Manholes are serious business.
Corolinth wrote:
[...] in the event that we have a square tube, we'll still want to use round lids for it because they're already available on the market. I mean, we could design a square lid, but there's this perfectly good round one sitting in the back of the truck already.
This part is true, and is largely the reason why rectangular storm drain vaults still have round access covers.
However...
Corolinth wrote:
Consider that you could still put a square hole on the end of a cylinder.
Actually, not really. By and large, manholes aren't cylinders. The main body of the manhole is cylindrical, but the upper most segment is a (usually) concentric cone. There is no "top" of the manhole into which you can cut any kind of shape. The rim of the manhole is nothing more than the chopped off tip of that cone. Transitioning from the manhole's circular cross-section to a square rim just doesn't make any sense at all. The shape would be really bizarre, and probably not structurally sound. You could probably manage with a fiberglass manhole, but most sanitary sewer manholes are concrete, and the rebar pattern would be like...M. C. Escher. Doing this back in ye olden dayes with brick sewers would be basically impossible.
If the manhole is some other shape, the lid shape will follow suit. For instance, the telecom industry is mostly using cylindrical fiberglass manholes these days, but prior to that, it was fairly common to use square manholes with square rims (no taper). The ones I've seen while surveying usually just have some kind of diamond plate square hatch set on top. I'm not sure how they keep water out.
Corolinth wrote:
It's generally easier to get stuff through a round aperture than through a square aperture. That's as true of people as it is of water.
Only if we can't predict the shape of the thing going through the opening. A square object does not fit more easily through a round hole than a square one. And for a square object, a square opening is the most efficient. The cross section of the human body is irregular, of course, but it's roughly elliptical. If we're going to make an ease-of-access argument, manhole lids should probably be elliptical. In the absence of an ellipse, it's not clear to me (without more data about the shape of the human body) whether a rectangle or a circle would be a better approximation.
Corolinth wrote:
It takes less metal to make a round hole, because the corners of the square are waste area.
Again, in the case of manholes, this is not how it works. You aren't cutting a hole out of a solid surface. You're chopping the end off a tapered tube. Material waste could only be a factor where you're carving the shape out of a solid surface. This
might be the case with rectangular drainage structures like I mentioned earlier, although I'm fairly sure the manway opening is cast along with the rest of the vault, not cut out of it later.
But even if it were, the manway opening is significantly smaller than the surface you're cutting into, so "waste area" wouldn't be at issue. The material efficiency would be purely a function of the total area of the opening; shape being irrelevant. Of course, a 26" diameter round opening would waste less than a 26" x 26" square opening. However, this is still largely irrelevant. The cost of the lid and the cost of a few cubic inches of concrete is a trivial component of the cost of a manhole or concrete vault. It's not something you would bother taking into consideration.