Undrained shear strength of soft clay from the vane torque (IS 2720 Part 30).
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Vane Shear Test Calculator: Free Online Tool
The vane shear test measures the undrained shear strength of soft, saturated clay. A cruciform vane is pushed into the soil and turned slowly, and the torque needed to shear a cylinder of soil is recorded. The test can be done in the laboratory on a small sample or in the field on soft deposits, where taking undisturbed samples is difficult.
Geotechnical engineers, site technicians and students use the result, Cu, to check the bearing capacity of soft clay, the stability of embankments on soft ground and the safe rate of loading. Repeating the test on remoulded soil gives the sensitivity, which shows how much strength is lost when the clay is disturbed. This calculator follows IS 2720 Part 30 and converts your vane size and torque readings into Cu in kPa and kgf/cm2.
When to use this calculator
Use this calculator when you have a vane size and a torque reading from a laboratory vane apparatus or a field vane test in soft clay. It is helpful for soft-soil investigations, checking embankment stability on marshy ground and comparing soil layers with depth. Enter the vane diameter D and height H in mm, choose N·m or kgf·cm for the torque, and enter the failure torque of the undisturbed soil. Add the remoulded torque if you want sensitivity. The defaults are a 50 mm by 100 mm vane with 14 torque units.
How to use the Vane Shear Test Calculator
- Open the calculator above on your phone, tablet or computer. Nothing needs to be installed and no sign-up is required.
- Pick the option you need from the drop-down list (N·m; kgf·cm).
- Enter your values in the input boxes: Vane diameter D (mm), Vane height H (mm), Torque unit, Torque at failure (undisturbed), Torque, remoulded (optional). The boxes already hold sample values, so overwrite them with your own.
- Press Calculate. The result appears straight away under the button.
- Change any value and press Calculate again to compare options. Press Reset to go back to the sample values.
- Check the result against your drawings, project specification and local code before you order material or approve work.
How the calculation works
The calculator uses Cu = T / [pi x D2 x (H/2 + D/6)], which assumes the vane is pushed fully into the soil so that both ends of the sheared cylinder resist the turning. T is the torque at failure, D the vane diameter and H the vane height. The vane dimensions in mm are converted to metres. If you choose kgf·cm the torque is converted to N·m, and the result is given in kPa and in kgf/cm2. If you enter a remoulded torque greater than zero, the same formula gives Cu remoulded, and sensitivity is the undisturbed Cu divided by the remoulded Cu. Leave the remoulded torque at 0 to skip sensitivity.
Worked example
When you open the page the calculator is filled with sample values. Pressing Calculate with those values gives the result below, which you can use to check that you are reading the output correctly.
| Undrained shear strength | |
|---|---|
| Cu, undisturbed (kPa) | 30.56 |
| Cu, undisturbed (kgf/cm²) | 0.312 |
| Cu, remoulded (kPa) | 9.17 |
| Sensitivity (St = Cu / Cu remoulded) | 3.33 |
Tips and common mistakes
- Check the torque unit selector before calculating. Entering kgf·cm values while N·m is selected gives a result almost ten times too large.
- Use the peak torque at failure for the undisturbed value, and the steady lower torque after rapid rotation for the remoulded value.
- Measure the vane diameter and height from the actual blades, because D is squared in the formula and errors have a strong effect.
- The formula assumes the vane is fully embedded. Do not use it if only part of the vane height was in the soil.
- Use the test only for soft clay, and compare the calculated Cu with the consistency description of the soil in your log.
Frequently asked questions
What does Cu mean?
Cu is the undrained shear strength of clay, the shear stress at failure when no drainage occurs. It is the main strength input for short-term stability analyses of soft clay and is reported here in kPa and kgf/cm2.
How is sensitivity calculated?
Sensitivity is the undisturbed Cu divided by the remoulded Cu. With the default torques of 14 and 4.2 N·m, Cu values are 30.56 and 9.17 kPa, so sensitivity is 3.33. A higher value means greater strength loss on disturbance.
Can I enter torque in kgf·cm?
Yes. Pick kgf·cm in the torque unit box and enter your readings in that unit. The calculator converts them to N·m before applying the formula, and the result is still shown in kPa and kgf/cm2.
Why does the formula use D/6?
The term comes from the shearing of the top and bottom ends of the soil cylinder in addition to its curved side. The formula in this calculator is valid when both ends shear, meaning the vane is fully embedded.
Is the vane test suitable for stiff clay or sand?
No. The vane test is intended for soft, saturated clay. Stiff clay can damage the vane and sandy soils drain during shearing, so the undrained assumption in the formula does not hold.
Important note
This tool gives estimates for planning, study and quick checks. It does not replace design by a qualified engineer, site-specific testing, or the requirements of your project specification and local building code. Always confirm final quantities, test results and acceptance limits with the responsible engineer before using them for construction or payment.
