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There is no immutable law demanding that manhole covers have to be round—so why are they? While some sewer covers are square or rectangular, these types are rare and are generally only found in cases where there is a need for heavy equipment to go through them. The answer to why manhole covers are round is relatively straightforward but requires an understanding of their purpose and history.
Where Does the Name “Manhole” Come From?
What even is a manhole, exactly? It’s just as the name would suggest—no funny business here. Manholes emerged in the mid-nineteenth century as a way to access sewer systems, which had begun to be built underground to accommodate the growth in sewer systems. Originally, the access points to the sewer system were only large enough to shine a light through the hole to see if the sewage was flowing as it should. Eventually, though, humans needed to be able to enter the sewer system.
So, simply put, the term “manhole” is short for a man-sized hole.
Why Are Manhole Covers Round?
So what exactly is the reason why manhole covers are round and not square in today’s world? Is it simply because they always have been round? Is there some complex manufacturing reason that round manhole covers prevailed against their less popular square counterparts? Did the original inventor of manhole covers have some sort of grudge against right angles?
If you’ve ever thought to yourself that there has to be some logic to it, your hunch is correct. After all, it was engineers that were responsible for the original design! There are actually quite a few good reasons why sewer covers are circular, all of which have to do with structure, safety, and efficiency.
Firstly, there is the geometric fact that a round manhole cover cannot fall through its own opening. For example, if you have a square cover, the diagonal distance across the length of the square is 1.414 times the length of the sides. This means that if you were to drop a square cover onto its opening at a diagonal angle, it could easily fall through. For more visual thinkers, envision a children’s toy that has a series of shapes like triangles, rectangles, squares, and circles, as well as a bucket that has the same shapes punched out at the top. The square shape could fit into the square box not only when lined up with the corners but also when tilted. The circle shape, however, has to be lined up around the edges in order to fit.
Bonus shape: All the geometry-inclined readers out there might be quick to point out that, in fact, a circle is not the only shape that would solve this problem. A particular type of triangle called a Reuleaux Triangle also fits the criteria and would be just as suitable for stopping its cover from falling in.
Depending on the size of the hole being covered and the material from which the cover is constructed, manhole covers may be heavy and difficult to remove. A round cover can be rolled away relatively easily. This is where the Reuleaux triangle fails its usability test—it can’t roll with the same ease as a circular shape.
When you want to replace the cover of a manhole, there is no need to worry about the alignment of a round cover—it can go on at any angle, unlike covers of other shapes. Another factor in favor of circular manhole covers is that they are the most likely to drop back into position if they are bounced off by the forces of a heavy vehicle passing by.
Believe it or not, there’s also an economic reason that manhole covers are round. There is less surface area on a circular sewer cover than there is on an equivalent-sized square cover, which means that the circular cover also requires less material to manufacture.
5. More Geometry
Another geometrical reason why manhole covers are round is that circular coverings offer the strongest shape, especially because the cover only rests on a shallow lip around the perimeter of the hole. Each point of its edge is the same distance from the center of the cover. In many ways, each of these points has the same strength as the corners of a square or rectangular cover.
A cylinder is the strongest shape for a hole because it holds against the compression of the earth surrounding it. Thus, it is easier to dig and maintain round holes. However, not all manhole covers lead directly down to round holes—they often lead to a larger cavern. Many underground structures are square or rectangular because they meet up with an intersection of pipes, which makes it easier to have straight, flat sides rather than circular ones. Despite this singular case for a square or rectangular sewer cover, the other benefits of a circular manhole cover far outweigh this argument to the contrary.
It’s not just tradition that dictates why manholes are round—there are many practical reasons why manufacturers produce round sewer covers. Next time you find yourself staring down at the sidewalk in your neighborhood, take note of the manhole covers. You now have five reasons to appreciate their circular shape!