct access to the soil is available
The test is not appropriate for granular soils (sands, gravels), hard or stiff clays where the vane cannot be pushed in cleanly, or situations where drainage conditions during testing are uncertain.
Field Vane Shear Test vs Other In-Situ Methods
| Test Method | Measures | Best For | Limitation |
|---|---|---|---|
| Field Vane Shear Tester | Undrained shear strength (Su) in kPa | Soft to firm clays, in-situ Su profiling | Cohesive soils only |
| Pocket Penetrometer | Estimated unconfined compressive strength | Quick field index check on cohesive soils | Low accuracy; index only |
| DCP Test | CBR, penetration resistance | Granular subgrades, compaction QA | No direct Su measurement |
| CPT / CPTU | Cone resistance, sleeve friction, pore pressure | Detailed soil profiling on large projects | Heavy rig required; expensive |
Equipment Overview
A standard field vane shear tester kit includes:
- Calibrated torque head with scale ring (retains peak reading)
- Extension rods for testing at depth (typically 0.5m increments)
- Interchangeable vane blades in multiple sizes
- Push-in guide shoe for borehole use
- Carrying case
Vane dimensions are critical — the height-to-diameter ratio must be 2:1 in accordance with ASTM D2573. Standard vane sizes are defined in the standard and must be selected based on expected soil strength.
Vane Size Selection Guide
| Vane Size | Soil Shear Strength | Typical Su Range | Typical Soil Type |
|---|---|---|---|
| Large vane | Very soft to soft | < 25 kPa | Estuarine clays, peats, soft alluvium |
| Standard vane | Soft to firm | 25–75 kPa | Most alluvial and lacustrine clays |
| Small vane | Firm to stiff | 75–150 kPa | Overconsolidated clays, glacial tills |
Test Procedure (ASTM D2573)
The following procedure applies to borehole deployment, which is the most common configuration:
1. Advance the borehole
Drill or auger the borehole to approximately 0.5m above the test depth. Avoid disturbing the soil in advance of the vane — this is critical for obtaining representative peak strength values.
2. Insert the vane
Lower the vane assembly through the drill rods and push it rapidly into the undisturbed soil below the borehole base. The vane should be inserted at a continuous rate to minimise disturbance. Push to at least four vane diameters below the borehole base.
3. Wait period
Allow a minimum of 5 minutes between insertion and the start of rotation. This allows dissipation of any excess pore pressures generated during insertion.
4. Apply torque
Rotate the vane at a rate of 6° per minute as specified in ASTM D2573. This controlled rate is critical — rotating too fast generates excess pore pressure; too slow allows partial drainage. Note the peak torque reading on the scale ring.
5. Record peak strength
The peak torque reading corresponds to the peak undrained shear strength. Note this before any further rotation.
6. Remould (if required)
For sensitivity assessment, rapidly rotate the vane through a minimum of 10 complete revolutions to fully remould the soil. Then re-run the slow rotation to measure the remoulded shear strength.
7. Calculate Su
Undrained shear strength is calculated from the peak torque using the standard formula for a cylindrical failure surface. Most torque heads include a calibrated scale that reads Su directly in kPa, removing the need for manual calculation in the field.
Interpreting Your Results
Undrained Shear Strength (Su)
The peak Su value from the field vane test is used directly in bearing capacity calculations, slope stability analysis, and wall design. It represents the shear strength of the soil in its in-situ, undrained condition — the relevant condition for short-term loading scenarios.
Soil Sensitivity
Sensitivity (St) = Su (peak) / Su (remoulded). This ratio characterises how much strength the soil loses when disturbed. Sensitive clays (St > 8) can lose most of their strength on disturbance and require careful consideration in foundation design. Quick clays (St > 16) are encountered in some glaciated environments and are a significant geotechnical hazard.
Correction Factors
ASTM D2573 and published research (notably Bjerrum, 1972) recognise that field vane tests in highly plastic or sensitive clays may overestimate Su relative to actual failure conditions. Correction factors (μ) based on plasticity index are commonly applied in design — your geotechnical engineer should confirm whether correction is warranted for your site conditions.
Field Notes and Best Practice
- Always record the vane size used — Su calculations are dimension-specific
- Note the insertion depth and distance below borehole base for each test
- Maintain the ASTM D2573 rotation rate of 6°/minute consistently across tests
- Record the peak torque reading before resetting the scale ring
- Log soil description (colour, consistency, structure) alongside every test
- If the vane meets resistance during insertion, do not force it — the soil may be too stiff for vane testing
- Allow the full 5-minute wait period after insertion without exception
Field Vane Shear Tester from GeoSite Supply
The GeoSite Supply Field Vane Shear Tester is built to ASTM D2573 and BS 1377 specifications, supplied as a complete field-ready kit. It includes a calibrated torque head with scale ring, multiple extension rods, interchangeable vane blades, and a robust carry case. Ships directly to the US with no minimum order.
View full specifications and order online: GeoSite Supply Field Vane Shear Tester