Why are manholes called manholes?
The term "manhole" comes from the simple idea of how the hole was used--by men who entered the hole to locate the tunneled area beneath the ground. A manhole may also be called an access chamber, utility hole, maintenance hole or inspection chamber.
A manhole (alternatively utility hole, cable chamber, maintenance hole, inspection chamber, access chamber, sewer hole, smellhole, flabhole, ding-dong 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.
What is Manhole ?
Manholes are masonry or RCC chambers constructed at suitable intervals along the sewer lines for providing access into them.
Purposes of Manhole
- They are used to carry out inspection, cleaning and removing obstruction in the sewer line.
- Manhole allows joining of sewers or changing the direction of sewer or alignment of sewer or both.
- They allow the escape of considerable gases through perforated cover and thus help in ventilation of sewage.
- They facilitate the laying of sewer line in convenient lengths.
Location of Manhole
- Manhole is provided when
- There is change in grade of sewer
- There is change in alignment
- There is change in size of sewer
- At junction of two or more sewers
- 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.
Usages of Manhole
Manhole closings are protected by a manhole cover, a flat plug designed to prevent accidental or unauthorized access to the manhole. Those plugs are traditionally made of metal, but may be constructed from precast concrete, glass reinforced plastic or other composite material (especially in Europe, or where cover theft is of concern).
Manholes are usually outfitted with metal, polypropylene, or fiberglass steps installed in the inner side of the wall to allow easy descent into the utility space. Because of legislation restricting acceptable manual handling weights, Europe has seen a move toward lighter weight composite manhole cover materials, which also have the benefits of greater slip resistance and electrical insulating properties.
The access openings are usually circular in shape to prevent accidental fall of the cover into the hole.
Manholes are generally found in urban areas, in streets and occasionally under sidewalks. In rural and undeveloped areas, services such as telephone and electricity are usually carried on utility poles or even pylons rather than underground.
Composite manholes
Composite (fiberglass) manholes are commonly used in applications where infiltration, exfiltration, or corrosion by hydrogen sulfide (from sewer gas) are a concern, or where structures need to be factory integrated into a manhole before placement.
Structures commonly integrated into composite manholes include:
- Flow inverts
- Flumes
- Drop structures from higher elevation flows to lower elevation discharge pipes
Occasionally, composite manholes will integrate:
- Weirs
- Storm water screening structures
- Sewage grinders
- Energy absorbing structures to dissipate undesirable flow stream turbulence or velocity
Hazards caused by stray voltage in manholes
In urban areas, stray voltage issues have become a significant concern for utilities. In 2004, Jodie S. Lane was electrocuted after stepping on a metal manhole cover, while walking her dog in New York City.
Source: wikipedia.org
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