Earth Retaining Structures MCQ with answer and solution

Concept

Uniform surcharge produces a rectangular lateral earth pressure diagram whose resultant acts at mid-height (H/2) from the base of the wall.

Formula

Height of action = H / 2 = 0.5 H

Step-by-step solution

1. Uniform surcharge 'q' transmits a constant lateral pressure p_a = Ka × q throughout the wall height H. 2. The lateral pressure distribution over the wall height is rectangular. 3. The centroid of a rectangle of height H lies at half the height from the base, which is H/2 = 0.5 H.

Explanation

A uniform surcharge load 'q' per unit area on the ground surface induces a uniform lateral pressure of magnitude Ka × q over the entire height of the retaining wall. Since the resulting lateral pressure distribution diagram is rectangular over height H, the resultant thrust acts at the centroid of the rectangle, which is at a height of H/2 or 0.5 H from the base.

Why the other options are wrong

A: 0.66 H corresponds to 2H/3, which is measured from the top of the wall for triangular distribution, not from the base for surcharge. B: 0.33 H (or H/3) is the point of application of the thrust caused by the soil's self-weight (triangular distribution). D: 0.25 H (or H/4) is not a standard line of action for simple uniform surcharge or hydrostatic soil pressure profiles.

Shortcut

Rectangular pressure distribution (surcharge) acts at H/2 (0.5 H); Triangular distribution (soil self-weight) acts at H/3 (0.33 H).

Common mistake

Confusing the lateral thrust due to soil self-weight (triangular distribution acting at H/3 = 0.33 H from the base) with that due to surcharge (rectangular distribution acting at H/2 = 0.5 H from the base).