Shear Strength MCQ with answer and solution

Concept

Shear strength parameters of granular soils including drained friction angle and zero shear resistance under liquefaction/undrained conditions.

Formula

τ = c' + σ'_n × tan(φ')

Step-by-step solution

1. Analyze Statement 1: Sands and gravels are coarse-grained soils with internal friction. Their drained friction angle φ' typically lies between 30° and 45°, rarely dropping below 30°. 2. Analyze Statement 2: Saturated sands, particularly loose sands subjected to quicksand conditions or undrained loading leading to liquefaction, can temporarily exhibit an angle of shearing resistance (or undrained shear strength) of zero. 3. Conclude that both statements describe valid phenomena in soil mechanics.

Explanation

Statement 1 is correct because standard sands and gravels generally have a drained friction angle (φ') ranging from 27° to 45°, rarely falling below 30° for dense or standard compacted states. Statement 2 is also correct because under undrained conditions or specific loose/critical states (such as loose saturated sand undergoing liquefaction or initial undrained shear), the shear resistance can temporarily drop to zero (residual or undrained shear strength under zero effective stress).

Why the other options are wrong

A: Statement 2 is also valid under undrained liquefaction/zero effective stress conditions. B: Statement 1 is a well-established empirical fact in soil mechanics. D: Both statements are correct based on soil mechanics principles.

Shortcut

Remember that coarse-grained soils have high friction angles (>30°), but saturated loose sands can lose shear strength completely under undrained undrained pore pressure build-up (zero shear resistance).

Common mistake

Assuming saturated sand always has a friction angle, forgetting that under undrained liquefaction conditions, the shear resistance can approach zero.