Manhole (Inspection Chamber) | Features & Types of Manhole | Construction of Manhole | Advantages & Disadvantages of Manhole

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.

Inspection chamber can be seen frequently in streets and sidewalks, particularly in urban areas.

  2. Purposes  

The main purpose of constructing an inspection chamber 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.

  3. Location  

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.

  4. 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.

The access shaft consists of a vertical chamber of size 0.75m * 0.6m for a 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 1.2 m.

For rectangular manholes, the size of the manhole must not be less than 1.2 m*0.9 m.

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 1.8 m.

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 be less than:

a. 15 cm for manholes up to 0.8 m 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:

where,

t= thickness of the wall in cm.

d= depth of manhole in m.

Plastering is done on both the inner and outer sides of the brick wall with 1:3  cement mortar.

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 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 center-to-center distance of 38 cm and a vertical centre-to-centre distance of 30 cm.

It is desirable to place the uppermost step 45 cm below the  manhole cover and the lowermost step less than 30 cm above the benching.

6.  Manhole Cover and Frame:

The top opening of a manhole is provided with a set of covers 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 the cover including the frame ranges from 90 to 270 kg.

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.

  5. Types of Manhole  

Manholes can be classified into the following categories:

A. Based on the depth of the manhole

Depending upon the depth of the manhole, they can be classified into the
following:

i. Shallow Manholes:

These are the type of manholes whose depth generally ranges from 0.75 to 0.90m.

Shallow chambers are mostly constructed in areas with low traffic or at the beginning of branch sewers.

They are commonly rectangular.

ii. 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 a thick cover on the top and are square in shape.

iii. 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 materials used for the construction of the inspection chamber, they can be classified as:

i. Precast  Concrete Inspection Chamber:

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 inspection chambers are directly manufactured and laid on site.

Thus, allows a rapid and quick installation.

Such manholes are extensively used worldwide because of their long life span.

ii. Plastic Manholes:

Plastic manholes are manufactured using the polyethene material with durable one-piece construction.

That means a plastic chamber is manufactured directly with accessories like a cover and ladder.

Such manholes are designed and manufactured in such a way that doesn’t harm the environment.

Moreover, such chambers do not cause any adverse effect on the soil or groundwater where it is installed.

Plastic chambers require lesser maintenance and have high resistance to corrosion.

iii.  Fibreglass Manholes:

Such chambers are manufactured using  fiberglass technology.

They essentially consist of a barrel, cover, weirs, flumes, separation units for stormwater, etc.

The light weight of such inspection chambers makes their handling and installation relatively easy.

Also,  fiberglass manholes are durable and environment-friendly.

  6. Construction  

In general, the construction of a typical inspection chamber involves the following steps that are mentioned below:

1. Firstly, the size of the manhole must be determined. It must be noted that the size of the manhole depends upon the pipe diameter and depth between the road level and the crown of the pipe.

2. Then, a suitable type of construction must be decided.

3. The excavation of the ground is then carried out at the site. Compaction may be required depending on the type of soil.

4. The next step involves the laying of the foundation.

5. Then, the walls of the inspection chamber are constructed and the benching is formed.

6. Roofing of the inspection chamber is done and the cover is provided.

  7. Advantages  

a. It helps to maintain environmental hygiene with proper management of drainages and sewage 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.

  8. Disadvantages  

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.

  9. FAQ  

1. Why are  manhole covers round?

It would be harder to adjust with a cover. The worker needs to rotate the cover exactly the right way.

Therefore, various covers are round so they don’t require 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 an  inspection chamber cover weigh?

The weight of the  manhole cover is 113 kg.

3. How heavy is an  inspection chamber cover?

4. What is an Inspection Chamber?

An inspection chamber may be defined as an important component of a water supply and sanitary system that is constructed underground to provide access to underground utilities.

Read Also: System of Sanitation

Read More: Water Distribution System