* A round manhole cover cannot fall through its circular opening, whereas a square manhole cover might fall in if it were inserted diagonally in the hole. 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. (A Reuleaux triangle or other curve of constant width would also serve this purpose, but round covers are much easier to manufacture.)
* Round tubes are the strongest and most material-efficient shape against the compression of the earth around them.
* A round manhole cover of a given diameter has a smaller surface area than a square cover of the same width, thus less material is needed to cast the manhole cover, meaning lower cost.
* 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 with the manhole.
* A round manhole cover can be more easily moved by being rolled.
* A round manhole cover can be easily locked in place with a quarter turn (as is done in countries like France), which makes them hard to open without a special tool. Lockable covers do not have to be made as heavy, because traffic passing over them cannot lift them up by suction.
15 years ago I was interviewing for an engineering job and the VP of engineering at a company asked this question. I happened to have heard this question previously so spouted off a bunch of answers and impressed the heck out of the VP. And that is the real problem with this kind of questions. They largely depend on if you heard the question before or not.
I got into Oxford like that. Basically the interviews are a bunch of interesting questions like "How could I take the log of -1?" or "How is e defined?".
If you already know the answer, your main problem is acting like you're thinking it through.
Yeah for both Oxbridge unis the grades get you the interview but they could just fill the whole uni several times over with kids who got top grades.
So they reckon a 45 min quiz/chat is what will tell them who to admit.
Gets pretty open ended, favors people who can talk well.
I ended up not applying to MIT because you get your acceptance before Christmas and the US applications are huge. But my understanding is the US school interviews are more cultural fit?
Don't forget also: round manhole covers are a Schelling point.
Making a manhole cover round is one of the first intuitive shapes you'd think to make them (along with squares and rectangles); and so you'd guess, as a construction company gearing up to sell the concept of concrete manholes with removable iron-plate covers to a municipality (back in the 1800s or whenever we first got them) that what the municipality will ask you to make is round manhole covers. So you provisionally set up your tooling for round manhole covers, and build a few for the demo. And the municipality assumes you're the expert about good manhole-cover shapes, and so just goes with the flow, rather than trying to figure out the best shape themselves. So you end up with "the first intuitive thing that worked" spreading via network/"system compatibility" effects.
More like, why we got four-way intersections for so long before anyone tried roundabouts. It was intuitive to just drive one road through another, and then we added traffic laws and signalling on top to try to make it work, rather than anyone sitting down at the start to try to work out what intersection shape would optimize for traffic flow and [non-]accident rate.
It’s actually several things. It’s stronger, cheaper, rolls, lighter, no need for alignment etc. You can make sure any shape does not fall in, but strength to weight ratio is important as you want people to open them with hand tools while supporting dump trucks.
PS: At ~250 lb you can see why a less optimal shape might be a problem.
Less raw materials to produce, easier to transport by rolling, easier to place onto the manhole, easier to stack when transporting/storing in bulk, etc
