In the heart of our urban landscapes, manholes serve as crucial access points to the labyrinth of underground utilities. But what is a manhole exactly? In this comprehensive guide, we will delve into the world beneath our feet to explore the purpose, design, and history of manholes in detail.
Defining a Manhole
A manhole, also known as a utility hole, maintenance hole, or sewer hole, is a vertical underground access point designed to facilitate the inspection, maintenance, and upgrading of public utilities. These utilities encompass a variety of services, including water, sewers, electricity, telecommunications, gas, and storm drains.
The Purpose of Manholes
The main function of a manhole is to provide maintenance personnel with an entry point into the underground utility system. They are critical for inspecting, cleaning, and removing any obstructions in the sewer lines. Beyond this, these access points also aid in the joining of sewers and facilitate changes in sewer direction or alignment. Manholes also serve a vital role in the ventilation of underground sewage systems, thanks to their perforated covers that allow foul gases to escape.
Components of a Manhole
A typical manhole consists of several parts, each playing a crucial role in its overall function. Key components include:
- Inlet/Outlet: These are the entry and exit points for the flow within the manhole
- Channel/Invert: The channel is located at the center of the benching. It is the lowest part of the manhole, specifically designed to allow waste to flow by gravity to its intended location
- Bench/Table: This flat concrete surface is designed to provide a stable standing area for maintenance crew within the manhole, above the flow of waste
- Shaft/Chamber: Located below the manhole cover, the shaft leads down to the base of the manhole, providing access to the underground utilities
- Cover: This is the removable lid that prevents unauthorized access and accidental falls into the manhole. It is traditionally made of metal, but can also be constructed from precast concrete, glass-reinforced plastic, or other composite materials
Types of Manholes
Manholes can be categorized into three types based on their depth: shallow manhole, normal manhole, and deep manhole.
- Deep Manhole: Any manhole with a depth greater than 150 cm falls into this category. These structures usually have a heavier cover and enhanced access facilities
- Normal Manhole: These have depths of around 150 cm and often take a square shape
- Shallow Manhole: These are typically between 75 to 90 cm deep and are usually located at the beginning of a branch sewer or in areas with less traffic
Manhole Construction Materials
Manholes can be constructed from a variety of materials, each with its unique benefits and drawbacks:
- Plastic Manhole: Manufactured using polyethylene material, these manholes are both durable and environmentally friendly. They resist corrosion and require minimal maintenance over time
- Precast Concrete Manhole: This is the traditional method of constructing manholes. The units are manufactured offsite in a factory and then assembled at the location
- Fiberglass Manhole: These manholes are engineered to include a barrel and a cover. They also incorporate additional features like grinder channels, weirs, flumes, and separation units for stormwater
Manhole Cover Design
The design of manhole covers is primarily focused on preventing accidents and unauthorized access. While most manhole covers are round, they can also be rectangular or square. The round shape is favored as it is easier to manufacture, move by rolling, and can’t fall into the opening. The material of the covers can vary from metal to precast concrete, or even composite materials.
Placement of Manholes
Manholes are strategically placed in locations where they provide optimal access to the underground utility network. They are typically installed where there are changes in the sewer line’s direction, grade, invert level, or at the intersection of sewer branches. While they are prevalent in urban areas, they can also be found in rural and undeveloped areas where services such as electricity and telephone are carried underground.
The Evolution of Manholes
Manholes have come a long way from their initial inception in the mid-19th century. While originally constructed from bricks, manholes are today primarily built using concrete, plastic, or fiberglass. Precast manholes, which are constructed aboveground before being lowered into a dug-out space, have also become increasingly common due to their ease of installation and durability.
Modern-Day Applications
In the modern era, manholes are not just access points for utility personnel. They are increasingly utilized for deploying inspection equipment like CCTV cameras and other maintenance tools. This shift towards automated equipment has improved efficiency and reduced costs, while also enhancing worker safety by reducing their exposure to hazardous conditions underground.
Manhole Covers
Manhole covers, which serve as the removable lids to manholes, are vital for preventing accidents and unauthorized access. Traditionally, they have been made of cast iron or concrete due to their durability, weight, and cost-effectiveness. However, with advancements in technology, newer materials like fiberglass, plastic, and composite have also been used, offering benefits such as reduced weight and improved resistance to corrosion.
Final Words
In conclusion, understanding what is a manhole helps us appreciate the critical role they play in maintaining our urban infrastructures. These access points ensure the smooth functioning of our underground utilities while keeping our streets safe and accessible. They play an important role in the drainage systme of the city.
