What Is A Manhole?
Manhole can be defined as an access point inside the sewer line for material and workers. It is the most important component of the sewer line because it completely eliminates the need for the digging process and it is also known as an Inspection chamber or Unit.
The main purpose of the construction of a manhole is trenchless rehabilitation for the sewer system, an inspection of a drainage system, cleaning the clogged line, and maintenance purpose, also it an initial step for entering the inside sewer line and finding out a problem with it and it also helps to replace the damaged pipe without need of digging work.
Manholes are placed throughout the sewer line at regular intervals till the end of the main sewer line or drainage point.
Generally, the manhole is consisting of a vertical circular chamber of different sizes and depths which is used to reach the inspection point.
Manhole construction is done at that point where the change direction of the sewer line and the gradient of a sewer line. The manhole is typically installed 0.5 m away from the curb line of the road.
Also, the manhole covers are available in different size, material, and shape such as rectangular, circular, and square; it is made from metal, precast concrete, and composite material.
If manhole chamber depth is more than 2.5 m then a ladder must be fitted inside and if the depth is less than 1 m then a step ladder must be used.
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Purpose of Manhole
- For inspection purposes, maintenance purposes, and removal of any obstruction part present in the sewer line.
- For connecting sewer lines at a junction point.
- For changing the direction or alignment of a sewer line.
- For changing the gradient.
Types of Manhole
The following types of manholes are given below,
- Shallow Manhole
- Deep Manhole
- Normal Manhole
A shallow manhole is constructed at the start of a branch sewer line where there is no heavy traffic and it has a depth range from 75 to 90 cm.
A Normal Manhole has a depth of 150 cm and generally, it is square in shape with a heavy cover on its top surface.
A deep manhole is provided at greater than 150 cm with a heavy cover on its top surface. Also, the depth of the manhole is increased same as the facility for going down is increased.
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Component of Manhole
The following are major manhole parts,
- Access Shaft
- Working Chamber
- Base and Sidewall
- Bottom or Invert
- Ladder or Steps
- Cover and Frame
An access shaft is a vertical passage and upper portion of the manhole which provides access to the working chamber. The minimum size of the access shaft for a rectangular manhole is about 0.75m x 0.60m and for the circular shape manhole 0.70m in diameter.
In some cases, the access shaft may be covered by a reinforced cement concrete slab with a suitable dimension
A working chamber means a lower portion of the manhole which is provided working space for cleaning and inspection of the sewer line.
The minimum size of the rectangular chamber is 1.2m x 0.9m with the flow provided larger side. The minimum size of the Circular chamber is 1.2m and the height of the working chamber should be not less than 1.8m.
Generally, the size of the working chamber is larger than the access shaft. So the working chamber is constructed by enlarging the bottom area of the access shaft.
Base and Side Walls
Generally, the base of the manhole is made of plain cement concrete and it is placed at the base level, for supporting the sidewall of the ma hole and also it is preventing the entry of groundwater.
|Minimum thickness of bed of concrete||Depth of manhole|
|15 cm||Up to 0.8 m|
|23 cm||Above 0.8 m to 2.10m|
|30 cm||Above 2.10 m|
The manhole side walls are made of stone masonry, brick masonry, and reinforced cement concrete. But commonly used brick masonry as a sidewall.
|Minimum Thickness of Manhole||Depth of Manhole|
|20cm||up to 1.5m|
|30cm||more than 1.5m|
The following rule may be used for determining the thickness of brick walls,
t = 10 + 4d……. (1)
t = Thickness of wall in cm
d = Depth of manhole in m
The outside brickwork and inside brickwork are plastered with the cement mortar 1:3 means 1 part of cement and 3 part of coarse sand and inside apply a smooth finished cement coat.
The thickness of reinforced cement concrete walls will be much less than compared brick walls. However, the R.C.C. manhole wall is costly and is used in special cases.
Bottom or Invert
The bottom portion of the manhole is semicircular or U-shaped. It is a cement concrete channel also the diameter of the concrete channel is equal to the sewer line.
The horizontal diameter is extended vertically nearly up to the crown point of the sewer or manhole, and their top edge is rounded off and made to slop towards the channel to form benching.
For benching, the slope is provided from 1 in 10 to 1 in 6. When two or more sewer lines are entered into a manhole at the same level as the bottom portion of a manhole, the main channel branch is constructed similarly with respect to the benching. At the junction point of the sewer line, easy curves are provided.
Steps or Ladder
Ladders are provided in that case when the depth of the manhole is more than 0.8 m. These steps are made of cast iron and placed inside the manhole; also the arrangement of a ladder is staggered type.
The distance between the two steps is 38cm center to center horizontally and vertically 30 cm center to center.
The width of all ladders is 15cm and the first ladder is placed 45cm below the cover of the manhole and the lowest ladder is not placed more than 30 cm above the benching level. these steps are sufficiently embedded in the chamber wall so that they do not overturn.
Cover and Frame
The top open portion of the manhole is close to the set of frame and cover, generally, the opening of the manhole is in a circular shape so that the cover of the manhole is a circular shape.
The standard size of the cover is 56cm in diameter and the depth is more than 0.9.
Both the frame and cover are made of cast iron, the frame is supporting the cover also the weight of the cover varies from 90 to 270 kg.
The heavy type cover and frame are adopted where heavy traffic load is to be borne and similarly the light type is adopted for light traffic load is to be born.
The frame is slightly embedded in the concrete surface on the top of the wall and the cover is placed in the groove which is provided inside the frame. To avoid becoming top surface being slippery makes the rough surface of the manhole cover.
Materials Used for Construction of Manhole
The following types of materials are used for the construction of manholes,
- Plastic Manhole
- Pre Cast Manhole
- Fiberglass Manhole
Plastic manholes are manufactured by using polyethylene material. It is environmentally friendly and sustainable and also does not ill affect the soil or the ground where it is placed.
The main advantage of using plastic manholes is they do not require frequent rehabilitation and maintenance.
Precast Concrete Manholes
Precast concrete manhole is a traditional method for the construction of a manhole. This method is easy to install and easy to assemble on the site. Also, its life span is high so it is a more popular type of manhole.
Fiberglass manhole is having additional features like a grinder channel, weirs, separation unit for stormwater, etc.
The fiberglass manhole is easy to handle and place. Its weight is much less than compared of a concrete manhole. Also, it is environmentally friendly and highly durable in nature.
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Types of Manhole Cover
The main aim of providing a manhole cover is it serves as a lid to close the manhole also that cover is removable .generally manhole cover is made of concrete, cast iron, and a combination of the two.
These materials are commonly used because it is inexpensive, more durable, and heavy enough to stay in place properly.
Due to modernization manhole covers are available in various materials like fiberglass, plastic, composite, etc.
Ductile Iron Cover
Ductile-type manhole covers are made of ductile iron and nodular iron it is a unique form of iron. This type of cover is more popular due to its strength, solidness, durability, and resilient features. A ductile manhole cover will not corrode as quickly as a cast-iron manhole cover.
Recessed Manhole Cover
Recessed manhole cover as the name suggests is placed in recessed areas suitable for laying paver block, cement, and a layer of screed for aesthetic appearance in a pedestrian way.
These types of manhole covers are more suitable for internal manholes such as workshops, hospitals, domestic, and garden apartments.
Solid Top Cover
The solid Top manhole cover is made of lighter galvanized material and is designed to be prominently visible on the floor.
What is a Manhole?
A manhole can be defined as an access point inside the sewer line for materials and workers. It is the most important component of the sewer line because it completely eliminates the need for the digging process and it is also known as an Inspection chamber or Unit.
What are the types of manholes?
The following types of manholes are given below,
What is the Purpose of a Manhole?
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