The vane shear test is used to find the shear strength of:

Asked in SSC JE 03/03/2017 (Morning)

Detailed Explanation

In the vane test, thin blades are pushed into the soil and rotated. This only works if the soil lets the blades in without disturbing it much and shears undrained around them.

Why “clayey soil” Is Correct

Soft clays meet both conditions, so the vane test is used for clayey soils. In sands and gravels the blades cannot be pushed in cleanly, the soil drains during the test, and particles jam the blades, so the results would be meaningless.

Vane shear test. Four thin blades on a rod are pushed into soft clay and rotated. The torque needed to shear the cylinder of soil swept by the blades gives the undrained shear strength c_u. It is done in the field (in boreholes) and in the laboratory.

Why the Other Options Are Wrong

B. gravelly soil: Gravels would block and damage the blades.
C. sandy soil: Sands are too permeable (drain during rotation) and too resistant to vane insertion.
D. all of the above: It is not suitable for gravel or sand.

Key Point

Vane test: soft to medium clays only.

Key Concepts for Students

Gives undrained strength cu directly from the torque.
Can be done in boreholes to great depths.
Lab vane is used on tube samples of very soft clay that cannot stand in a UCS test.

Practice more: this is question 34 in Shear Strength of Soils MCQs – Part 3. Try the whole set and check every answer instantly.