Hey perry62:
Don’t you think your continued use of the term “manhole” is a tad sexist and perhaps points to some latent homosapien tendencies?
From the article:
[q] the corners might tend to pop up when cars drove over them (I get the latter from my friend David Feldman’s book, When Do Fish Sleep?); [/q]
This shameless name-dropping just has to stop! 
Ah, Mr. Erlvis…
Perhaps you would prefer “workerhole” or “personorifice”? And keep my species out of this!
I posted this previously, but my thread seems to have been ignored. Did I break some etiquette ruke that I am not aware of? Anyway, here it is, form a sanitary services/sanitary maintenance engineer:
Manhole covers are often round as it stops them being accidentaly dropped down the manhole. Period.
Many round covers have a notch in one side that fits over a protruding piece in the rim, so the reason given for easy replacement does not stand up.
Covers in Europe sometimes consist of two right-angled triangles attached by a loose bolt in the middle. These 2 pieces together form a square. This stops rattling as traffic passes over loose covers, as teh triangle will sit firmly on a slightly uneven rim, unlike a square or circle, which will rock in the same situation.
For anyone who has tried, a full-strength cover, intended for use in heavy traffic situations, is quite heavy and it is easy to drop down the manhole. Nothing to do with being a Sanitation moron, rather to do with being a lightly-muscled engineer!
I’m with Richard Feynman. Manhole covers are not round.
I live in the London suburbs, and work in Docklands. I have a 5-10 minute walk to the station at each end of my commute. I must pass about 50 inspection covers of one kind or another on that walk - I will count them as I go home. I don’t know what the definition of a manhole cover is in this discussion - most of these will be electric services of one kind or another - but a fair proportion must be drains and sewers.
I don’t think there is a round cover over any of the holes. The vast majority are rectangular. Many of the heavy-duty ones in the road are twin triangles - probably to cut down on the weight that must be lifted.
London engineers have a reasonably long experience with sewers - I know we still operate some Roman ones in this country - so they have had time to try out a lot of options, and we don’t get many sewers collapsing.
Watch this space for experimental evidence!
erlvis and perry62
Stockton’s Public Works Department calls them Maintenance Holes (MH), if that makes you feel any better. At least that’s how it’s written on the plans and in the specifications. The word Maintenance is often contracted to Ma’n’ when read out by the engineers.
Someone is taking the piss. 'Nuff said.
Right. Some actual data.
On a 300yd walk along a main road in London’s Docklands I counted 109 major inspection covers. The area is full of offices - most were probably telecommunications. The ones on the pavement were mainly rectangular concrete - those on the road were almost all the ‘twin triangle’ design. This is a square hole, about 2ft x 2ft, covered by two triangular iron heavy-duty covers, capable of taking continuous heavy lorry traffic. Sometimes several are merged together, covering a hole perhaps 2ft x 6ft.
Out in the suburbs, I took a four-mile bus ride home, and counted 227 inspection covers in the road (I ignored the pavement). Almost all were the twin triangle design. A few were just square, one was a large metal circular base with a square hatch in the centre, and two were oddly shaped pentagons. There were no circular ones at all.
What I think is happening is that the discussion above is about 20 years out-of-date. Modern manholes in a big city street need to be very strong to resist heavy articulated lorries. If such a cover was a single piece of metal it would be too heavy to move - so the manholes are made in triangular sections. Those on the pavement can be lighter.
They are usually rectangular, because that is the shape of the hole. A circular hole may be the ideal shape for strength, but modern materials and techniques are strong enough for this not to matter. If a cast concrete lining is used it may be circular, but I suspect that is more to save material than to gain strength. Most of the holes I have seen are brick lined, because it is easier to carry bricks to a hole than a big concrete liner, and you don’t have to wait for them to be delivered. If you are using bricks, a square or rectangular hole is the easiest to make - you would have to be a very skilled brickie to do a perfect circle!
The very small inspection hatches I saw - 6 inch to 9 inch - tended to be circular. They were probably lined with a plastic pipe, which would have been round.
So, yes, the English do have triangular manhole covers, and either any benefits from having circular ones are very small, or the ability to lift the manhole in sections outweighs all other considerations!
109 covers in 300 yards? Wow, that’s a lot of holes!
“109 covers in 300 yards? Wow, that’s a lot of holes!”
Yes, and that was just major ones - man sized. I estimated that you could not draw a circle bigger that about 10 ft radius anywhere on that road without including an inspection cover. But we are talking about a major world financial centre here - there are a lot of services to provide.
There’s one problem with any manhole cover-- designing one that’s smaller than the hole it’s supposed to cover.
My dad troubleshoots mines, and one engineer one made this near-fatal flaw (well, nearly fatal for the chap who nearly fell in) recommending a 16" manhole cover for an 18" hole. No one noticed until they went to install the first one. CLANG!