What is a Manhole- Function, Construction, Materials, and Types of Manhole - Civil Engineering Portal (2024)

Introduction:
A manhole is an opening in a street or sidewalk that provides access to an underground utility or infrastructure, typically a sewer, storm drain, or utility vault. It is usually covered with a heavy metal lid that can be removed to allow entry for maintenance or inspection purposes. Manholes are designed to provide a safe and convenient way for workers to access underground systems for repairs, cleaning, or installation of new infrastructure. The term “manhole” is derived from the fact that historically, the openings were large enough for a person to enter. Still, nowadays, many manholes are smaller and may only allow equipment or specialized personnel to enter.


Fig: Manhole
Courtesy: civillead.com

What is Manhole?
A manhole is a covered opening or access point on the surface of a road, sidewalk, or other paved area that provides entry into an underground utility or infrastructure system. It is typically a round or rectangular hole large enough for a person to enter and perform maintenance or inspection tasks. Manholes are common in urban areas and serve as access points to underground networks, including sewer systems, storm water drains, electrical and communication cables, water pipelines, and gas lines. They allow workers to enter these underground structures for repairs, cleaning, inspections, or installations.

The cover of a manhole is usually made of durable materials such as cast iron or concrete to withstand the weight of vehicles and foot traffic. It is designed to be removable, providing access to the underground infrastructure when necessary. The cover is typically flush with the surrounding pavement to prevent vehicle accidents or damage. Manholes are often marked with warning signs or painted in bright colors to increase visibility and awareness for safety. They may also be secured with locks or other devices to prevent unauthorized access.

The Functions of Manholes:

• Check the sewage line, clean it, and get rid of any blockages that are there.
• With the help of a manhole, pipes can be joined together, turned around, or put in the right place.
• These have a cover with holes in it that lets the bad gases escape. So, it is a good way for the underground sewage system to get air.
• With the help of manholes, sewer lines can be laid at the right lengths.

How Are Manholes Constructed?
Manholes are typically constructed using several steps and materials to ensure durability and functionality. Here is a summary of how manholes are usually made.

(i) Planning and Design: The first step involves planning and designing the manhole according to the project’s specific requirements. Factors such as size, shape, depth, and load-bearing capacity are considered during this phase.

(ii) Excavation: A hole is excavated in the ground at the designated location to accommodate the manhole structure. The depth of the excavation will depend on the required access and the depth of the utility lines or sewer system being accessed.

(iii) Base Preparation: Once the excavation is complete, the base of the manhole is prepared. It typically involves leveling and compacting the soil or adding a layer of granular material like crushed stone or gravel to create a stable foundation.

(iii) Installation of the Base Section: The base section of the manhole, often made of precast concrete, is installed in the excavated hole. This section forms the bottom of the manhole structure and supports the subsequent sections.

(iv) Wall Construction: The walls of the manhole are constructed using precast concrete rings or segments. These rings are placed on top of each other to build the height of the manhole. The joints between the rings are sealed with mortar or specialized sealants to ensure water-tightness.

(v) Connection to Utilities: During the construction of the manhole walls, connections are made to the utility lines, such as sewer pipes or storm water drains. These connections allow for access to the underground systems and facilitate maintenance and repairs.

(vi) Installation of the Cover: Once the desired height of the manhole is reached, a cover or top section is installed. This cover is usually made of cast iron or concrete and is designed to support the weight of vehicles or pedestrians passing over it.

(vi) Finishing and Accessory Installation: The manhole is finished by applying protective coatings or linings to enhance durability and prevent corrosion. Various accessories like ladders, steps, and safety features are also installed to ensure safe entry and exit from the manhole.

Materials Used for Manhole Construction:
Manholes are commonly constructed using various materials to ensure durability, stability, and ease of maintenance. Here are some of the typical materials used in manhole construction:

i) Concrete: Concrete is the primary material used for constructing manholes. It is known for its strength, durability, and resistance to environmental factors. Reinforced concrete is often used to provide additional strength and load-bearing capacity.

(ii) Cast Iron: Cast iron is frequently used for manhole covers and frames. It is known for its durability, resistance to heavy loads, and protection against unauthorized access. Cast iron covers are often designed with specific load ratings to withstand the weight of vehicles or pedestrians.

(iii) Brick or Masonry: In some cases, brick or masonry construction may be used for manholes, especially in older systems. Bricks are commonly used for the walls and benching inside the manhole, providing structural support and stability.

(iv) Plastic: Certain manholes may use high-density polyethylene (HDPE) or other plastic materials, particularly in corrosive soil or chemical environments. Plastic manholes offer resistance to corrosion and are lightweight, making them easier to handle and install.

(v) Composite Materials: Fiberglass-reinforced plastic (FRP) or composite materials are gaining popularity in manhole construction due to their corrosion resistance, lightweight nature, and durability. These materials offer advantages over traditional materials in terms of longevity and reduced maintenance.

(vi) Grout or Mortar: Grout or mortar fills gaps between precast concrete sections or bricks in manhole construction. It helps to provide stability, prevent water infiltration, and ensure a tight seal between different components.

(vii) Bituminous Materials: In some cases, bituminous materials like asphalt or bitumen may be used for the base or floor of manholes. These materials provide a smooth surface, help to prevent groundwater infiltration, and offer resistance to chemical attacks.

Types of Manholes:
1. Shallow Manhole:
A shallow manhole typically refers to a manhole that is not very deep or has a relatively shallow depth compared to standard manholes. Manholes are access points built into underground utility systems, such as sewer lines, storm drains, or telecommunications networks, to allow maintenance workers to access and inspect these systems. The depth of manholes can vary depending on the specific infrastructure they access and the regulations or standards in a particular region. Standard manholes are usually several feet deep to allow workers to descend safely and perform their tasks.

A shallow manhole may have less than the standard depth, making it easier and quicker to access but potentially limiting the space available for workers and equipment. Shallow manholes may be used in cases where the underground infrastructure does not require a deeper access point or when construction or space constraints make it impractical to build a deeper manhole.

2. Deep Manhole:
A deep manhole is usually a vertical shaft or opening in the ground that gives entry to underground utility systems like sewage or storm water drainage systems. Manholes are built so maintenance workers can check, fix, or clean the area. The depth of a manhole can vary depending on the specific purpose and location. In urban areas, manholes are generally not extremely deep, typically ranging from a few feet to around 20 feet deep. However, in certain situations, such as in areas with complex underground infrastructure or larger industrial settings, manholes can be much deeper, sometimes exceeding 50 feet or more.

Safety is of utmost importance when working in deep manholes. Workers must follow specific protocols and use proper protective equipment to mitigate the risks associated with confined spaces, such as poor air quality, limited visibility, and potential hazards like moving water or equipment.

3. Normal Manhole:
A normal manhole, also known as a utility hole or maintenance hole, is a covered opening in the street or sidewalk that provides access to underground utilities such as sewer lines, storm drains, or telecommunications infrastructure. Manholes are typically made of cast iron, concrete, or a combination of materials. The main purpose of a manhole is to allow workers to enter the underground infrastructure for inspection, maintenance, or repairs. They are usually round or rectangular and have a removable cover that can be lifted to provide access. The covers are designed to be heavy and secure to prevent unauthorized access and to ensure the safety of pedestrians and vehicles passing over them.

Manholes are an essential part of urban infrastructure, allowing access to underground systems for various purposes. They often have ladders or steps to facilitate entry and exit for workers. Manholes are typically marked with warning signs or symbols to alert people to their presence and to prevent accidents.

Maintenance of Manhole:
Maintaining manholes is crucial to ensure sewer systems’ proper functioning and preventing potential hazards. Here are some guidelines for manhole maintenance:

(i) Regular Inspections: Conduct routine inspections to identify any signs of damage, deterioration, or blockages. Inspect the manhole cover, frame, walls, and base for cracks, corrosion, or structural weaknesses.

(ii) Cleaning: Remove debris, sediment, and any obstructions from the manhole to prevent blockages and maintain the wastewater flow. Use appropriate tools, such as vacuum trucks or high-pressure water jetting, to clean the interior of the manhole.

(iii) Manhole Covers and Frames: Check the condition of the manhole cover and frame. Ensure they are secure and in good working condition. Replace any damaged or missing covers to prevent accidents or unauthorized access.

(iv) Sealing: Inspect the seals around the manhole cover and ensure they are intact. Replace damaged or worn-out seals to prevent the entry of surface water, gases, or odors into the sewer system.

(v) Repairs: If any structural damage is identified, such as cracks or fractures in the manhole walls or base, repair them promptly. Follow proper repair procedures, which may involve applying coatings or patching materials to restore structural integrity.

(vi) Odor Control: Manholes can emit unpleasant odors due to the presence of gases. To minimize odors in residential spaces, consider implementing odor control techniques, such as installing odor-reducing inserts or chemical treatments.

(vii) Vegetation Management: Regularly trim or remove vegetation near manholes to prevent root intrusion. Tree roots can penetrate manhole walls and cause blockages or structural damage.

(vii) Safety Measures: Ensure manholes are adequately secured with locked covers to prevent unauthorized entry. Provide appropriate safety signage and markings to warn pedestrians and drivers about the presence of manholes.

(viii) Record Keeping: Maintain a comprehensive record of manhole inspections, cleaning activities, repairs, and maintenance schedules. This information can help track the condition of manholes, identify recurring issues, and plan future maintenance activities.

Conclusion:
Manholes are essential to our infrastructure, providing access to underground utilities and facilitating maintenance and inspection. They symbolize the hidden systems that support our daily lives, and advancements in technology continue to improve their functionality. With proper attention and investment, manholes will continue to play a crucial role in our evolving urban landscape.

References:
1. Steffen, J. (2022, May 6). What is Manholes – Importance, Purpose of Manholes, Types of Manholes – ConstructUpdate.com. ConstructUpdate.com. https://www.constructupdate.com/what-is-manholes-and-its-types/
2. A. (2020, December 31). 3 Different Types of Manholes Used for Different Applications – Crescent Foundry. Crescent Foundry. https://www.crescentfoundry.com/blog/3-different-types-of-manholes-used-for-different-applications/
3. Types of Manholes | Sewer Appurtenances. (2023, January 17). PRODYOGI. https://www.prodyogi.com/2023/01/types-of-manholes-sewer-appurtenances.html
4. Manhole Construction | How to Construct a Manhole. (n.d.). Manhole Construction | How to Construct a Manhole. https://www.constructioncost.co/manhole-construction.html
5. Drainage, K. (2023, March 1). What is a Manhole? KD Drainage. https://www.kent-drainage.co.uk/what-is-a-manhole/
6. Desk, H. N. (2022, December 20). Sewer Manhole: Types, Purpose, and Materials Used. Housing News. https://housing.com/news/how-does-a-manhole-operate/

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