A manhole is one of the most common civil engineering structures we see in daily life, yet most people never think about its importance. Those circular or square covers on roads and pavements are not just pieces of metal or concrete—they are gateways to essential underground networks like sewage, stormwater drains, and utility cables.

This blog gives you everything you need to know about manholes—their history, design, construction, types, safety, global standards, modern innovations, and frequently asked questions.

What is a Manhole?

A manhole is a vertical chamber built underground that provides access to sewer lines, stormwater drains, water pipelines, or utility cables. Its top is sealed with a strong manhole cover to ensure safety and prevent odours.

Historical Background

Manholes have existed since the earliest sewer systems. The Roman Empire developed stone-lined underground drains with access points, while modern cast-iron manhole covers first appeared in the 19th century during rapid urbanisation in Europe. Today, manholes are an essential part of any city’s infrastructure worldwide.

Functions of a Manhole

Inspection and Cleaning – Allows workers to maintain underground systems.
Ventilation – Releases trapped gases from sewers.
Change of Direction – Built wherever pipes bend or slope changes.
Joining of Lines – Where multiple sewers meet.
Emergency Access – Quick repair without large-scale excavation.
Flood Control – Ensures excess rainwater can be managed.

Types of Manholes

Based on depth:

Shallow Manhole (up to 0.9 m) – Found in low-traffic areas.
Normal Manhole (0.9 m to 1.5 m) – Most common on city streets.
Deep Manhole (over 1.5 m) – Requires ladders or steps for worker entry.

Based on location and use:

Sewer Manhole – For drainage and sewage inspection.
Cable Manhole – Used by telecom or electrical companies.
Stormwater Manhole – Built into rainwater drains.

Materials Used in Manhole Construction

Brick Masonry – Traditional, still used in many towns.
Reinforced Concrete (RCC) – Long-lasting and strong, ideal for highways.
Precast Concrete – Ready-made units for faster installation.
Polyethylene/Plastic – Lightweight, resistant to corrosion, eco-friendly.
Composite Manholes – New technology combining fibre and resin for high strength.

Manhole Covers

The most visible part of a manhole:

Cast Iron Covers – Heavy, durable, used in roads.
RCC Covers – Cheaper, for light loads.
Composite Covers – Theft-proof, lightweight, resistant to rust.

Why round? Round covers do not fall inside, are easy to roll, and distribute load evenly.

Design Standards

To ensure safety and durability, manholes follow certain standards:

Size: Opening between 56–90 cm for worker access.
Steps: Provided if depth exceeds 1.5 m.
Strength: Covers must withstand traffic load (light, medium, or heavy duty).
Ventilation: Covers often have holes to release gases.
IS Codes (India): IS 12592 and IS 458 for manhole covers and concrete pipes.

Construction Process

Excavation – Digging to the required depth.
Base Slab Construction – Made of RCC for strength.
Wall Construction – Using brick or concrete rings.
Benching – Sloped floor to guide sewage.
Steps/Ladder Fixing – For deep manholes.
Cover Installation – Strong cover fitted at road level.

Maintenance and Common Issues

Blockages – Caused by waste, debris, or tree roots.
Gas Accumulation – Methane or hydrogen sulphide build-up.
Structural Damage – Cracks due to traffic load or poor construction.
Manhole Theft – Metal covers stolen for resale.

Regular inspection is necessary to avoid health hazards and road accidents.

Safety Precautions for Workers

Test air quality before entering.
Wear protective gear (helmets, boots, gloves, masks).
Use safety harnesses for deep manholes.
Ensure ventilation and lighting.
Always work in pairs with a supervisor.

Modern Developments

Smart Manholes: Fitted with IoT sensors to monitor gas, water levels, and theft.
Precast Fibre-Reinforced Plastic (FRP) Manholes: Lightweight and corrosion resistant.
Manhole Detection Systems: Used in traffic planning and road mapping.

Cost Aspects

The cost of a manhole depends on size, material, and location:

Small RCC manhole: ₹10,000 – ₹15,000
Medium-sized manhole: ₹20,000 – ₹30,000
Heavy-duty precast manhole for highways: ₹40,000+

Strange Cases and “Curses” Linked to Manholes

Manholes are not only about engineering—they have also been part of unusual events, accidents, and even superstitions. Here are some interesting cases:

1. The “Exploding Manhole” Cases

In cities like New York, London, and Mumbai, sudden manhole explosions have been reported. These happen when gases such as methane or hydrogen sulphide build up in sewers and get ignited by sparks from cables.

In New York (2004), more than 200 manhole explosions were reported in a single year.
In Mumbai (2018), a municipal worker tragically lost his life when a manhole cover burst open due to gas accumulation.

This is one reason why manholes are often seen as dangerous and cursed spaces by workers.

2. The “Manhole Cover Curse” in Japan

In Japan, manhole covers are considered lucky or unlucky depending on where you step.

Some towns have beautifully painted manhole covers with symbols of nature or culture, believed to bring good luck if you step over them.
In contrast, older sewage manholes are avoided at night, as there are myths that stepping on them brings bad health or bad luck due to the trapped underground spirits.

3. The Paris Catacombs Connection

Beneath Paris, many manholes lead directly into the catacombs. Local legends say that opening certain manholes at midnight invites curses from wandering souls buried underground. Urban explorers have often shared eerie stories of hearing voices while entering through manholes into the tunnels.

4. The “Missing Manhole Covers” Mystery

In some countries, especially China and India, manhole cover theft is a major issue. Stolen covers lead to open pits, causing accidents.

In Beijing (2004), over 240,000 manhole covers were reported stolen in one year. Locals called it the “Curse of the Hungry Roads” because so many accidents followed.
This has led to many municipalities shifting to composite or plastic covers, which have no resale value.

5. The Superstitions of Utility Workers

Sewer workers often have their own beliefs about manholes:

Some refuse to open manholes on Amavasya (new moon day), believing underground gases are strongest then.
In parts of Eastern Europe, it’s believed that saying prayers before entering a manhole keeps workers safe from the “curses of the underground.”

6. Manhole Number Superstitions in London

Londoners used to believe that if you step on three manhole covers in a row, it brings bad luck. This old superstition is still remembered today, especially among teenagers who avoid stepping on triple manholes for fun.

Why Do Manholes Have Such Myths?

Manholes represent the hidden, dark, and dangerous underground world—a place of gases, waste, and even death for careless workers. Since they are risky and mysterious, people in many cultures built myths, curses, and superstitions around them.

Manhole Construction

Constructing a manhole requires proper planning, engineering design, and skilled workmanship. The process usually follows these steps:

1. Site Preparation and Excavation

Mark the location where the manhole is required (at pipe junctions, changes in slope, or inspection points).
Excavate the soil to the required depth, ensuring the pit is wide enough for workers and concrete formwork.

2. Base Slab

A strong RCC (Reinforced Cement Concrete) base slab is laid at the bottom to provide a firm foundation.
The thickness depends on the manhole size and depth, generally between 150 mm and 300 mm.

3. Walls / Shaft Construction

Manhole walls can be built using brick masonry or precast RCC rings.
Walls are usually circular or rectangular in shape and lined with plaster for smooth finish.

4. Benching and Channels

The floor inside is sloped (called benching) to guide sewage flow smoothly.
Channels are formed to direct wastewater along the intended path, reducing blockages.

5. Steps or Ladders

For manholes deeper than 1.5 m, steps or metal rungs are fixed into the wall for safe worker entry.
In modern designs, FRP or galvanised ladders are installed.

6. Cover Slab and Manhole Cover

The top is finished with a cover slab, fitted with a manhole cover.
Covers are selected based on traffic load (light, medium, or heavy duty).

Manhole Maintenance

Manholes, like all utility structures, require regular maintenance to function properly. Poorly maintained manholes can lead to flooding, sewage leaks, and even accidents.

1. Routine Inspection

Manholes should be inspected every 3–6 months in urban areas.
Inspections include checking for cracks, water leakage, gas accumulation, and loose covers.

2. Cleaning

Desilting is carried out to remove sludge and debris.
High-pressure water jetting or suction machines are often used.
In older systems, workers may still manually clean, though this is being phased out due to safety risks.

3. Gas Testing

Before workers enter, gases like methane, carbon monoxide, and hydrogen sulphide are tested.
Ventilators or blowers may be used to clear the air.

4. Repair Works

Damaged covers are replaced immediately to prevent accidents.
Cracks in walls or base slabs are patched with cement mortar or epoxy.
Corroded steps are replaced with FRP or galvanised alternatives.

5. Record Keeping

Municipalities maintain a logbook for each manhole to record inspection, cleaning, and repairs.
This helps in planning preventive maintenance instead of emergency repairs.

Challenges in Manhole Maintenance

Worker Safety: Toxic gases and risk of drowning make it dangerous.
Theft of Covers: Leads to open manholes and accidents.
Urban Load: Heavy traffic damages covers quickly.
Flooding: During monsoons, blocked manholes cause road waterlogging.

Modern Maintenance Approaches

To improve safety and efficiency, modern cities are adopting:

Robotic Sewer Cleaners – Machines that clean without human entry.
CCTV Inspection Systems – Cameras inside manholes for condition monitoring.
Smart Sensors – Installed in manholes to detect blockages or rising water levels.
Composite Covers – Theft-proof and long-lasting.

Why Proper Construction and Maintenance Matters?

Without properly built and maintained manholes:

Sewage overflows can cause diseases and environmental pollution.
Flooded roads can disrupt traffic.
Accidents may occur due to broken or missing covers.
Repair costs increase when preventive maintenance is ignored.

Thus, good manhole construction and timely maintenance are essential for the smooth functioning of any city.

Global Practices

UK & Europe: Composite manholes are widely used for sustainability.
USA: Smart manholes are tested in smart city projects.
India: RCC and precast manholes are standard, with growing use of FRP.

Conclusion🎯

A manhole may appear as just a cover on the street, but it is a vital entry point into the underground world of utilities. From sewage management to telecom networks, manholes ensure smooth functioning of cities. With modern technologies like smart covers and precast systems, manholes are becoming safer, stronger, and more efficient.

FAQs on Manholes

Q1: Why are some manholes square and others round?
A: Round covers are safer for roads, while square ones are easier to fit in construction sites.

Q2: Can manholes flood during heavy rains?
A: Yes, if drainage is blocked. That’s why maintenance is essential.

Q3: Are manholes only for sewage?
A: No, they are also used for electricity, telecom cables, and stormwater drains.

Q4: How often should manholes be inspected?
A: In cities, ideally once every 3–6 months.

Q5: What is the life of a manhole?
A: RCC manholes last 40–50 years with proper maintenance, while plastic/composite ones can last even longer.