What is a manhole?
A manhole is a masonry or R.C.C. chamber constructed on a sewer line or drains in order to enable a man to enter or leave the sewer. The openings of the manhole are fitted with a suitable cover called a manhole cover.
If the manhole covers are perforated, they may assist in ventilating the sewers. The manholes are constructed with different materials in different shapes according to requirements.
Objectives of manhole
- The objectives of manholes are inspection cleaning and removal of obstructions from the sewer line.
- It allows the escape of considerable gases through perforated cover and thus helps in the ventilation of sewage.
- Manhole allows changing the direction of sewer or joining of sewer or alignment of the sewer.
Location of manhole
- Manholes are provided at every bend, junction, change in gradient, or change in diameter.
- The sewer line between two manholes is normally laid straight with an even gradient. Even if the sewer line runs straight then, manholes are provided at regular intervals.
- If the diameter of the sewer is larger, the spacing will be greater.
Components of manhole
a) Access shaft
- It is the upper portion of the manhole.
- Provides access to the working chamber
- Dimension: 0.6 m x 0.75 m for rectangular manhole and 0.6 m for circular manhole
b) Working chamber
- It is the lower portion of the manhole
- Provides working space to carry out maintenance works.
- Dimension: 0.9 m x 1.2 m for rectangular manhole and 1.2 m diameter for circular manhole.
c) Bottom
- The bottom portion is known as benching.
- It is constructed of cement concrete connected to a semi-circular or U-shaped main channel.
d) Side walls
- Side walls are made up of R.C.C. or masonry.
- It resists the earth’s pressure.
e) Steps or ladders
Steps are made up of cast iron placed at a distance (horizontal: 20 cm c/c, vertical: 30 cm c/c)
f) Top cover (Manholes cover)
- A heavy cast iron or R.C.C. structure that is used to cover a manhole is called a top cover.
- It is used for safety and prevents the spreading of foul odor.
Why are manhole covers round?
Manhole covers are round because of the following reasons;
- Mishandling the Manhole cover:- A round manhole cover can’t fall through its circular opening, whereas a square cover could fall in it if inserted diagonally in the hole.
- Rolling the Manhole Cover:- A round manhole cover is well moved and rolled.
- The strongest shape:- When a square or rectangular manhole cover is subjected to sudden vertical compression the weakest spot, the center, isn’t equidistant to all supporting sides, especially the corners, which give support from both directions and thus are the strongest points.
- With a circular cover, the center is equal in distance from all supporting sides,& due to the gradual curve, any point along the edge gives much support as a corner on a square cover.
- It is easier to dig a circular hole.
- Round shape manhole castings are easy to manufacture.
How much does a manhole cover weight?
The weight of the manhole cover depends on the density of materials used to make the manhole cover. By the way, most of the manhole covers are made up of Reinforced concrete cement and Iron.
The density of iron is 7800 kg/meter cube while the density of RCC can be taken as 2500 kg/meter cube.
Weight of manhole cover = Density x volume of Cover
The volume of cover depends on the thickness and shape of the manhole. i.e Volume is equal to area multiplied by its thickness.
Area of square-shaped manholes cover = l x l
Area of rectangular shape manholes cover = l x b
Area of Circular shape manholes cover = π r2 (Where r is the radius of manholes)
Now, Lets us calculate the weight of the circular iron manhole cover having a diameter of 1 meter. The thickness is 25 mm.
We have,
Weight = volume x density
Weight = Area x thickness x density
Weight = π r2 x (25/1000) x 7800
Weight = 3.14 x (1/2)2 x 0.025 x 7800
Weight = 153.075 Kg.
How to calculate Cast iron manhole cover weight?
Again, Lets us calculate the weight of the circular iron manhole cover having a diameter of 1 meter. The thickness is 25 mm.
We have,
Weight = volume x density
Weight = Area x thickness x density
Weight = π r2 x (25/1000) x 2500
Weight = 3.14 x (1/2)2 x 0.025 x 2500
Weight = 49 Kg.
Hence, in conclusion, the Iron manhole cover is approx. 3 times heavier than the RCC manhole cover.
Types of manhole
There are mainly three types of manholes depending on the depth. They are:
- Shallow Manhole
- Normal Manhole
- Deep Manhole
a) Shallow Manhole
In this manhole, the depth ranges between (75-90) cm. Shallow manholes are constructed either at the start of the branch sewer or in an area where there is no more traffic. These types of a manhole are suitable for maintenance work and are also called inspection chambers.
b) Normal Manhole
These types of manholes are about 150 cm deep. They are usually found in the sewer line. Generally, they are available in square or rectangular shapes.
c) Deep Manhole
If the depth of the manhole is greater than 150 cm and fitted with a very hard cover at its top is known as a deep manhole. Also, the depth of the manhole is increased same as the facility for going down is increased.
What is a Doghouse manhole?
Dog house manholes are those whose top shape is round and made such that it looks like the house of the dog. A small opening is placed at the curved surface area of the manhole as shown in the figure below.
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