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Which of the following statements about the circular sewer section is INCORRECT?
It has low structural stability.
Under the full-flow condition, a circular sewer has the maximum hydraulic radius.
Circular sewers are easy to construct.
It offers the maximum cross-sectional area for the amount of materials used in walls.
It has low structural stability.
A circular sewer section is considered the most structurally stable shape for underground applications because it distributes external loads uniformly. It offers the best geometric efficiency by minimizing the perimeter for a given cross-sectional area, leading to cost-effectiveness and optimal hydraulic performance.
CPHEEO Manual on Sewerage and Sewage Treatment Systems
A circular sewer section is considered the most structurally stable shape for underground applications because it distributes external loads uniformly. It offers the best geometric efficiency by minimizing the perimeter for a given cross-sectional area, leading to cost-effectiveness and optimal hydraulic performance.
A=4╧АD2тАЛ тАФ Cross-sectional area of a full circular pipe
P=╧АD тАФ Wetted perimeter for a full circular pipe
R=PAтАЛ=4DтАЛ тАФ Hydraulic radius for a full circular pipe
The circular shape provides maximum structural resistance against external earth pressures due to the arch effect. Hydraulically, it provides the maximum hydraulic radius (R=D/4) when flowing full, which minimizes frictional losses and maximizes flow velocity, keeping the sewer self-cleansing.
Circular sewers provide high structural stability against external soil loads.
They offer the least wetted perimeter for a given area, reducing friction.
Ease of manufacturing and standardized precast production makes them highly economical.
They are the preferred standard for most gravity-fed wastewater collection systems.
Maximum hydraulic efficiency at full capacity
High structural integrity against external pressure
Ease of construction and standardized precast availability
Optimal material efficiency (lowest area-to-perimeter ratio)
Poor hydraulic performance at very low flow depths
Requires careful alignment and slope management
Municipal wastewater collection networks
Stormwater drainage systems
Gravity flow sanitary sewers
Option A is incorrect because circular sewers possess high structural stability, not low stability.
Option B is true: at full flow, R=D/4, which is the maximum value for circular geometry.
Option C is true: standard precast circular pipes are mass-produced and easy to install.
Option D is true: the circle is the most efficient geometric shape for confining area relative to perimeter.
A is correct тАФ The statement that circular sewers have low structural stability is incorrect, as they are inherently robust and resistant to external loads.
In competitive exams, remember that while circular sewers are excellent for full-flow, egg-shaped sewers are often preferred in combined sewers to maintain higher velocities during low-flow (dry weather) conditions.
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