Earth Retaining Structures MCQ with answer and solution

Concept

Wall friction acts to resist the relative movement of the sliding soil mass, thereby reducing active thrust and increasing passive resistance.

Formula

Ka decreases with increase in δ; Kp increases with increase in δ

Step-by-step solution

1. Identify the relative movement of the backfill soil wedge in active and passive conditions. 2. In active state, the backfill slides downward and outward; friction acts upward along the wall face, supporting part of the soil weight and reducing the horizontal thrust (active earth pressure decreases). 3. In passive state, the backfill is pushed upward and inward; friction acts downward along the wall face, adding resistance against the push and increasing the horizontal thrust (passive earth pressure increases). 4. Therefore, wall friction decreases active earth pressure and increases passive earth pressure.

Explanation

Wall friction (δ) develops along the back of the retaining wall due to relative movement between the wall and the soil wedge. In the active state, the soil wedge moves downward relative to the wall, causing upward shear resistance that reduces the active earth pressure. In the passive state, the soil wedge is forced upward relative to the wall, causing downward shear resistance that increases the passive earth pressure.

Why the other options are wrong

B: Incorrect because passive resistance increases and active pressure decreases due to friction. C: Incorrect because passive earth pressure increases with wall friction. D: Incorrect because active earth pressure decreases with wall friction.

Shortcut

Friction always opposes motion: it helps you in active (reduces pressure) and resists you in passive (increases pressure).

Common mistake

Confusing Coulomb's theory with Rankine's theory (which assumes a smooth, frictionless wall) or thinking friction affects both pressures in the same direction.