Ask @HossamEhsan:

why manhole covers are round ?

Reasons for the shape include:
- A round manhole cover cannot fall through its circular opening, whereas a square manhole cover may fall in if it were inserted diagonally in the hole.
- A round manhole cover has a smaller surface than a square one, thus less material is needed to cast the manhole cover, meaning lower cost.
- The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic. Round castings are much easier to machine using a lathe.
- A round manhole cover can be more easily moved by being rolled.
- A round manhole cover can be easily locked in place with a quarter turn (as is done in countries like France). They are then hard to open without a special tool. Also then they do not have to be made so heavy, because traffic passing over them cannot lift them up by suction.
2y 5ed3a :D
refrence: http://en.wikipedia.org/wiki/Manhole_cover#Circular_shape

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