Manhole, Diversion Manhole – what is it?

Thomas Sim PEA2066

Q.
What is manhole, and its purpose? What is a diversion manhole?

A.
A manhole (alternatively utility hole, cable chamber, maintenance hole, inspection chamber, access chamber, sewer hole, or confined space) is the top opening to an underground utility vault used to house an access point for making connections, inspection, valve adjustments or performing maintenance on underground and buried public utility and other services including sewers, telephone, electricity, storm drains, district heating and gas.

Wikipedia search 'manhole', available at
https://en.wikipedia.org/wiki/Manhole
This is a sewer manhole, exposed during construction. Typically, only the top ring and manhole cover (not pictured) would be visible.
Manhole being used to access sewer

Role of manhole:

To provide access for humans to carry out operation and maintenance (surveys and inspections) of the pipe (main pipe) .

Installation location

  • Origin or junction of pipes
  • Points where the gradient or diameter changes

Made such that people can always have access (The method of opening the cover varies according to the type)

Another answer is more complete as provided below.

Manhole is provided when

  1. There is change in grade of sewer
  2. There is change in alignment
  3. There is change in size of sewer
  4. At junction of two or more sewers
  5. Manhole is also provided in straight alignment of sewers at regular intervals depending upon the diameters of sewers. It ranges from 90m to 150m (300' – 500') e.g. 75m for 60cmф, 120m for 90cmф and 150m for 120cmф.

Types of Manhole

Depending upon the depth the manhole can be classified as;

1. Shallow manhole
It is provided at shallow depth of 75-90cm (2'-3'). It is provided at the beginning of branch sewer or at a place not subjected to heavy traffic. It is provided with a light cover at its top it is also called inspection chamber.

2. Normal Manhole
It is provided in sewer line at depth of 150cm with a heavy cover on its top. It is generally of square shape (or rectangular shape).

3. Deep Manhole
They are provided at depth greater than 150cm with heavy cover at the top. The size is gradually increased and a facility for going down is provided.

Ref:
FOCUS, Manhole, available at,

Manhole

Diversion manhole

In a combined sewerage system, when the stormy season with more rain fall is causing the swelling of the main sewerage system, the excess rain water is diverted to the river with the diversion manhole. This system would address the issue of back flow of sewers and contamination of the overflow in public drains by sanitary waste from houses.

Ref:
Wikipedia search, Manhole, available at
https://en.wikipedia.org/wiki/Manhole
Diversion Manhole, from
Sewerage Operation and Maintenance, Tokyo Metropolitan Government Bureau of Sewerage Facilities Management Division, Pipeline Management Section, available at
http://www.asianhumannet.org/db/datas/11_anmc21_sche/4SewerageOperation.pdf