Shear Strength MCQ with answer and solution

Concept

Shear strength tests are categorized based on drainage conditions during consolidation and shearing. For dry sand, since pore water is absent, pore pressures are always zero, making any test effectively a drained test (CD test).

Formula

τ = c + σ' tan(φ)

Step-by-step solution

1. Identify the material as dry sand, meaning the degree of saturation S = 0 and pore water pressure u = 0. 2. Recognize that during the direct shear test on dry sand, volume changes occur freely as the soil is sheared slowly. 3. Since no pore pressure develops (u = 0), total stress equals effective stress (σ = σ'). 4. A test where drainage is allowed and no excess pore pressure exists during shearing is defined as a Consolidated-Drained (CD) test.

Explanation

In a direct shear test on dry sand, drainage is permitted freely throughout the test and no excess pore water pressure is generated because the soil is dry (degree of saturation is zero). Since the rate of shear is slow enough to prevent any pore pressure build-up and drainage occurs completely, it corresponds to a Consolidated-Drained (CD) test.

Why the other options are wrong

A: UU test is unconsolidated-undrained, where no drainage is allowed and pore pressures develop. B: CU test is consolidated-undrained, where pore pressures are measured during undrained shearing. D: UU test without pore pressure measurement is incorrect because dry sand inherently operates under drained conditions.

Shortcut

Dry soil + fully drained shear test = CD test always, because excess pore water pressure cannot be generated without water.

Common mistake

Students often confuse direct shear test setups with triaxial tests and assume it is always an unconsolidated test, ignoring that dry sand allows immediate drainage and has no pore pressure.