The useful method of finding the shear strength of very plastic cohesive soils is by means of the:

Asked in SSC JE 2015

Detailed Explanation

Very plastic, soft clays are difficult to sample and trim without disturbance, and may not stand up in an unconfined compression test.

Why “Vane shear test” Is Correct

The vane shear test measures their undrained strength in place (or in a sample tube) with minimal disturbance, which makes it the most useful method for such soils.

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

A. Cone test: The cone (penetrometer) test is mainly used to find the liquid limit or for quick strength index, not as the standard strength test for plastic clays.
B. Penetration test: Penetration tests such as SPT are crude in soft clays and are mainly used in sands.
D. Torsional shear test: Torsional (ring) shear is a specialised test for residual strength, not routine.

Key Point

Soft/very plastic clay → vane shear.

Key Concepts for Students

Torque T relates to cu through the vane dimensions.
Field vanes are pushed below the bottom of a borehole to avoid disturbed soil.
Results may need correction for plasticity index.

Practice more: this is question 42 in Shear Strength of Soils MCQs – Part 3. Try the whole set and check every answer instantly.