The risk the vane test reduces
In soft clay, stability is governed by the undrained shear strength. The vane test measures Su where sampling almost always disturbs the soil and distorts the result: right in the ground, with no sample taken.
The test, step by step
Vane insertion
The cruciform vane, with four blades at a height-to-diameter ratio of 2:1, is pushed from the bottom of the borehole down to the test depth, without rotation.
Torque to failure
Rotation is applied at a standard angular rate and the peak torque is measured. From that peak the undrained peak strength (Su) is calculated.
Remoulding and residual reading
After failure, the vane is rotated rapidly to remould the soil and the residual torque, the remoulded strength, is measured.
Sensitivity and profile
The ratio between peak and remoulded strength defines the sensitivity (St). Repeating the test at several depths yields the Su profile with depth.
In accordance with NBR 10905
| Standard | ABNT NBR 10905, international reference ASTM D2573 |
|---|---|
| Peak strength | Undrained Su, calculated from the peak torque |
| Remoulded strength | Residual Su, measured after remoulding the vane |
| Sensitivity | St, ratio between peak and remoulded strength |
| Design correction | Bjerrum factor (μ) as a function of the plasticity index, for stability analysis |
Quality and field operation
Data quality
- Calibrated vanes and torque head, with periodic verification and traceable certificates.
- Test depth below the zone disturbed by insertion, so as not to underestimate Su.
- Standardised rotation rate and controlled test timing.
- Log reviewed by an engineer before delivery, a process within the ISO system.
Safety and operation
- Ready for high-demand environments: mining, industry and ports.
- Organised, signed work front with an HSE procedure in place.
- Own crews and fleet, including night shifts.
The deliverable
| Field log | Peak and remoulded Su by depth, with the strength profile versus depth |
|---|---|
| Sensitivity | St calculated for each point tested |
| Design parameters | Su corrected by the Bjerrum factor, when interpretation is contracted |
| Typical turnaround | Preliminary log the same day; report according to scope |
Sectors and project types
Frequently asked questions about the Field vane test
What does the vane test measure?
It measures the undrained shear strength (Su) of the clay directly in the ground, both in the intact state and after remoulding. With the two readings it also yields the sensitivity (St) of the soil.
Does the vane test replace the CPTu?
It does not replace it, it complements it. The CPTu gives the continuous subsoil profile; the vane test gives a direct measure of Su, useful to calibrate piezocone correlations and to check critical points in the soft layer.
In which soils does the test apply?
It is suited to soft to medium, saturated clays. It does not apply to soils with sand, gravel or material that allows drainage during rotation, situations in which the undrained assumption no longer holds.
Why does the vane Su need to be corrected?
For use in stability analysis, the measured Su is adjusted by the Bjerrum factor, which depends on the plasticity index of the clay. The correction avoids overestimating the strength available in design.
Bulletin 26: Vane Probe
The technical bulletin on the vane shear test in soft clays, interpretation of Su, sensitivity and the correction for stability analysis.
Field Vane Test for your project
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