Manholes, which interconnect underground sewerage pipes, and
serve as a point of entry for cleaning the pipes, are located at
every major sewer pipe junction, and are capped with round manhole
covers. The reason for the circular construction of these covers
is, quite simply, that covers of any other shape would fall through
the manholes by virtue of their varying diameters. Circular manhole
covers do not vary in width, or in diameter, as is the case with
these other shapes, thus remaining in place despite the street
traffic running roughshod over them.
Manufacturers craft the manhole covers together with the smaller
sized lip, upon which they rest, at the same time to ensure a
tightly sealed fit. Additionally, the manufacture of circular
manhole covers is easier and more accurate than the manufacture of
covers of any other shape. Lastly, round manhole covers, once
removed, require less lifting and less manpower, as their shape
allows them to be rolled.
You can turn a square manhole cover sideways and drop it down the
diagonal of the manhole. You cannot drop a round manhole cover down
the manhole. Therefore, round manhole covers are safer and more
practical than square ones.
taken from coolquiz and discoveryschool.com
