A manhole or an inspection chamber is a unit constructed underground to provide access to the utilities like a sewer system, drainage system, etc. Hence, with the help of a manhole, underground utilities are inspected, modified, cleaned and maintained.
The main purpose, features, and types of a manhole are briefly explained in the article.
Purpose of Manhole
The main purpose of a manhole is:
- To perform inspection, cleaning, and removal of any obstruction present in the sewage line.
- The joining of sewers, the change of direction or the alignment of sewers can be performed with the help of manhole.
- These have a perforated cover which helps the foul gases to escape. Hence it is a good means of ventilation for the underground sewage system.
- Manholes help to lay the sewer line in the conventional lengths.
Features of Manhole
The main parts of a manhole are the chamber or ring and the Vertical Circular Pipe. The vertical circular pipe is available in varying depth and sizes. These pipes are used to access the inspection joints in the system.
Manholes are mainly positioned 0.5m away from the curb lines of the road. Mostly it is constructed such that it is away from the wheel line of the traffic.
The cover of a manhole is a plug that protects the manhole from any unauthorized access. The covers used for manholes can be either rectangular, square or circular in shape. The material of cover can be precast concrete, composite material or any glass-reinforced plastic material.
The provision for access through the manhole is performed through steps. If the depth of the manhole is less than 1 m, a step ladder is constructed. If the depth of the manhole is greater than 2.5m, a regular ladder is fitted. Now modern manholes do not demand physical entry.
Types of Manhole
The three main types of manhole depending on the depth are:
- Shallow Manhole
- Normal Manhole
- Deep Manhole
1. Shallow Manhole
A shallow manhole has a depth ranging between 75 to 90 cm. These are constructed at the start of a branch sewer or in an area where there is not much traffic. The shallow manhole is provided with a light cover called as the inspection chamber.
2. Normal Manhole
These are provided at the sewer line with a heavy cover on its top. It has a depth of 150cm. Normal manhole takes a square shape.
3. Deep Manhole
Deep manhole is provided at a depth greater than 150cm with a very heavy cover at its top. The size can be increased and the facility for going down is also increased.
Location of the Manhole
The manhole construction is performed under the following circumstances:
- Where a change of sewer line is necessary
- There is a change in sewer size and alignment.
- A junction is formed by two or more sewer lines
Materials Used for Manhole Construction
The three main types of manhole based on the material are:
- Plastic Manholes
- Precast Concrete Manholes
- Fiberglass Manholes
1. Plastic Manholes
Plastic manholes are manufactured by using the polyethylene material. This is manufactured with durable one-piece construction. This construction does not employ any seams or seals as they cause several maintenance issues.
Plastic manholes are environmentally friendly and sustainable. They do not contaminate or cause any adverse effect on the soil or the ground where it is placed.
Plastic manholes are extremely resistant to corrosion. These do not degrade with time and do not demand frequent rehabilitation and maintenance with time. These manholes are manufactured with additional accessories like ladders and manhole covers.
2. Precast Concrete Manholes
Construction of manholes by precast concrete is a traditional method. These manhole frames are engineered in the segment in a factory located offsite. This method hence ensures quality and also facilitates quick installation. The manufactured precast manholes are assembled on the site.
The increased durability with a life span of 100 years is one of the reasons for its continued popularity and widespread use.
3. Fiberglass Manholes
Fiberglass manholes are engineered such that it includes a manhole barrel and a cover. This basic structure also incorporates additional features like grinder channels, weirs, flumes, separation units for stormwater,etc.
This is a combination of several units. But the fiberglass technology seals the units together such that they behave monolithic, that there won’t be any seams or seals.
Fiberglass manholes are easy to handle and weigh one-tenth of the weight of the concrete manhole. As the unit is lightweight, the manhole is easy to install. Fiberglass manholes are environmentally friendly are highly durable.
Manhole Cover Materials
Two major materials used for the construction of manhole cover are cast iron and concrete. These two materials are also used in combination with the construction of manholes. Both the materials are durable, heavy and inexpensive. With technology, manhole covers are made from fiberglass, plastic, and composite.
Cast Iron Manhole Cover is cast out of gray cast iron. As ductile cast iron is higher in strength, it is used for manholes that are supposed to take heavy loads ( Airport areas). The gray cast iron used for manhole cover manufacture is a combination of carbon and silicon. The carbon gives strength and durability to the element. The addition of manganese to the molten iron gives ductile cast iron.