That was the situation until a Swedish geotechnical engineering expert came up with the in-situ vane shear test in 1919. Today, the field vane test is the most popular technique used to estimate the undrained strength of fine-grained soils, such as silts and soft clays – it is simple, fast and inexpensive.
The test works like this:
- A four-bladed vane is pushed into undisturbed soil at the bottom of a borehole. A general rule of thumb is to go down five borehole diameters to help ensure you’re in undisturbed territory. Be careful not to rotate the vane during this first procedure.
- The rod holding the vane is rotated slowly – 6 degrees to 12 degrees a minute – and the torque is measured several times a minute until a cylindrical volume of soil is completely sheared by the vane. The torque measured at failure is the undrained shear strength of the soil.
- A second test is run immediately after to measure the remolded strength of the soil, providing a measurement of its sensitivity.
Vane diameters vary according to the consistency of the soil, with larger vanes used in softer materials, and smaller vanes used in harder materials.
Because the test is taking place underground and out of sight, there is a need to take some factors of the test for granted. We have to assume:
Inserting the vane didn’t cause undue disturbance to the soil.
- The soil is homogeneous.
- No drainage occurs during the shear process.
- The soil fails on a cylindrical shear surface.
- The shear stress is uniform across the surface.
That’s a lot to have to assume with a high degree of certainty, but it’s what we have to do. Important factors that are within our control:
- Careful placement of the vane.
- The time lapse between the insertion of the vane and the test itself (it should be very short).
- The rate of rotation (slow and steady).
Whatever the results are from the field vane tests, they must be taken within the larger context of the surrounding environmental conditions. While an extremely useful tool, the field vane shear test can only do so much. Bridges have collapsed because of faulty tests and improper interpretations of test results. Thankfully, standardization of the test, and nearly a century of experience, have made the field vane shear test a most valuable instrument in the geotechnical arena.