Manhole or maintenance hole covers have been around at least since Rome made sewer grates from stone. These removeable plates allow access to underground vaults or pipes with an opening large enough for someone to pass through.
Manhole cover sits on a metal base, with a smaller inset rim/recess which fits the cover. The base and cover are sometimes called “castings”, because they are usually made by a casting process, typically sand-casting techniques. To lift the plates from the base/casting a hook handle tool is inserted into “pick holes”.
Shapes – Circular, square, rectangular with the Circular form most common (Fig 1).
The US Foundry Rings and Covers is an excellent resource for this subject as they provide drawings with dimensions. You choose the cover type from one column and then select the ring from the other column. I just randomly picked out three versions of base and cover (Fig 2). In all these the cover measures 22-3/4″ diameter with the base varying according to end use. The recess is about 1/2″ to 3/4″ over the plate diameter with the outside diameter of the base ranging from 28″ to 36″. Speaking very generally, the one on the left would be used under a paved road while the one on the right inserted into something like a concrete storm drain. (My ultimate purpose here is to make 3D prints so ‘generally’ is usually fine)
The sizes of the manhole covers varies widely:
I was talking about this on my Deep River Facebook forum and found “stuff” .. going to copy and paste for a bit as I do more research ..
Sanitary vs. storm sewers
Manhole sizing recommendations :
yeah .. some are exactly 24″. I was researching and found this company, US Foundry that has literally a TON of manhole covers and rings complete with dimensions on their website. That have some 24″ models but most seem to be 22-3/4″ dia covers. I covered this some on my website – http://www.deepriverrailroad.com/…/infr…/manhole-covers/
Model AC is 34-1/8″
From this we see that the driving factor behind the manhole cover size is based on the diameter of the sewer line connected to the manhole. This link – Sewer-System-Design-Standards Sunrise Arizona Sewer Design Standards .. about half-way down the page says ..
So. Pipe size increases as more and more pipes connect to the system. What this means for the modeler then is to make a guess how much flow you would have at a location. The sewer pipe down a street with small businesses and/or homes will be fed by laterals. I would think the ~24″ manhole cover so ubiquitous would indicate a 8-12″ pipe hidden from view, the cover sitting on a 48″ manhole. If modeling an area where presumably there might be many connections running into the system you might think of a larger manhole cover . an industrial area perhaps.