why is a manhole circle?

Answer 1

*Other shapes can fall into a hole if turned upright on an angle.
As long as it is just slightly larger than the hole, a circular cover will not fall down the shaft,no matter what angle it is turned at.
*Reasons for the shape include:

1.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 aperture (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture.)
2.Round tubes are the strongest and most material-efficient shape against the compression of the earth around them, and so it is natural that the cover of a round tube assume a circular shape.
3.Cylindrical holes are easier to dig.
4.The bearing surfaces of manhole frames and covers are machined to assure flatness and prevent them from becoming dislodged by traffic.
5.Round castings are much easier to machine using horizontal boring mills.
6.Circular covers do not need to be rotated to align them when covering a circular manhole.
7.Human beings have a roughly circular cross-section.
8.A round manhole cover can be more easily moved by being rolled.
9.If an automobile rolls over a dislodged manhole of another shape, the sharp corners could puncture the automobile's tire, whereas a circle doesn't have corners.
10.Tradition
11.Aesthetics

*One practical answer being, "Manhole covers are round because manholes are round.

Answer 2

One cool feature of Yahoo answers is that the keep old questions and answerers so if you have a question that has been asked before (or 57 times before) you can get an answer without waiting for a response.

http://answers.yahoo.com/search/search_result;_ylt=Akq17nf4LJ9cfvviUTVHNUgjzKIX?p=manhole+round

Answer 3

Because only a circular manhole cover cannot fall into a manhole. Depending on orientation, a square or rectangular cover could fall in, so could an elliptical cover.

Answer 4

I agree, but I would also like to add that with the lack of corners, they would be more structurally sound, and people would be less prone to serious injury (a round piece of metal landing on a steel-toed shoe versus the corner of a square shaped piece of metal hitting the same shoe)

Answer 5

corners are a waste of space.

Answer 6

Just like JohnV said . . .