Why are manhole's round?

So the covers can't fall into the hole. Any other shape could.

The manholes are round because is the only form that prevents the cover from falling down through the manhole.

They are round so that the very heavy steel manhole covers can be rolled out of the way, instead of having to be lifted.

Shape has nothing whatsoever to do with the cover "falling into the hole". There is a steel lip about 1 1/2" wide that supports the outer edge of the manhole cover, and that lip can be fabricated in any shape because it is made by welding steel angles together. One lip of the angle is vertical and forms the outer edge of the manway hole, and the other leg is horizontal and forms the lip that the cover sits on.

The steel angles are anchored into the concrete form that all manway entrances are made from.

Manholes and their covers are only the entrance to a large concrete vault which is usually rectangular in shape anyway , so having to fit certain pipe sizes has nothing to do with it either. The actual concrete vault buried beneath the street that the manway provides access to is much larger in size than the manway opening. It may be 10 feet square.

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 constantly over them.

A circular manhole is the only shape in which the cover cannot fall down into the hole under any circumstances.

Because a round disk of a smaller diameter will not pass a round disk of larger diameter. If they were triangular this would also be true, but the round hole is much more easily passed through by a human. It also adapts much more easily to the standardized sizing of pipe. If the disk were any other shape it could theoretically fit through a shape of a smaller diameter. Of course there are limits to size and thickness for this to be true.

They are not all round. The answer is mainly cost.

A circle is gives the largest area for a given perimeter.

Round ones are normally fitted where the chamber is small and access is restricted.

i.e. Personnel are not expected to enter the chamber.

But I expect there are exceptions to this.

The fact that round covers cannot fall down the hole would certainly have been considered.

The "doesnt fall in" seems good but how do you explain the square ones and the square ones made up of two triangles?

Next time you see somebody go down one of those, take a good look at the gut some of these guys are slogging around with. Mystery solved. You're disqualified as a candidate for employment if you have a regular build!

So they don't fall in the manhole.

I understand that Boston or some other city in Mass. uses an octagonal shape (that also "doesn't fall in the hole")

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