Which of the following statements about the circular sewer section is INCORRECT?
- AIt offers the maximum cross-sectional area for the amount of materials used in walls.
- BUnder the full-flow condition, a circular sewer has the maximum hydraulic radius.
- CIt has low structural stability.Correct
- DCircular sewers are easy to construct.
Solution
Concept
A circular sewer is strong and stable because it resists external loads mainly by compression (arch action).
Formula
R = A/P = D/4 (circular pipe running full)
Step-by-step solution
1. Check A: a circle gives the most area for the least wall perimeter, so the least material. True. 2. Check B: when running full, a circle has the highest hydraulic radius for a given area, R = D/4. True. 3. Check C: a circular shape carries soil and traffic load by arch action, so it has HIGH structural stability. Statement is false. 4. Check D: circular pipes are easy to make, precast and lay. True. 5. The incorrect statement is (C).
Explanation
A circular sewer has high structural strength because its uniform curved shape resists outside pressure well. So the statement 'low structural stability' is wrong. The other statements are true advantages of circular sewers.
Why the other options are wrong
A: True, a circle uses the least wall material for the area enclosed. B: True, a circle gives the maximum hydraulic radius at full flow. D: True, circular pipes are easy to construct, precast and lay.
Shortcut
Circle = least material, best hydraulics, strongest shape.
Common mistake
Thinking B is wrong. At very low flows the egg-shaped section is better, but at full flow the circular section has the maximum hydraulic radius.
Correct answer: C) It has low structural stability.