Manhole may be defined as an important component of 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. In simple terms, manholes refer to the RCC or masonry chambers that are constructed at suitable intervals along the sewer lines for providing access into them.
Manhole is also commonly known as the inspection chamber, utility hole, maintenance hole, inspection chamber, access chamber or the 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 the urban areas.
Purpose of Manhole
The main purpose of constructing manholes can be listed as follows:
1. Manholes are used for carrying out the cleaning, repair and maintenance works in the sewerage system.
2. Manholes permit the change of alignment and direction of sewers. Moreover, manholes also facilitate the joining of sewers.
3. The manholes with perforated covers help in the release of foul gases. Thus, manholes can serve as ventilation for the underground sewer system.
4. With the aid of manholes, sewer lines can be laid in convenient lengths.
Location of Manhole
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.
Features of Manhole
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:
1. Access Shaft:
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.
2. 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.
3. Base and Side Walls:
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
t= 10 + 4d
t= thickness of the wall in cm.
d= depth of manhole in m.
Plastering is done on both the inner and outer side of the brick wall with 1:3 cement
4. Invert/ Bottom:
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.
5. Ladder or Steps:
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.
6. Manhole Cover and Frame:
The top opening of a manhole is provided with a set of cover in a frame. Usually, the opening of manholes 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.
Types of Manhole
Manholes can be classified into the following categories:
A. Based on the depth of the manhole
Depending upon the depth of manholes, they can be classified into the
1. Shallow Manholes:
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. They are commonly rectangular. These manholes are also referred to as inspection chambers.
2. Normal Manholes:
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.
3. Deep Manholes:
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.
B. Based on Construction Material Used
Depending upon the different material used for the construction of manholes, they can be classified as:
1. Precast Concrete Manholes:
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 rapid and quick installation. Such manholes are extensively used worldwide because of its long life span.
2. Plastic Manholes:
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.
3. Fibreglass Manholes:
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.