______ is a method of measuring the undrained shear strength of a cohesive soil.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 09/10/2023 (Morning)
Detailed Explanation
The vane shear test uses four thin blades fixed at right angles on a rod. The vane is pushed into soft clay and rotated. The clay shears around the cylinder swept by the blades, and the maximum torque gives the undrained shear strength.
\[ T = c_u\,\pi D^2 \left(\frac{H}{2} + \frac{D}{6}\right) \quad\text{(both ends shearing)} \]
Why “Vane shear test” Is Correct
The test is so fast that no drainage can occur, so it gives the undrained strength cu directly. It is the classic method for measuring undrained strength of soft, sensitive clays, especially in the field where sampling would disturb them.
Why the Other Options Are Wrong
- B. Direct shear test: The direct shear test cannot control drainage, so it is not a reliable undrained test; it is mainly used for sands.
- C. Torsion test: ‘Torsion test’ is a test on shafts and materials, not a standard soil test for cu.
- D. Triaxial test: A UU triaxial test can also give undrained strength, but the method specifically known for measuring undrained shear strength of soft cohesive soil (and the expected answer) is the vane shear test.
Key Point
Vane shear test → cu of soft clays, in situ or in the lab. Not for sands or stiff, fissured clays.
Key Concepts for Students
- Sensitivity can be found by re-rotating the vane after failure (remoulded strength).
- Field vane results are often corrected (Bjerrum's correction) for design.
- Typical lab vane: 12 mm diameter, 24 mm high.
Practice more: this is question 12 in Shear Strength of Soils MCQs – Part 1. Try the whole set and check every answer instantly.