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What are manholes?

Articles > What are manholes?

In the previous article we have discussed the inspection chambers in detail. The subject of our today's article is to discuss in detail the manhole in a sewerage system. First of all we will differentiate manhole from the inspection chamber. An inspection chamber is usually small and individual access is possible only through camera and other cleaning apparatus, while a manhole is comparatively larger in size and it allows the physical access to the manhole for the sake of inspection and cleaning. Nevertheless function of both the manhole and inspection chamber is same.

Manhole is an opening in the underground drainage system, with a cover/lid on its mouth. Manholes are often used as an access point for an underground public utility, allowing inspection, maintenance, and system upgrades. The majority of underground services have manholes, including water, sewers, telephone, electricity, storm drains, district heating, and gas.

 

Parts of a manhole:

There are four major parts of a typical manhole. Each part of the manhole performs a separate function. Following are the main four parts of the manhole.

  1. The Groove
  2. RSJ
  3. Double Seal Terminal Manhole
  4. U-Trap with Roding Arms

(1). The Groove:

            The groove is used to facilitate the maintenance of manholes and sewer/drain pipes. Shutoff boards are erected on the grooves during maintenance operation so that water flow coming from upstream is terminated in the manhole and backwater from downstream is also blocked.

The groove of the manhole may be made up of precast concrete or it is constructed with bricks by masonry work.

(2). RSJ:

            Rolled Steel Joist is a small-scale size of universal beams and is used for resisting the high stresses incurred by heavy traffic loads acting directly on the upper narrow projected section of manholes.

(3). Double Seal Terminal Manhole:

            It is a lid/ cover which is placed on RSJ to prevent gases emitted in the underground sewer/drain from releasing out.

(4). U-Trap with Rodding Arms: 

The purpose of this part of the manhole is to allow the allow water streaming from homes and the rain water from the streets as well as to seal the unpleasant smell emitted with in the pipes. The rodding arm of the U-Trap is normally closed with rubber rings during normal operation. However, during maintenance operation, the rubber ring is removed and rodding can be carried out through the rodding arm.

Purpose of Manholes:

The main purpose of all types of the manholes is:

  1. to perform cleaning, inspection and to remove any kind of blockage in way of drain/ sewer.
  2. to join the sewers, change the direction of sewers etc.
  3. to prevent the unpleasant smelled and harmful gases to release out of the sewerage system, which are emitted in large quantity with the pipes and sewerage tunnels.
  4. to help laying the sewer lines in the convention length. The sewer line has opening at equal intervals in the form of manholes.

Purpose of Manholes:

The main purpose of all types of the manholes is:

  1. to perform cleaning, inspection and to remove any kind of blockage in way of drain/ sewer.
  2. to join the sewers, change the direction of sewers etc.
  3. to prevent the unpleasant smelled and harmful gases to release out of the sewerage system, which are emitted in large quantity with the pipes and sewerage tunnels.
  4. to help laying the sewer lines in the convention length. The sewer line has opening at equal intervals in the form of manholes.

Features of the manhole:

1. The main parts of a manhole are chamber and vertical circular pipe. The vertical circular pipe allows the person to enter the chamber to inspect the joints in the chamber. The vertical pipe may be of different length depending on how far deep the sewer pipes are installed.

2. Manholes are always installed away from the wheel line of traffic. For this purpose manholes are constructed at least 0.5 meters away from the roads. If the manholes are installed in the streets, then the location of a manhole is always in the middle of the street. This is because to prevent manhole from the heavy weight of vehicles which can be exerted through the wheels of traffic. Therefore, manholes are always installed in a way that they are must not be underneath of wheel line.

3. The cover of a manhole is a plug that protects the manhole from any unauthorized access.

4. The cover of a manhole is to protect from breakage. It is usually made up of precast concrete, glass reinforced plastic and iron or steel. The strength of the cover is basic thing.

Designs of a Manhole:

Manholes are installed in a variety of designs, but three man designs are prominent in urban and rural sewerage/ drainage system i.e. Round Manholes, Rectangular Manholes and the Square Shaped Manholes. Among these manholes the round one is the best choice of civil engineers and urban planners, becasue the round walls of the chamber and verticle pipe does has no corners and edges. The smooth look of the manhole allows the sewers and the water flow with fluency. Moreover, the round walls of the manhole increase the strength of the manhole, and the burden of traffic does not demage the walls, verticle hole/ groove and moth of with round stell ring, because each part of the manhole exerts pressure on the other part. This interdependence of different parts of the manhole increase its strenght. If a manhole is round in shape, no matter if its is constructed with simple masonry of bricks. 

 


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