Shear Strength MCQ with answer and solution

Concept

A stress state cannot lie above the Mohr-Coulomb failure envelope. The soil fails before that point can be reached.

Formula

τ_f = c + σ tanφ

Step-by-step solution

1. The failure envelope gives the maximum shear strength τ_f = c + σ tanφ. 2. A point below the envelope means τ < τ_f, so the soil is safe. 3. A point on the envelope means τ = τ_f, so failure is just starting. 4. A point above the envelope means τ > τ_f, which is not possible because the soil would already have failed. 5. So the condition is imaginary.

Explanation

The Mohr envelope shows the highest shear stress the soil can carry. A stress point above it cannot really exist, because the soil fails as soon as it touches the envelope. So such a point shows an imaginary or impossible condition.

Why the other options are wrong

B: The safe condition is a point below the envelope. C: Imminent failure is a point exactly on the envelope. D: Maximum obliquity also occurs at the point where Mohr's circle touches the envelope, not above it.

Shortcut

Below = safe, On = failure, Above = impossible (imaginary).

Common mistake

Thinking a point above the envelope means failure. Failure happens on the envelope, and above it is not possible.