Shear Strength MCQ with answer and solution

Concept

Shear strength of soil comes from cohesion between particles and internal friction between particles.

Formula

τ_f = c + σ tan φ

Step-by-step solution

1. Use the Mohr-Coulomb equation: τ_f = c + σ tan φ. 2. The term c is cohesion. 3. The term σ tan φ is frictional resistance. 4. So shear resistance comes from cohesion and friction. The answer is (C).

Explanation

As per Mohr-Coulomb theory, soil resists shear by cohesion (c) and internal friction (φ). Sands mainly have friction, and clays mainly have cohesion. Most soils have both.

Why the other options are wrong

A: Pore pressure reduces effective stress and so reduces strength. It is not a source of strength. B: Plasticity is a property of clay, not a component of shear strength. D: Only friction is true only for cohesionless soil like clean sand, not for soil in general.

Shortcut

Remember τ = c + σ tan φ: c is cohesion and φ is friction.

Common mistake

Choosing pore pressure, which actually lowers effective shear strength.