Dams MCQ with answer and solution

Concept

When the reservoir is full, the resultant force moves toward the toe. So the vertical stress at the heel falls, and tension first appears at the heel.

Formula

p = (ΣV/B) × (1 ± 6e/B); tension at heel when e > B/6

Step-by-step solution

1. Full reservoir: water pressure pushes the resultant toward the toe. 2. Eccentricity e is measured from the base centre toward the toe. 3. Stress at heel = (ΣV/B)(1 - 6e/B). 4. If e > B/6, the heel stress becomes negative, which means tension. 5. So tension first develops at the heel.

Explanation

In a full reservoir, horizontal water pressure shifts the resultant toward the toe. Compression rises at the toe and falls at the heel. When the resultant goes outside the middle third, tension starts at the heel.

Why the other options are wrong

A: The toe gets the maximum compression, not tension. C: A gallery is an inspection opening, not the critical base point. D: The mid-point has only the average stress ΣV/B.

Shortcut

Full reservoir = tension at heel. Empty reservoir = tension at toe.

Common mistake

Mixing up the cases. When the reservoir is empty, the resultant shifts toward the heel and tension can occur at the toe.