Shear Strength MCQ with answer and solution

Concept

Angle of internal friction for purely cohesive soils

Formula

τ = c_u

Step-by-step solution

1. Purely cohesive soils (such as saturated clays under undrained conditions) exhibit constant shear strength. 2. The undrained shear strength equation is τ = c_u + σ × tan(φ_u). 3. Under undrained conditions, undrained angle of internal friction φ_u = 0°. 4. The failure envelope is a horizontal line parallel to the normal stress axis. Answer: A

Explanation

For a purely cohesive soil (such as saturated clay under undrained conditions, φ_u = 0), the shear strength envelope is horizontal. The shear strength is entirely derived from undrained cohesion (c_u), and the angle of internal shearing resistance (φ) is 0°.

Why the other options are wrong

B) 30° is typical for sands or normally consolidated clays under drained effective stress conditions. C) 45° is common for dense interlocking angular sands. D) 90° is physically impossible for internal friction angles.

Shortcut

Purely cohesive clay (undrained) = φ = 0°; failure envelope is horizontal (τ = cu).

Common mistake

Mistaking the friction angle of clay in drained conditions (φ') with the purely cohesive φ_u = 0 condition.