Sewage systems have been around almost since the dawn of civilization. The ancient Roman Empire built aqueducts to transport not only water to the city, but waste away from it. Once mankind began burying their pipes to carry sewage, they needed a way to access these lines, and these access points came to be known as manholes.
Despite how long sewer lines have been around, manholes were not the first access points for workers. Designers knew that a new underground system would need a way to access the pipes. To fulfill this need, they added points along the plumbing known as lamp-holes. These holes were between 10 and 13 inches in diameter, depending on the materials used.
Due to the restricted size, the lamp-holes didn’t accommodate an average person. They were only good for shining a light down to verify the system was flowing normally. (For more on manholes, see A Look at Structural Manhole Rehabilitation.)
Realizing they needed a way to access the gravity sewer, engineers began adding man-sized holes or “manholes” to the sewage lines. It is common to see these gaps in the streets and sidewalks. They are covered by manhole covers to keep pedestrians from falling in or injuring themselves on the open holes.
In the early age of manholes in Europe, they decided that having the manholes on the streets and walkways was inconvenient. Instead, they built large structures to the side of the main sewage way and connected them through an underground passage. The Europeans discovered that these connections were not only expensive, but also a place where debris seemed to accumulate, making it difficult to access the underground lines.
The primary purpose of a manhole is to allow workers into the sewers. Like all infrastructures, drains need to be inspected, repaired and cleaned on a regular basis. However, without access workers would not be able to ensure the system is working correctly. The opening is 3 feet wide, which is large enough for an average size person, and they are included at regular intervals in most systems.
Manholes are also a key component in sewage system design. These service pipes are used to join different parts of sewers together. Engineers sometimes employ them to help change the direction of sewer flow. Due to their placement, they help to facilitate laying sewer pipes in convenient lengths.
While inspection, cleaning and repairing sewage systems is essential, it is not the sole function of a manhole. A secondary purpose is to allow the sewer to “breathe.” Without proper ventilation through manholes, the odor would back up into homes and businesses. Without ventilation, the air inside becomes toxic to workers who may need to access the sewer for maintenance. (To learn more about sewers, see The Complex World of Sewer Networks.)
Manholes are classified based on the depth of the access point. If the extent is only 2 to 3 feet down, it is known as a shallow manhole. These are found at the beginning of a sewer branch and not subject to heavy traffic.
A typical manhole has a depth of 4 to 5 feet. Most manholes are round, but they may be rectangular or square instead.
Finally, deep manholes have a depth greater than 5 feet. They have heavy covers and have a gradually increasing diameter as they descend.
What Are They Made From?
When manholes were first employed, workers used brick and mortar to create the entryway. Sometimes they would seal the outside of the brick with a plastic coating. The rim of the assembly was framed with an iron casting about eight to twelve inches deep. This lip allowed construction workers to add ventilated paving stones or bricks over the opening.
Modern manholes are typically made from concrete. However, there are some areas where a composite material, such as fiberglass, is more suitable. Generally, regions using composite liners have a high probability of corrosion from hydrogen sulfide or have seen higher incidents of exfiltration or infiltration into the line.
The rim and cover assemblies are often still made with cast iron. The covers fit down in the rim so that they are flush with the walkway or street. The heavy plug prevents unauthorized access while still offering ventilation.
Why Are Manhole Covers Round?
Manhole covers are the most noticeable part of the sewer system. A question often asked is, “Why make them round?” Even if the opening below becomes rectangular, the covers are still round. The reasoning is simple: With a square or rectangular shape, the cover may end up falling through the opening into the sewer below; however, with a round cap, there is no way to turn it so that it might fall through.
Not much has changed with manholes over the last century. These massive concrete or fiberglass portals serve as an access point for workers to help maintain the sewer systems. Some areas use advanced technology to survey the pipelines so that workers do not have to climb into the dark depths. However, the manholes still play an essential role in keeping the environment sanitary.