Question

27. Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : Direct shear test is not of much use for fine grained soils where drainage conditions play an important role influencing the shear strength.
Reason (R) : In direct shear test drainage conditions can not be controlled and pore water pressure can not be measured.
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

For cohesive fine-grained soils, the Triaxial Shear Test is preferred over the Direct Shear Test because it allows full control over drainage and precise measurement of pore water pressure.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The direct shear test is a common laboratory method used to determine the shear strength parameters (cohesion \(c\) and internal friction angle \(\phi\)) of soil.
However, its application is limited when dealing with saturated fine-grained soils (like clay) because their shearing behavior depends heavily on pore water pressure changes and drainage conditions.

Step 2: Detailed Explanation:
Let us evaluate the validity of both the assertion and reason statements:
Assertion (A): Fine-grained soils have low permeability, and their shear strength parameters depend significantly on whether they are tested under drained (slow shearing) or undrained (rapid shearing) conditions.
In engineering practice, we must design foundations using drained or undrained parameters depending on the loading rate.
Because drainage conditions cannot be properly simulated in a direct shear test to obtain precise undrained or consolidated undrained parameters, this test is not suitable for fine-grained soils.
Thus, Assertion (A) is true.
Reason (R): In a direct shear apparatus, the soil sample is held in a split metal box with porous stones at the top and bottom.
Since the box is split and exposed to the atmosphere, there is no way to seal the specimen to prevent drainage during a rapid shear test.
Additionally, there is no provision or instrumentation in a standard direct shear apparatus to measure the pore water pressure generated inside the soil specimen during shearing.
Thus, we cannot determine the effective stresses acting on the shear plane.
Hence, Reason (R) is true.
Because the inability to control drainage and measure pore water pressure (R) is the precise technical reason why the test is unsuitable for assessing drainage-sensitive fine-grained soils (A), the reason is the correct explanation of the assertion.

Step 3: Final Answer:
Both (A) and (R) are true and (R) is the correct explanation of (A).