For very large trunk and out-fall sewers carrying high discharges, which cross-sectional shape is generally preferred in practice?

Introduction / Context:
Sewer cross-sections are selected based on hydraulic performance, structural feasibility, construction practicality, and maintenance. While egg-shaped sewers are excellent for combined systems at small to medium sizes, very large trunk and out-fall conduits often use alternative shapes.

Given Data / Assumptions:

• Out-fall/trunk mains with very large discharge.
• Gravity flow with wide variation in flows.
• Constructability and structural stability are key considerations.

Concept / Approach:

The horse-shoe section offers a broad invert for structural stability and ease of construction at very large sizes, while still providing favorable hydraulic characteristics. Circular sections are common overall, but at very large spans, the horse-shoe provides better roof and sidewall behavior under loads. Egg-shaped is ideal for small to medium combined flows (wet-weather performance at low dry-weather flows) but is not the standard for the largest out-fall sizes.

Step-by-Step Solution:

Verification / Alternative check:

Historic large out-falls and interceptors (masonry or concrete) frequently adopt horse-shoe geometry due to formwork practicality and load paths.

Why Other Options Are Wrong:

(a) Best for combined small/medium conduits; (b) feasible but less common at very large spans compared to horse-shoe; (d) and (e) are uncommon choices for main out-falls.

Common Pitfalls:

Assuming one shape is optimal at all scales; ignoring maintenance access and ventilation needs in very large conduits.

Final Answer:

Horse-shoe section