The most suitable test for finding the shear strength of very plastic cohesive soils is
- ACone test
- BPenetration test
- CVane shear testCorrect
- DTorsional shear test
Solution
Concept
The vane shear test gives the undrained shear strength of soft, very plastic and sensitive clays directly in the field or in the lab.
Formula
T = π D2 cu (H/2 + D/6) (both ends of vane shearing)
Step-by-step solution
1. Very plastic cohesive soils are soft clays. 2. Undisturbed samples of soft clay are hard to get and test. 3. The vane shear test is done in place and measures undrained shear strength cu from the torque. 4. So the vane shear test is the most suitable.
Explanation
Soft and very plastic clays get disturbed easily during sampling. The vane shear test measures shear strength in place, so disturbance is small. It gives undrained cohesion cu from the measured torque.
Why the other options are wrong
A: The cone test is mainly for liquid limit or consistency, not field shear strength. B: Penetration tests like SPT are not reliable in very soft plastic clay. D: The torsional shear test is a special lab test and is not commonly used for this purpose.
Shortcut
Soft, sensitive or plastic clay means vane shear test.
Common mistake
Choosing the penetration test because it is a common field test.
Correct answer: C) Vane shear test