Why are manholes round?

Philippines
February 20, 2007 6:21am CST
I couldn't believe it when I met someone who has just been in a job interview and was actually asked why are manholes round? He wasn't even applying for a construction firm to be asked such a question. On the other hand, this actually made me very curious as to whether there is actually a logical explanation to this. I myself haven't seen a manhole that is not round, not at least here in our country. There are only two somehow logical answers I could think of. First, it's probably easier to make holes that are circular in shape than those with edges, so our manholes are always round. And second, it's probably because of the usual perception of people that holes are always associated as something round in shape since we seldom holes that have different shapes, like bottle holes for example. So, what do you think? Do you think that there's actually a purpose for the round or circular shape of a manhole? I'm not just looking for logical explanations but bizarre ones are also accepted here. Wouldn't you find it more interesting if manholes have different shapes like rectangle or hexagon or any weird shapes that you could think of? heheh!!
7 people like this
24 responses
• India
20 Feb 07
i think there is a very simple and scientific explanation to it. a man's cross section is round. so naturally the most fitting shape to slide through would be round. an other shape would leave a lot of wasted area at the corners! try fitting a round shape into a square shape - you can see a lot of wasted space at the corners! so any other shape than a round shape would leave a much larger hole than is necessary - making the defect in the pavement larger and thus the pavement can be theoretically weaker in strength. in addition, a larger hole means a larger cover for the manhole, which translates into a heavier cover - more difficult to handle manually! also, a round (or oval) opening has less chances of sharp or pointed edges in either the opening or the cover, whereas any other shape, which has corners, would have pointed edges - potential of hurting unsuspecting people or workers. there might be more reasons, but i could think of these few only. the most convincing is, in my opinion, the fact that the shape of the human torso in cross section is round!!
4 people like this
• Philippines
20 Feb 07
hmmm...i haven't actually thought of it that way. it was great to hear your explanation! tnx!
@sedel1027 (17855)
• United States
20 Feb 07
One reason is because the manhole cover would not be able to be accidentally dropped into the hole. A logical asnwer is that the manhole is around, so the cover is round. The reason your friend was asked this question is because the company want to know how he resolved an issue that has multiple solutions.
3 people like this
@sedel1027 (17855)
• United States
20 Feb 07
No there are only a handful of geometric shapes that could not fall through. Another shape that would work is the rouleaux triangle. Almost all other shapes if positioned incorrectly would fall into the hole. You need something that has a constant diameter shape.
1 person likes this
@sigma77 (5385)
• United States
20 Feb 07
A square or rectangle manhole cover could be turned diagonally and dropped through the hole. This is true of any size square. Simple geometry proves the fact. Then again, if the inner lip that the cover sets in was wide enough, the cover might not be able to fit through.
1 person likes this
• India
20 Feb 07
There is a logical mathematical expalnation to that: A man hole is round bcoz its the only geometrical shape which will not fall inside itself. Say for ex:if it was aquare it will fall inside through the diagnal,if its rectangulat it will fall inside through its longer side.
2 people like this
• United States
20 Feb 07
That is so true. I was asked this question once in a job interview as well. I had no idea how to respond! Now I will have an answer for next time. LOL
@sigma77 (5385)
• United States
20 Feb 07
I have never worked with manholes. The reason they are round is so they can't fall into the hole they are covering. If they were a different shape, the could be twisted and turned to fit through the hole. If someone was working down there in the system, a manhole cover could accidently fall on them. It also prevent pranksters from dropping them into the holes. It could take thousands of man-hours to recover them, a waste of manpower. The shape is the way it is for practical reasons.
1 person likes this
• Philippines
21 Feb 07
yeah i agree with you hihihi!
• Philippines
22 Feb 07
same here. tnx!
@nowment (1758)
• United States
20 Feb 07
This question was first asked by someone at microsoft to access how a person would approach a question with more than one answer. 1 Manhole covers are round because man holes are round 2 A round manhole cover can not fall through whereas most other shapes such as square may have this problem 3 Round tubes are strongest and most material efficiant shape against the compression of the earth 4 Cylindrical holes are easier to dig. 5 Human beings have a roughly circular cross section 6 A round manhole can more easily be moved by being rolled. 7 If manhole cover is displaced one with sharp edges of corners could puncture a tire driving over it, a round one doesn't have those edges. Other shapes can be found but over all they are being phased out.
1 person likes this
• Philippines
20 Feb 07
wow! it's like you summarized all their answers here! great post! tnx!
@nowment (1758)
• United States
28 Jul 07
Thanks for the best response, I had been curious about things that crop up and lol those who know me, know that means I am going to check into it find out what I can on the topic. When a friend asked this question I thought about it and said ok there are reasons, but basically who created the first man hole and what shape was used and why? the original why may be never know other than an educated guess. It is not like we can go back and ask. But there does seem to be some very interesting reasons.
20 Feb 07
Everyone is kind of going to say the sme thing but my first thought on why man hole covers are round is because they can fit on anyway. The last thing you want is one that isn't on right or difficult to slot back on. Theere is no corners you have to mathc up. Over it the UK I think we only have big square ones and big triangles ones. But its like an isosceles triangle. ill keep an eye out anyway.
1 person likes this
• Philippines
20 Feb 07
Cool! At least you have different shaped manholes unlike here in the Phil. We only have circular ones (or at least I haven't seen one that isn't round). hehe!
1 person likes this
@WebMann (4732)
20 Feb 07
I would have said manholes are round because then you don't have corners and the covers never need to be aligned to corners, much easier to put back on. If you had a square hole you would always have to line up corners.
• Philippines
20 Feb 07
couldn't agree more! it is definitely much more convenient to cover up round holes.
• India
20 Feb 07
actually they have been made round cause a circle is the one which gives you maximum surface area with a fixed perimeter , as the strength of manhole is affected by the holes which you make , so want maximum surface area for minimum perimeter.
1 person likes this
• Philippines
20 Feb 07
great mathematical perspective! tnx!
@nvtellan (1907)
• Philippines
23 Feb 07
Wow! I learned a bunch of things today with this post. Kudos to chrmdarcher!
1 person likes this
• Philippines
22 Feb 07
hmm... your logical answers is same as mine... holes = circular or round in shape... but ive send man holes that have edges... such retangular in shape...O_o
1 person likes this
• United Arab Emirates
22 Feb 07
The reason is quite simple: * For the construction:- Covers of any other shape would fall through the manholes by virtue of their varying diameters in the opposite circular manhole covers don't vary in width,or in diameter (only one standard diameter). * For the manufacture:- the manufacture of circular shape is easier and more accurate than any other shape. * For the maintenance:- Round manholes requires less lifting and less manpower, as their shape allows them to be rolled when there is a maintaenance process for the sanitary network. At last, I can tell you as I guess why this question is always been asked in the interviews: because you see these round manholes in the steet everyday so the question is did you asked yourself before why this is a default case in our life? (for manholes to be round in cover)
1 person likes this
• India
21 Feb 07
In my opinion manholes need not be circular,it can be of any shape like rectangular,square,hexagon and etc.I think the the interviewer has already selected somebody for the post and just playing with everybody,and thats why asking such silly questions.
1 person likes this
• Sri Lanka
21 Feb 07
I think many logical reasons were given in the previous threads. The best reason I think is the cover will never fall into the hold. It will happen in any other shape. Experiment with a paper if you like. In a job interview they don't always need the correct answer. What they want to know is how you face the problem. I was asked once the height of a building in coins. Now who can tell the answer. I said, the I can hold 50 coins in my palm, my body is about 10 palms and I am 500 coins high. In the building one floor is 2 people high so one floor is 1000 coins. It has 30 storeys and the height in coins will be 30000. The answer may not be correct, but I faced it.
1 person likes this
@brians (679)
• India
21 Feb 07
there might be some logic behind it thats why he asked him the question.. well the answer is 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.
1 person likes this
@pinknic (307)
21 Feb 07
I know the answer! Manholes are round so that the manhole cover won't fall back into the hole and crush the workers. Square or rectangular ones would if twisted at the wrong angle. The kind of question is becoming more popular and it's to test people's analytical skills.
1 person likes this
@hellboi (664)
• Philippines
21 Feb 07
I think it was Microsoft who generally asks this supposedly stupid questions. Stupid in a sense that you cannot see their connection to the job you are applying for. But the main reason why they ask these questions is to assess the person's ability to think out of the ordinary which to them is most important skill. As to you question of why manholes are round, it is the only shape possible where the manhole could not slip through the hole it is supposed to be covering.
1 person likes this
@TinWolf (184)
• United States
21 Feb 07
The answer simply is so that they cannot fall through the hole. This Q has been asked in other Q & A sites as well. Steven Wolf
1 person likes this