Manhole: Definition, Purpose, Types, Construction & Accessories

What is Manhole?

manholes

Manholes are chambers made of masonry or RCC, designed at convenient intervals around the sewage lines for entry to them.

Purposes of Manhole

  • These are used to perform sewage pipe inspection, washing, and remove the obstruction.
  • Manhole helps sewers to connect or change sewer path or sewer orientation, or both.
  • They require significant gases to escape via perforated cover and thus aid in sewage ventilation.
  • Promotes the easy duration construction of sewer lines.

Types of Manhole

  • Shallow manhole.
  • Normal Manhole.
  • Deep Manhole.

Shallow manhole: It is given 75-90cm (2′-3′) of shallow depth. It is given at the start of branch sewer or at a site not subject to heavy traffic. It is also named the inspection chamber with a light cover at its end.

Normal Manhole: It is supported at a depth of 150 cm in a sewage pipe, with a hardcover on top. It is normally of a square (or rectangular) form.

Deep Manhole: They are fitted with thick cover at the top at a distance greater than 150 cm. The scale is gradually expanded and downward facilities are given.

How to Construct a Manhole?

manholes

A manhole or chamber of inspection is an underground unit built to provide access to utilities such as a sewer system, drainage system, etc. Thus, underground utilities are inspected, modified, cleaned, and maintained with the help of a manhole.

For More Details: Civil engineering

Bed Concrete: A layer of 1:4:8 concrete is spread on which the manhole is to be constructed. For manholes of diameter up to 4.25 m and over 4.25 m, the required thickness of bed concrete shall be 20 cm and 30 cm respectively. In the case of loose dirt, special supports to protect the manhole shall be built.

Manholes below the depth of 4.25 m in ordinary soil shall be built using one and a half brick, and if the manhole is below 9.75 m, the walls of the manhole shall be built with two brick thicknesses. The decoration of the brick masonry on the exterior side shall be completed easily.

Brick Masonry: The basic density of bricks used in manholes building must be of Class 7.5N with 1:4 ratio cement mortar. The ratio of cement mortar shall be 1:3 for arched form and circular manholes (1 cement: 3 fine sand). The walls of the manhole shall be built using one brick thickness to a depth of 4.25 m.

Excavation: For manhole excavation, the measurements and rates defined in the design or drawings shall be as required. The excavation width shall provide the required workspace for the mason to function comfortably below the ground for usability.

Plaster and Pointing: The interior of the manhole brick masonry shall be plastered with a 1:3 cement mortar thickness of 12 mm. For the triangular or arched form of the manhole, the plastering shall be completely up to the cement mortar standard of 1:2.

Usually, the outside side of the manhole shall not be plastered, although, in the case of contaminated earth, the outside surface of the manhole shall also be plastered with a cement plaster of 12 mm thick.

Footrests: In the case of cleaning and maintenance, the footrests in the maintenance holes are provided for easy accessibility. Footrests in the manholes are given if the depth of the manhole reaches 0.8 m. The footrest must be constructed of PVC or of stainless steel. Every form of the footrest, whether PVC or mild steel, must be inserted 200 mm deep into the 1:3:6 concrete block measuring 200x200x100 mm.

  • The footrest shall be laterally spaced and vertically separated 40 cm apart.
  • Beyond the inner surface of the manhole walls, the footrest shall protect 100 mm.
  • The first footrest is positioned 450 mm underneath the cover of the manhole.
  • To shield it from corrosion the mild iron footrest shall be coated with coal tar.

Channel and Benching: Benching in manholes refers to the route built from cement mortar to guide the wastewater in a certain direction and avoid waste splashing, which in effect destroys the manhole.

  • The channels and benching shall be installed in a manhole using 1:2:4 ratio cement concrete.
  • The channel finish and benching shall be completed easily and neatly.

What is a Sewer Manhole?

manholes

A manhole or chamber of inspection is an underground structure built to provide access to services such as a sewage network, drainage system, etc. Thus underground utilities are inspected, modified, cleaned, and maintained with the help of a manhole.

What is benching or channelling in a manhole?

Benching in manholes refers to the route built from cement mortar to guide the wastewater in a certain direction and avoid waste splashing, which in effect destroys the manhole.

What is the type of brick used in the construction of manholes?

The basic density of bricks used in manholes building must be of Class 7.5N with a 1:4 ratio cement mortar. The ratio of cement mortar shall be 1:3 for arched form and circular manholes (1 cement: 3 fine sand).

What is the thickness of the walls in a manhole?

Manholes up to a depth of 4.25 m, the walls of the manhole shall be formed using one layer of cement. The manholes shall be built using one and a half bricks to the depth of 4.25 m, and if the manhole is below 9.75 m, the manhole walls shall be made with two brick thicknesses.

Manhole Accessories

Manhole Locks, Security Systems, and Barriers: Many cities around the world rely on bolts when using cast iron manholes to reduce manhole cover theft issues. However, whether you opt for a plastic manhole, fibreglass manhole, or composite manhole, it is also necessary to have a lock or protection mechanism in place to ensure that your manhole stays protected and secure in the case of a sewage or stormwater flood, theft, or even a truck tyre knocking it out. There are a number of solutions out there to ensure that you hold the manhole in place safely.

Manhole Maintenance and Rehabilitation: There are about 12 million sewer or stormwater manholes throughout the country, according to the Environmental Protection Authority. Typically these manholes correspond to the lengths of blocks within cities and suburbs, spaced from 100 to 500 feet apart anywhere. However, you may be surprised to find that the Public Works Magazine estimates that 80 percent of these nearly 12 million manholes require some level of maintenance or rehabilitation.

Common Manhole Maintenance Issues: Garbage or litter on the top of the door, in front of the manhole, or inside the manhole.  Test whether the construction of the manhole is “sitting flat on the riser rings or top slab and tightly fixed” ensuring that the system itself is safe and that all pipes are properly linked without holes. You need to make sure that no contaminants are found in the manhole (including fuel or oil) ensuring that the manhole cover is in place, safely secured and simple to pull out.

  • Ensure that the interior of the building is free of dead creatures, trees, roots, or sediments.
  • Test for some waste or debris inlet and drain drains.
  • Ensure there are no holes or defects to the manhole cover.

Manhole Covers: Functions, Materials, and Engineering

manholes

What is a Manhole Cover?

Chances are you walk frequently on a daily basis over manhole covers, particularly if you’re a resident of the area. Manhole covers are essentially the flexible covering that acts as a manhole cap. This cover serves many functions, the most important of which is to avoid pedestrians from slipping through such sometimes very deep points of entry.

Manhole covers often have the function of preventing unauthorized workers from entering manholes and maintaining sewage or stormwater services secure in addition. Manhole covers often serve as a drainage outlet to avoid the entrance of wastewater or stormwater gasses into homes, and as a simple entry point for sanitation, staff to enter sewage and stormwater.

Cast Iron Manhole Covers

Historically, manhole covers are crafted from cast iron, which is iron – molten and poured into a mould. Grey cast iron and ductile cast iron are preferred when it comes to manhole cover material since they are both economical and extremely durable. Grey cast iron consists of iron with alloys that contain carbon and silicon.

Manhole Cover Materials

Manhole coverings have historically been made of either cast iron, concrete, or even a mixture of the two. That is because, both are inexpensive, strong, and powerful enough to remain in place. However, development has contributed to a range of alternate manhole surface products, including carbon, fibreglass, and rubber, close to manholes. We’ve addressed several common styles of manhole coverings in this segment including cast iron manhole coverings, plastic manhole coverings, and composite manhole coverings.

Common Issues with Cast Iron Manhole Covers

A number of substitute products have risen in popularity in more recent years as it comes to producing manhole coverings. That is one of the other issues linked to manhole cast iron covers. Some of the iron cast manhole covers’ top drawbacks include scrap metal theft, weight-related injury to workers, and corrosion.

How Manholes Are Made?

Cast iron manhole covers are made in so-called foundries factories, with a casting method consisting of five basic steps:

  • Making the pattern
  • Preparing the mold
  • Melting and pouring
  • Cooling the metal
  • Finishing

Alternative Materials for Manufacturing Manhole Covers

manholes

Over recent years, a number of alternate manhole cover forms, including synthetic materials, fabrics, and fibreglass, have increased over common owing to the infrastructure and safety problems associated with cast iron manholes.

Do Not Contain Scrap Metal: For manhole fraud that due to scrap metal value of typical cast-iron models, several communities are searching for substitute materials which have no scrap value. Alternative products such as composites, fibreglass, and concrete manholes save costs by cracking back on vandalism and thus reducing the chance of damage from lost manhole covers to pedestrians and animals.

Keep Out Inflow and Storm Water: Although cast iron manhole coverings are designed to fit flexible, manhole coverings constructed from composites, plastics, and fibreglass appear to provide a more stable seal fit. That, in effect, prevents pollutants like wastewater and oil from going into our sewage or stormwater networks, and finally our local water sources.

Lightweight: One of the main issues with cast iron manhole coverings is their weight, which not only renders them bulky and hard to deal with but has also given rise to questions about workers’ health. Composite manholes will be up to one-third smaller than their equivalents in cast iron and can weigh between 100 and 200 pounds anywhere.

Corrosion Resistant: Another factor composites, fibreglass, and plastic manhole covers are common is that they don’t corrode like cast iron ones. Since sewage or stormwater systems are considered to be extremely corrosive due to the existence of hydrogen sulfide gasses in them, this is a crucial factor in the manufacture of a long-lasting solution that does not need regular repair or replacement. It is also possible to build concrete, fiberglass, and metal manholes to avoid erosion induced by saltwater in coastal areas, as well as salt used to maintain roads in colder parts of the world.

Final thoughts:

We have provided the basic definition, the major purpose of manholes along with the types and construction procedure. Along with the basic info, we’ve also covered the brief section regarding manhole covers, their functions and materials used. Hope this article helped you.

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