Overview

Fig 1

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).

US Foundry

Fig 2

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:

  • Model AC is 34-1/8″
  • Model AD is 37″ !!!!
  • Model AJ is 23-3/4″
  • Model AR is 24″
  • Model AS is 25-3/4″
  • Model AV is 27″
  • Model AZ is 38″

Intermission

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

There are two types of sewer systems: storm water systems and sanitary sewers. Storm water systems or drains carry rainwater, ground water and road runoff water to an open body of water, such as streams, lakes and oceans. Sanitary sewers, however, carry wastewater from residential and commercial buildings to a treatment plant where the contaminants in the water can be removed. [1]angieslist.com : Sewer and main drains

Manhole sizing recommendations :

Manholes are used for connecting two or more converging storm or sanitary sewers, permitting pipe size changes, accommodating abrupt changes in alignment or grade and allowing for direct surface flow interception. The largest impact on the size of these structures is the diameter and angle of entrance of the intersecting pipe. [2]precast.org : Manhole Sizing Recommendations Note: The size of the pipes depends on location and the size of the manhole cover depends on manhole size. It is all interrelated.

Manhole Spacing

Manholes are placed in gravity flow (storm and sanitary) sewer systems at changes in pipe size and slope, as a node where sewer lines join, and wherever single lines change direction. Additionally, there are general rules for manhole spacing in sewer systems based on pipe diameter. For sewer pipes of 15 inches or less, manhole spacing is typically 300-400 feet (O scale 6′ to 8′) or less. For larger pipe diameters, recommended manhole spacing increases to about 500 feet (O scale over 10′) minimum. The recommended distance may be further increased if the pipes between manholes are of large enough dimensions to allow a person to walk through. [3]buildpedia.com : Manholes and Structures Note: I just tossed in the spacing to give an idea how far apart the manhole access should be on a layout. HO would be half.

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/
For example they have a model AA with a 32-1/8″ dia cover
https://www.usfoundry.com/…/ringsandcovers/covertypes/287
If you look the cover types are down the left column and then the available ring choice on the right.
Manhole Covers Manhole here refers to the the pipe underground.

Model AC is 34-1/8″
Model AD is 37″ !!!!
Model AJ is 23-3/4″
Model AR is 24″
Model AS is 25-3/4″
Model AV is 27″
Model AZ is 38″

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 [4]Sunrise Arizona Sewer Design Standards .. about half-way down the page says ..
Pipe diameter 8-12 inches has a 48″ dia manhole with a 24″ manhole cover
Pipe diameter >= 15 inches has a 60″ dia manhole with a 30″ manhole cover

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.

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References

References
1 angieslist.com : Sewer and main drains
2 precast.org : Manhole Sizing Recommendations
3 buildpedia.com : Manholes and Structures
4 Sunrise Arizona Sewer Design Standards
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