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The main purpose, features, and types of a manhole are briefly explained in the article.
The main parts of a manhole are the chamber or ring and the Vertical Circular Pipe. The vertical circular pipe is available in varying depth and sizes. These pipes are used to access the inspection joints in the system.
Manholes are mainly positioned 0.5m away from the curb lines of the road. Mostly it is constructed such that it is away from the wheel line of the traffic.
The cover of a manhole is a plug that protects the manhole from any unauthorized access. The covers used for manholes can be either rectangular, square or circular in shape. The material of cover can be precast concrete, composite material or any glass-reinforced plastic material.
The provision for access through the manhole is performed through steps. If the depth of the manhole is less than 1 m, a step ladder is constructed. If the depth of the manhole is greater than 2.5m, a regular ladder is fitted. Now modern manholes do not demand physical entry.
The three main types of manhole depending on the depth are:
A shallow manhole has a depth ranging between 75 to 90 cm. These are constructed at the start of a branch sewer or in an area where there is not much traffic. The shallow manhole is provided with a light cover called as the inspection chamber.
These are provided at the sewer line with a heavy cover on its top. It has a depth of 150cm. Normal manhole takes a square shape.
Deep manhole is provided at a depth greater than 150cm with a very heavy cover at its top. The size can be increased and the facility for going down is also increased.
The manhole construction is performed under the following circumstances:
The three main types of manhole based on the material
are:
Plastic manholes are manufactured by using the polyethylene material. This is manufactured with durable one-piece construction. This construction does not employ any seams or seals as they cause several maintenance issues.
Plastic manholes are environmentally friendly and sustainable. They do not contaminate or cause any adverse effect on the soil or the ground where it is placed.
Plastic manholes are extremely resistant to corrosion.
These do not degrade with time and do not demand frequent rehabilitation and
maintenance with time. These manholes are manufactured with additional accessories
like ladders and manhole covers.
Construction of manholes by precast concrete is a traditional method. These manhole frames are engineered in the segment in a factory located offsite. This method hence ensures quality and also facilitates quick installation. The manufactured precast manholes are assembled on the site.
The increased durability with a life span of 100 years
is one of the reasons for its continued popularity and widespread use.
Fiberglass manholes are engineered such that it includes a manhole barrel and a cover. This basic structure also incorporates additional features like grinder channels, weirs, flumes, separation units for stormwater,etc.
This is a combination of several units. But the
fiberglass technology seals the units together such that they behave
monolithic, that there won’t be any seams or seals.
Fiberglass manholes are easy to handle and weigh one-tenth of the weight of the concrete manhole. As the unit is lightweight, the manhole is easy to install. Fiberglass manholes are environmentally friendly are highly durable.
Two major materials used for the construction of manhole cover are cast iron and concrete. These two materials are also used in combination with the construction of manholes. Both the materials are durable, heavy and inexpensive. With technology, manhole covers are made from fiberglass, plastic, and composite.
Cast Iron Manhole Cover is cast out of gray cast iron. As ductile cast iron is higher in strength, it is used for manholes that are supposed to take heavy loads ( Airport areas). The gray cast iron used for manhole cover manufacture is a combination of carbon and silicon. The carbon gives strength and durability to the element. The addition of manganese to the molten iron gives ductile cast iron.
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The weight of the manhole cover is 113 kg. So, it is considered heavy to carry by a single person.
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A manhole or Inspection chamber may be defined as an important component of a water supply and sanitary system that is constructed underground to provide access to the underground utilities.
The underground utilities mostly include the sewer system and the drainage system.
A manhole is also commonly known as the inspection chamber, utility hole, maintenance hole, inspection chamber, access chamber, or sewer hole.
The primary purpose of constructing a manhole is to provide access to the underground systems for cleaning, modifying, repairing, and inspection works.
Manholes can be seen frequently in streets and sidewalks particularly in urban areas.
The main purpose of constructing a manhole can be listed as follows:
1. It is used for carrying out the cleaning, repair and maintenance works in the sewerage system.
2. It permits the change of alignment and direction of sewers. Moreover, It also facilitates the joining of sewers.
3. The manhole with perforated cover helps in the release of foul gases. Thus, It can serve as ventilation for the underground sewer system.
4. With the aid of a manhole, sewer lines can be laid in convenient lengths.
Manholes are generally provided at the following locations:
1. When there is a change in the alignment of the sewer.
2. When there is a change in direction or grade of the sewer.
3. When there is a change in the size of the sewer.
4. At the junction of two or more sewers.
5. At regular intervals in the straight alignment of sewers.
A manhole is constructed 0.5m from the curb line such that it is sufficiently away from the wheel line of the road.
A manhole usually consists of an underground chamber of varying size and depth.
A typical manhole consists of the following components:
The top upper portion of a manhole is known as the access shaft.
As the name itself implies, it provides access to the sewer.
Access shaft consists of a vertical chamber of size 0.75m * 0.6m for rectangular chamber and 0.70m diameter for a circular manhole.
In the case of rectangular manholes, the access shaft is corbelled inwards on three sides. This is done to provide easy access by reducing the size of the chamber to that of the upper opening of the frame.
For circular manholes, the access shaft is slanted inwards. The access shaft leads to the working chamber.
The lower portion of the manhole is known as the working chamber.
It provides the actual working space for carrying out inspection, repair and maintenance activities.
The working chamber of a manhole is usually larger than the access shaft to provide adequate space for working.
The minimum diameter of the working chamber for a circular manhole is 1.2m.
For rectangular manholes, the size of the manhole must not be less than 1.2m*0.9m.
It must be noted that the larger dimension is towards the direction of flow.
The height of the working chamber must not be less than 1.8m.
Since the size of the working chamber is larger than the access shaft, the working chamber is constructed by enlarging the bottom portion of the access shaft.
Usually, an offset constructed of the brick arch or R.C.C slab is provided at the enlarged portion of the access shaft.
The base of a manhole is mostly constructed of plain cement concrete. This prevents the ingress of underground water inside the chamber as well as provides support to the side walls.
The bed of concrete may also be reinforced to provide additional strength. The minimum thickness of the concrete bed should preferably not less than:
a. 15cm for manholes up to 0.8m deep;
b. 23cm for manholes of depth more than 0.8m up to 2.1m;
c. 30cm for manholes more than 2.1m deep.
The side walls are generally constructed of stone or brick masonry or reinforced cement concrete.
Most commonly the side walls are made up of brick masonry. The recommended minimum thickness of the brick wall is:
a. 20cm or one brick for manholes up to 1.5m deep;
b. 30cm or one and a half bricks for manholes with a depth greater than 1.5m.
In general, the following thumb rule can be used for determining the thickness of
brick wall:
d= depth of manhole in m.
Plastering is done on both the inner and outer side of the brick wall with 1:3 cement mortar.
The bottom of the invert is provided with a semi-circular or U-shaped channel of cement concrete.
The diameter is kept the same as that of the sewer.
The sides of this channel are extended vertically above the horizontal diameter nearly up to the crown.
The top edge is rounded off and sloped towards the channel to form benching.
The slope of benching can range from 1 in 10 to 1 in 6.
Benching facilitates the drainage in the sewer.
When two or more sewers meet at the same manhole, additional channels must be provided concerning the benching.
Usually, a series of steps are provided for the entry and exit of the workers.
Steps are usually made up of cast iron.
They are staggered at a horizontal centre to centre distance of 38cm and vertical centre to centre distance of 30cm.
It is desirable to place the uppermost step 45cm below the manhole cover and the lowermost step less than 30cm above the benching.
The top opening of a manhole is provided with a set of cover in a frame.
Usually, the opening of the manhole is circular so circular covers made of cast iron are commonly used.
The frame provides support to the cover of the manhole.
The frame should preferably be 20 to 25 cm high and its base must be 10 to 12 cm wide.
The total weight of cover including the frame ranges from 90 to 270kg.
The frame is generally laid in a mass of cement concrete in the plane of alignment so that it doesn’t interrupt the traffic flow.
The top surface of the cover is usually provided with small projections to make it rough. This is done to ensure that the surface isn’t too slippery.
Manhole can be classified into the following categories:
Depending upon the depth of manhole, they can be classified into the
following:
These are the type of manholes whose depth generally ranges from 0.75 to 0.90m.
Shallow manholes are mostly constructed in areas with low traffic or at the beginning of branch sewers.
These manholes are also referred to as inspection chambers.
Normal manholes are the type of manholes with a depth greater than 0.90m but limited to 2m.
Such manholes are usually provided with the thick cover on the top and square in shape.
Deep manholes are the type of manholes with a depth greater than 2m.
These manholes are commonly circular where the diameter depends upon the depth.
Depending upon the different material used for the construction of manhole, they can be classified as:
These are the type of manholes that are constructed with precast concrete.
It is the traditional type of construction of the manhole.
The precast concrete manholes are directly manufactured and laid on site.
Thus, allows a rapid and quick installation.
Such manholes are extensively used worldwide because of its long life span.
Plastic manholes are manufactured using the polyethene material with durable one-piece construction.
That means a plastic manhole is manufactured directly with accessories like cover and ladder.
Such manholes are designed and manufactured in such a way that doesn’t harm the environment.
Moreover, such manholes do not cause any adverse effect on the soil or groundwater where it is installed.
Plastic manholes require lesser maintenance and have high resistance to corrosion.
Such manholes are manufactured using fibreglass technology.
They essentially consist of a manhole barrel, cover, weirs, flumes, separation units for stormwater etc.
The lightweight of such manholes makes its handling and installation relatively easy.
Also, fibreglass manholes are durable and environment-friendly.
In general, the construction of a typical manhole involves the following steps:
1. Firstly, the size of the manhole must be determined. It must be noted that the size of manhole depends upon the pipe diameter and depth between road level and crown of the pipe.
2. Then, a suitable type of construction must be decided.
3. The excavation of ground is then carried out at the site. Compaction may be required depending upon the type of soil.
4. The next step involves the laying of the foundation.
5. Then, the walls of manholes are constructed and the benching is formed.
6. Roofing of the manhole is done and the cover is provided.
a. It helps to maintain environmental hygiene with proper management of drainages and sewages waste products.
b. Its construction is a cheaper way of managing the wastages and drainages within the accessible budget.
e. It improves the excess infiltration, inflow and overall sewer system operation.
f. It helps to reduce the length of pipes and diameter of the chamber used for sewage lines and sidewalks of roads.
a. Its walls can be affected by microbiologically induced corrosion (MIC).
b. Installing and rehabilitating a manhole in a congested area is quite difficult.
c. Chance of mixing of groundwater and drainage water if proper care is not taken.
It would be harder to adjust with a cover. The worker needs to rotate the cover exactly the right way. Therefore, various manhole covers are round so they don’t requires to be rotated. it’s safer because a round manhole cover won’t fall down into a hole because it was rotated the wrong way.
2. How much does a inspection chamber cover weigh?
The weight of the manhole cover is 113 kg.
A manhole or Inspection chamber may be defined as an important component of a water supply and sanitary system that is constructed underground to provide access to the underground utilities.
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Home What Is A Manhole | Purpose of Manhole | Types of Manholes | Manhole Construction Details
Welcome to civiconcepts.com, this website is authored and maintained by Bhushan Mahajan . He is a Bachelor in Civil Engineering and the founder of Civiconepts – Make Your House Perfect With Us. He has work experience in bridge construction, Railway steel girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a diploma engineering college for more than 3 years.
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 manhole is trenchless rehabilitation for the sewer system , an inspection of a drainage system , cleaning the clogged line , and maintenance purpose , also it is an initial step for entering the inside sewer line and finding out a problem on 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 ma nhole is consisting of a vertical circular chamber of different sizes and depths which is used to reach the inspection point .
Manholes construct ion is don at that point where the change direction of sewer line and gradient of a sewer line. The man hole is typically installed 0.5 m away from the curb line of the road .
Also, the ma nhole covers are available in different size , material , and shape such as rectangular , circular , and square ; it is made from metal , precast co
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