What is a Manhole? | Functions, Components, & Location

Manholes are underground inspection chambers that provide access to underground utilities like a sewer system. Manholes enable operations and the utility personnel an access point to undertake operations like inspection, modification, cleaning, and maintenance.

Types of manholes vary based on the depth and function. Based on depth, it can be shallow, normal, and deep manhole. Based on function, it can be drop-manhole, flush manholes and junction chamber. 

Watch the Video Here:  What is a Manhole? Components & Functions - YouTube

Initially, manholes were built to provide humans with access to pipes and other underground assets. Now, its modern use is to employ equipment like inspection cameras in the manhole to gather CCTV footage to clean and maintain.

Functions of Manhole

The main functions of a manhole are:

1. To provide access for inspecting sewers or stormwater systems.
2. To clean and maintain sewer or stormwater line
3. To assist in ventilating the sewage system by allowing gases to escape
4. To facilitate joining sewer or stormwater systems, change in direction, change the alignment of sewer or stormwater system.
5. To ensure that the sewer or stormwater line is laid at convenient heights

Parts & Working of a Manhole

The components of a manhole can be understood by studying the below figure. The figure below shows the cross-section of brick masonry and pre-cast-made manhole.

Fig.2. Cross-Section of a Manhole

1. Cover of a Manhole:  The cover of a manhole provides and controls access from the ground surface into the manhole. Manhole covers can be made of metal or cast iron and are less commonly made of precast concrete, glass-reinforced plastic, or composite material.
2. Frame: The frame supports the manhole cover and forms the enclosing cover of the manhole. It forms the major body of the manhole. It is either made of brick, precast concrete, plastic, or fiberglass.
3. Chimney of a Manhole: Chimney is the component that connects the frame to the underlying cone of a manhole. It is made out of brick, concrete, or polymer material. 
4. Cone of a Manhole: Cone of a manhole is a tapered transition section of the manhole as shown in figure (2). It is installed between the smaller diameter of the chimney and the larger diameter of the wall for a certain depth of the manhole. 
5. Wall of a Manhole: This is the vertical section of the manhole from the cone to the bottom of the manhole.
6. Manhole Step: These are ladders provided on the inside of the manhole wall to allow easy access. A step ladder is necessary if the manhole is less than 1m. If the manhole depth exceeds 2.5 m, then a regular ladder is fitted. Modern manholes are nowadays designed to avoid the need for physical entry.                                                           

   

   Fig.3. Top View of a Manhole- Benching & Channel

   
   
7. Channel of a Manhole: A channel is a conduit that is located at the bottom of the manhole to let the water or sewage pass through the manhole inlet to the outlet. 
8. Bench of a Manhole: A manhole bench is the bottom section of the manhole made of mortar or brick. It is poured between the walls of the manhole and the sewer pipes as shown in figure-3. The main purpose of benching in the manhole is to direct flow back into the channel during any blockage issues. It is also used by maintenance personnel to stand on the bottom during servicing.  Note: Benching should be self-cleansing and formed with high-strength concrete at a gradient of between 1:10-1:30. To maintain a smooth flow within the main channel, the benching must be formed vertically from the edge of the channel to at least the crown of the pipe.
9. Invert/Bottom of a Manhole: The lowest point of the channel inside the manhole forms the invert. Its characteristics govern the function or flowline of a pipe. Invert elevation is the distance between the sewer pipe and a given benchmark ( mostly the top of the street or the road). Invert elevation is measured before installing a sewer pipe. (Figure-2)
10. Drop Inlet: These pipe drop inlets are special pipe systems that connect to the incoming pipe, and turn 90 degrees to the bottom of the manhole, then turn horizontally into the manhole. (Figure-2)

Location & Spacing of Manhole Construction

Manholes are located generally at a point where there is

1. A change in direction or change in gradient of the utilities
2. Need for access for specific maintenance purposes

The standard manhole requirements during the design of sewer as recommended by the City of New York Department of Environmental Protection are as follows. 

1. Manhole Location on Pipe Sewers

• At all changes in grade or elevation for all sizes of sewers.
• At all changes in alignment for all sizes of sewers.
• At all street intersections for sewers up to and including 24" diameter.
• At all junctions of 2 or more sewers. Recommended maximum spacing 250' 400' 600'
• At all catch basin connections where it is not practical to connect directly to the sewer. A direct connection shall not be made to a sewer less than 60" in diameter.
• The term "dry location" shall mean any location where the entire manhole is located above the water table and is in normally dry soil.
• The term "wet location" shall mean any location where the manhole is located in whole or in part below the water table or in normally wet soil.
• *Special consideration will be required for situations not covered herein.

2. Maximum Spacing of Manholes in a Sewer Pipe System

Sewer Pipe Size in Inches	Recommended Maximum Spacing of Manholes		Absolute Maximum Spacing of Manholes
	Feet	Metres	Feet	Metres
10" DIA. TO 36" DIA. CIRCULAR PIPE 14"H x 23"W TO 29"H x 45"W HORIZONTAL ELLIPTICAL PIPE 23"H x 14"W TO 45"H x 29'W VERTICAL ELLIPTICAL PIPE	250	76	300	91
42" DIA. TO 72" DIA. CIRCULAR PIPE 34"H x 53"W TO 58"H x 91'W HORIZONTAL ELLIPTICAL PIPE 53"H x 34"W TO 91"H x 58"W VERTICAL ELLIPTICAL PIPE	400	121	500	152
78" DIA. AND LARGER CIRCULAR PIPE 63"H x 98"W AND LARGER HORIZONTAL ELLIPTICAL PIPE 98"H x 63"W AND LARGER VERTICAL ELLIPTICAL PIPE	600	182	800	243

Link to the Documents: sewer-design-standards.pdf (nyc.gov)

Also Read: What is Wastewater? - Sullage, Sewage, and Sewers