Why manholes are round

have ever asked this think why manholes and covers are designed in circular shape.There are multiple answers to this question.

you can read more about this: http://en.wikipedia.org/wiki/Manhole_cover

• 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. 
• Round tubes are the strongest and most material-efficient shape 
• 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.
• Circular covers do not need to be rotated to align with the manhole.
• 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 which makes them hard to open without a special tool. Lockable covers do not have to be made as heavy, because traffic passing over them cannot lift them up by suction.courtesy: