Table of Contents
What is A Manhole?
A unit built underground to enable access to services such as a sewer system, drainage system, etc. is referred to as a manhole or an inspection chamber.
Thus, subsurface utilities are examined, altered, cleaned, and maintained with the aid of a manhole.
A manhole is a point of access for supplies and persons inside a sewer network.
It is the most crucial part of the sewage line and is also referred to as an inspection chamber or unit since it fully removes the necessity for digging.
Manholes are primarily built for trenchless sewage system rehabilitation, drainage system inspections, blocked line cleaning, and maintenance requirements.
It is also a first step in getting inside the internal sewer system to identify any issues, and it aids in replacing the broken pipe without having to dig.
Manholes are installed at regular intervals along the sewer line until it reaches the primary sewer line or sewage point.
The manhole typically consists of an inspection point-accessible vertical circular chamber with a range of sizes and depths.
The intersection of a sewer line’s gradient and direction change is where manhole construction takes place. Typically, the manhole is placed 0.5 m from the road’s curb line.
The manhole covers are composed of metal, precast concrete, and composite material and come in a variety of sizes, materials, and designs, including rectangular, circular, and square.
If the depth of the manhole chamber exceeds 2.5 m, a ladder must be installed inside; if the depth is little than 1 m, a step ladder is required.
Purpose of Manhole
- The covers of these have perforations that allow the unpleasant gases to escape. Because of this, it provides the underground sewage system with effective ventilation.
- The sewer line can be laid in standard lengths with the aid of manholes.
- For sewer line maintenance, cleaning, and the removal of any obstructions that are discovered.
- For tying up sewer pipes at a junction.
- For adjusting a sewer line’s alignment or direction.
- For modifying a sewer line’s gradient.
Types of Manholes
The following manhole types are listed below:
- Shallow Manhole
- Normal Manhole
- Deep Manhole
Shallow Manhole
A shallow manhole with a depth range of 75 to 90 cm is built at the beginning of a branch sewage line where there isn’t a lot of traffic. They are also called inspection chambers.
Normal Manhole
A typical manhole is square in design, has a 150 cm depth, and typically has a thick cover over its top.
They are constructed either in a square(1m × 1m) or rectangular(0.8m × 1.2m) in cross-section. The section of the such manhole is not changed with depth.
Deep Manhole
Manholes deeper than 1.5 meters are referred to as deep manholes. Such a manhole is smaller at the top because the size of the manhole is larger at the bottom than at the top.
By offering an RCC offset, the size is decreased. In order to allow the employees to descend all the way to the bottom, steps are also installed on one of the manholes’ vertical walls. At the top, a solid cover with a proper C.I. frame is provided.
Location of Manholes
Every bend, intersection, change in grade, and change in diameter has a manhole.
Manholes are provided on straight reaches at a convenient spacing that varies depending on the size of the sewer. The following conditions apply to the building of the manhole:
- Where a new sewer line is required
- The size and alignment of the sewer have changed.
- Two or more sewer lines come together to create a junction.
The greater the diameter of the sewer, the greater the manhole spacing. The type of sewer cleaning mechanism being used will also affect how far apart the manholes are placed.
The spacing between manholes is provided in the table below according to Indian Standard IS1742-1960.
| Size of Sewer | Recommended Spacing On Straight Reaches |
|---|---|
| Diameter up to 0.3 Meter | 45 Meter |
| Diameter up to 0.6 Meter | 75 Meter |
| Diameter up to 0.9 Meter | 90 Meter |
| Diameter up to 1.2 Meters | 120 Meters |
| Diameter up to 1.5 Meters | 250 Meters |
| Diameter greater than 1.5 Meters | 300 Meters |
Dimensions of Manholes
The following table gives the minimum internal dimensions for manhole chambers as recommended by Indian Standard IS 1742-1960.
| Depth | Minimum Specified Size |
|---|---|
| 0.8 meters or less | 0.75-meter × 0.75 meter |
| 0.8 meters and 2.1 meters | 1.25-meter × 0.9 meter |
| Greater than 2.1 meter | Circular chamber with a minimum diameter of 1.4 meters or rectangular chamber with a minimum of 1.2 × 0.9 meter |
Components of Manhole
Major manhole components include the following:
- Access Shaft
- Working Chamber
- Sidewall and Base
- Bottom or Invert
- A Ladder or Steps
- Cover and frame.
Access Shaft
The upper part of the manhole, known as the access shaft, is a vertical conduit that gives access to the working chamber.
The minimum access shaft dimensions for a circular manhole are 0.70m in diameter and approximately 0.75m x 0.60m for a rectangular manhole.
In rare circumstances, a reinforced cement concrete slab with the appropriate dimensions may be used to cover the access shaft.
Working Chamber
The lower part of the manhole known as a “working chamber” is used as a workspace for cleaning and inspecting the sewer line.
The rectangular chamber must be at least 1.2 m by 0.9 m, with the flow delivered on the larger side. The working chamber must be at least 1.8 meters tall and have a minimum diameter of 1.2 meters.
The working chamber is often bigger than the access shaft in size, so that the working chamber is built by extending the access shaft’s bottom section.
Base And Side Walls
The foundation of the manhole is typically composed of plain cement concrete and is positioned at the base level to support the manhole’s sidewall and deter groundwater intrusion.
| Manhole Depth | Minimum Thickness of Concrete Bed |
|---|---|
| Up to 0.8 meter | 15 cm |
| 0.8 meters to 2.1 meters | 23 cm |
| Greater than 2.1 meters | 30 cm |
Reinforced cement concrete, brick masonry, and stone masonry are used to construct the side walls of the manhole. But for sidewalls, they frequently employed brick masonry.
| Manhole Depth | Minimum Wall Thickness |
|---|---|
| Up to 1.5 meters | 20 cm |
| Greater than 1.5 meters | 30 cm |
The appropriate thickness may be determined from the following thumb rule:
t = 10 + 4d
where t = wall thickness in cm, subject to a minimum of 225.
d = depth of manhole in meters.
Apply a smooth finished cement coat inside after plastering the inside and outside brickwork with cement mortar 1:3, which is made up of 1 part cement and 3 parts coarse sand.
Compared to brick walls, RCC walls will be much thinner. The R.C.C. wall, however, is pricey and only employed in unique circumstances.
Bottom or Invert
The manhole’s bottom is either semicircular or U-shaped. It is a concrete-cement channel.
The sewer line’s diameter is the same as the diameter of the concrete channel.
In order to create benching, the top edge of the horizontal diameter is rounded off and made to slope in the direction of the channel, almost reaching the crown point of the sewer or manhole.
The slope for benching ranges from 1 in 10 to 1 in 6. The benching and main channel branches are constructed similarly when two or more sewer lines reach a manhole at the same level.
There are simple curves available at the sewage line’s intersection.
A Ladder or Steps
In that situation, ladders are offered if the manhole depth is greater than 0.8 m. These cast iron steps are located inside a manhole and have a staggered ladder design.
38 centimeters from center to center horizontally and 30 centimeters from center to center vertically separate the two stairs.
The first ladder is positioned 45 cm below the manhole cover, and the lowest ladder is not positioned higher than 30 cm above bench level.
All ladders are 15 cm wide. These stairs won’t tip over because they are firmly attached to the chamber wall.
Frame And Cover.
The pair of frames and coverings is not far from the top open part of the manhole. In most cases, the manhole’s opening is circular, causing the cover to be circular as well.
The cover’s usual dimensions are 56 cm in diameter and more than 0.9 cm in depth.
Cast iron is utilized to make the cover and the frame. The cover is supported by the frame; its weight ranges from 90 to 270 kg.
The heavy type cover and frame are used to handle large traffic loads; similarly, the light type is used to support mild traffic loads.
The cover is put into the frame’s provided groove, which is slightly recessed into the top concrete surface of the wall.
The manhole cover’s rough surface was created to prevent the top from becoming slippery.
Materials Used in Manhole Construction
Manhole construction involves the usage of the following materials:
- Fiberglass Manhole
- Precast Manhole
- Plastic Manhole
Manhole Made of Plastic
Polyethylene is the material used to make plastic manholes. It is sustainable and environmentally friendly, and it has no adverse impacts on the soil or ground where it is installed.
The biggest benefit of adopting plastic manholes is that they don’t require frequent rehabilitation or maintenance.
Manholes Made of Precast Concrete
Precast concrete manholes are a traditional method of manhole construction. This method is simple to install and assemble on the job site. It is a more common type of manhole due to its long lifespan.
Manhole Made of Fiberglass
Fiberglass manholes have additional features such as grinder channels, weirs, stormwater separation units, and so on.
The fiberglass manhole is simple to handle and install. It weighs significantly less than a concrete manhole. It is also environmentally friendly and long-lasting in nature.
Manhole Cover Types
The main purpose of providing a manhole cover is that it serves as a lid to close the manhole and that it is removable.
Concrete, cast iron, or a combination of these materials is commonly used to construct manhole covers.
These materials are frequently used because they are less expensive, more durable, and heavy enough to stay in place.
Manhole covers are now available in a variety of materials, including fiberglass, plastic, and composite.
Cover Made of Ductile Iron
Manhole covers of the ductile type are made of ductile iron and nodular iron. It is a distinct type of iron.
Because of its strength, solidity, durability, and resilience, this type of cover is more popular.
A cast-iron manhole cover will corrode faster than a ductile manhole cover.
Recessed Cover
Recessed manhole covers, as the name implies, are installed in recessed areas suitable for laying paver blocks, cement, and a layer of screed for a pedestrian-friendly appearance.
These manhole covers are better suited for internal manholes found in hospitals, domestic, and garden apartments, and workshops.
Solid Top Cover
The Solid Top manhole cover is made of lighter galvanized material and is intended to be visible from the ground.
FAQs
What exactly is a manhole?
A manhole is an access point inside the sewer line for materials and workers. It is the most important part of the sewer line because it eliminates the need for digging, and it is also known as an inspection chamber or unit.
What is a sewer hole?
It is an opening, typically covered, that allows access to a sewer or drainage system, especially one that is situated on a city roadway.
What are the various types of manholes?
The following manhole types are listed below:
Shallow Manhole
Normal Manhole
Deep Manhole
What is the Function of a Manhole?
To inspect, maintain, and remove any obstruction parts discovered in the sewer line.
At a junction point, for connecting sewer lines for reversing the direction or alignment of a sewer line
To alter the gradient of a sewer line