Why sewer covers are circular
Question
Why are manhole covers made round?
Short answer
Mainly so the cover cannot drop through its own hole. A circle has the same width in every direction, so no orientation lets it slip in, whereas a square cover can fall through along its diagonal. Round covers also need no alignment when replaced, can be rolled rather than carried, and match the round shaft that best resists the surrounding soil pressure.
Adapted from Crack, Heard on the Street, Q5.5.23, p.55; question and answer rewritten by SelfMadeQuant.com.
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