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There is a whole culture fascinated with the history, artistry, and function of manhole covers一after all, that’s the part of the manhole we can easily see! But what about the mysterious manhole itself, down under the surface? What is its function, and how do you build one?
The function of manholes
Historically, manholes go back to around 500 BC, when the Etruscans excavated the first underground sewers in the city of Rome to siphon away excess water and sewage. Manholes as we know them have been around since the 19th century, giving workers access to sewage systems and allowing the system to “breathe” and ventilate without noxious fumes backing up into homes and buildings.
So they’re just for sewage, right?
From the time of the Roman aqueducts to the manholes of the 19th century, manholes provided access to underground sewage systems. But as technology advanced, we needed a place to put gas lines, phone lines, water lines, and more. Think what a mess it would be if those were on the surface! As city planners and builders were already used to putting sewage lines underground, it wasn’t much of a leap for them to begin placing other utility lines underground, all accessed through the cover of a manhole.
More than a hole in the ground
When sewer systems and other utility systems are designed and mapped, industrial water professionals and engineers determine manhole placement and the number of manholes to provide adequate access to underground utilities. Manholes are much more than holes in the ground protected by an interesting cover. Maintenance workers have to use these manholes regularly, which means they need to be built for safety as well as accessibility. For that reason, manholes are formed structures made of materials that ensure the integrity of the chamber in ways that provide safe accommodation for maintenance workers and the work they do.
The parts of a manhole
From top to bottom, the components of a manhole are:
- Cover - the round, square, or rectangular cover that protects the opening to the manhole.
- Frame - the frame of the manhole cover.
- Rings or Risers - tructures that adjust to match the pavement grade.
- Cone Top - the tapered opening to the manhole beneath the cover leading to the wall or barrel.
- Wall or Barrel - the main cylinder that forms the manhole, usually about 4 feet in diameter.
- Steps - horizontal structures that allow workers to safely climb down into the manhole.
- Bench - a floor at the bottom of the manhole that directs flow into the channel and provides a surface on which maintenance workers can stand.
- Channel - an opening at the bottom of the manhole that allows waste to flow into the system.
The process of building a manhole
Here’s how manholes are built, step by step:
As you probably guessed, building a manhole starts with digging a hole. The hole should be large enough to hold the body of the manhole and provide a safe environment for the workers installing the manhole. For this step, workers should be familiar with local construction codes for open pit construction.
Pouring the slab
Once the site is excavated, the workers install a concrete foundation on which the manhole will rest if it is pre-constructed. The foundation can be poured on-site or, if line downtime is an issue, it can be lowered into the manhole after being poured off-site. The foundation should be level and have a smooth, troweled surface. If the surrounding soil is loose, the foundation may need to be constructed with that in mind.
Setting the manhole
When the slab is poured and cured, the manhole can be set or built. Manholes can be pre-constructed or built in place.
Kicking it old-school: Manholes built in place
Historically, manholes were built in place, although technological advances and ease of installation have made pre-constructed manholes more popular. Manholes constructed in place may be from poured concrete or built with bricks and mortar.
For brick manholes, shallower manholes were one brick thick, but deeper ones sometimes needed a thickness of a brick and a half or two bricks. Cement mortar was used to plaster the manhole on the inside, and was also used to channel moisture to the outlet at the bottom. If surrounding soil was often wet, plaster might be used on the outside of the manhole as well to protect from moisture. Once the plaster dried, the inside of the manhole was sealed to waterproof it.
Concrete manholes poured in place use forms to control the shape of the concrete pour. The “pour-in-place” approach requires an extensive drying time to ensure the concrete is stable and can resist moisture. The installation can be delayed by weather; rain slows drying time, wind can blow debris into the forms, and extreme temperatures can affect pouring and hardening.
Concrete manholes poured in place may initially be less expensive than pre-constructed manholes, but most contractors and engineers are choosing pre-constructed manholes for their convenience and to save time.
Manholes can be pre-constructed of plastic, concrete, or fiberglass. They can be built and stored off-site until the time comes to install them.
To install pre-constructed manholes, first the site is excavated and the foundation poured. Once the slab is poured and cured the pre-constructed manhole can be set. Usually the precast manhole is set on some type of bedding, such as a foam pad for fiberglass.
Slow, smooth movements with guide lines will lower the manhole into place safely. It’s important to use guide lines that won’t damage the manhole form一for example, fabric slings rather than cables or chains when installing a fiberglass manhole.
Securing the pre-constructed manhole
Once the manhole is set on the slab it can be secured into place. Wedge style, preset, or cast anchor bolts may be placed and adjusted to get the manhole into position and secure it there.
Make the piping connections
When the constructed or pre-constructed manhole is set, piping connections are secured.
With the manhole set and secured, the excavation can be backfilled to provide stability. Backfill, such as sand, pea gravel, or crushed stone, is placed between the outside of the manhole and the soil. Fill is applied starting at the lowest point, under the piping, and then extended to the outside of the barrel.
And that’s it! You started with a hole in the ground, and through a series of steps you now have a stable, secure manhole to allow access to utilities. Fit the cover into place to protect the manhole from unwanted access or from gathering debris, and you’re ready to walk away. Your trusty manhole will be doing its job, out of sight and generally out of mind, until workers need to go below-ground to perform repairs, expansion, or maintenance.