Question

Why are sewer covers round?

Answer 1

what class is this for? TROL500?

Answer 2

its a mathematical question

Answer 3

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 Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture. The existence of a "lip" holding up the lid means that the underlying hole is smaller than the cover, so that other shapes might suffice.)
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.
Similarly, it is easier to dig a circular hole and thus the cover is also circular.
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 them when covering a circular manhole.
A round manhole cover can be more easily moved by being rolled.
A round manhole cover is cheapest to manufacture relative to other shapes because it requires the least amount of metal to cover an opening wide enough for a person to get through.
Most manhole covers are made by a few large companies. A different shape would have to be custom made.
If a manhole cover were to be displaced by a car or anything else, it could easily slip back into position and would not have to have any angles matched up.
If a manhole cover were to be displaced it would not pop or damage the tires of an automobile.