Introduction
Manhole covers are an essential but often overlooked part of modern
infrastructure. Designed to provide access to sewer systems, water lines,
and electrical vaults, they also ensure safety for pedestrians and vehicles.
However, manhole cover failure can lead to accidents, traffic disruptions,
and costly repairs. Understanding why manhole covers fail and how to prevent
it is crucial for municipalities, contractors, and infrastructure planners.
Common Causes of Manhole Cover Failure
1. Corrosion and Environmental Exposure
Corrosion is one of the most frequent causes of failure, especially in
areas with high humidity, salinity, or chemical runoff. When manhole
covers are not properly treated or coated, exposure to the elements can
degrade their metal over time.
2. Improper Installation Practices
Even a high-quality manhole cover can fail if it’s installed incorrectly.
Poor leveling, misalignment, and lack of bedding material can cause
uneven load distribution, leading to cracks and early failure.
3. Overloading and Load-Bearing Limits
Every manhole cover is designed for a specific load-bearing capacity,
classified under ratings like A15, B125, D400, etc. Using a low-rated
cover in a high-traffic zone can cause it to collapse or crack under
pressure.
4. Material Cracking and Fatigue
Over time, materials can degrade due to repeated stress, temperature
fluctuations, and traffic vibrations. This leads to manhole cover
cracking, a common form of fatigue failure that compromises safety.
5. Vandalism and External Forces
Intentional tampering or unintentional damage from vehicles and
construction activities can displace or damage manhole covers. Loose or
missing covers pose immediate dangers to vehicles and pedestrians.
Understanding Load-Bearing Capacity
Each manhole cover is rated based on its load-bearing class, typically
following international standards such as EN 124. Here's a quick overview:
| Class |
Load Capacity |
Recommended Use |
| A15 |
1.5 tonnes |
Pedestrian areas |
| B125 |
12.5 tonnes |
Residential driveways |
| C250 |
25 tonnes |
Light traffic areas |
| D400 |
40 tonnes |
Roads and highways |
| E600 |
60 tonnes |
Industrial zones, ports |
| F900 |
90 tonnes |
Airports, extreme load zones |
Incorrect use of class ratings is a leading contributor to premature manhole
failure.
Manhole Cover Maintenance Best Practices
To extend the life of manhole covers and reduce failures, regular manhole
cover maintenance is essential.
1. Routine Cleaning
Clearing debris prevents water pooling and reduces corrosion. It also
ensures the cover fits snugly and doesn't become a hazard.
2. Anti-Corrosion Coating
Applying protective coatings or using corrosion-resistant materials like
ductile iron helps extend the product's lifespan.
3. Lubrication of Hinges and Locking Mechanisms
For hinged or locked covers, lubrication prevents seizing, ensures proper
sealing, and allows safe access when needed.
Manhole Cover Inspection Checklist
A structured manhole cover inspection checklist helps identify issues before
they become failures:
1. Check for visible cracks or chips
2. Ensure the cover sits flush with the frame
3. Look for corrosion, rust, or pitting
4. Inspect for signs of displacement or a loose fit
5. Confirm markings and class rating visibility
6. Test the frame and fasteners for stability
Documentation during inspections aids in maintenance scheduling and safety
audits.
Recommended Inspection Frequency
How often should manhole covers be inspected? It depends on the location,
traffic, and environmental exposure.
Suggested Frequencies:
High-traffic urban zones: Every 3 months
Industrial/commercial areas: Semi-annually
Low-traffic residential zones: Annually
Post-extreme weather or construction: Immediate inspection
Smart sensors and IoT monitoring are emerging as tools for real-time
condition tracking.
Preventing Manhole Cover Failure
1. Select the Right Load Class
Ensure the manhole cover is correctly rated for the environment. For
highways, use D400 or higher.
2. Use Ductile Iron Over Grey Iron
Ductile iron provides better strength, flexibility, and resistance to
cracking under stress then Grey Iron.
3. Install Properly With Supportive Bedding
Levelling the frame, using the right mortar, and sealing the edges are
key to a stable manhole cover installation.
4. Maintain a Maintenance Log
Track each inspection, cleaning, and repair to ensure accountability and
early issue detection.
Conclusion
Manhole cover failure isn't just a maintenance issue - it's a public safety
risk. Fortunately, most failures are preventable through routine inspection,
proper installation, and material selection.
By following load-rating guidelines, adopting preventive maintenance
practices, and using advanced materials like ductile iron, infrastructure
managers can reduce risks, cut costs, and protect both people and property.
FAQs
1. What causes manhole covers to fail most often?
Corrosion, overloading, improper installation, and material fatigue are
the leading causes.
2. How do you prevent manhole cover cracking?
Use ductile iron, install according to load specs, and inspect regularly
for early signs of stress.
3. What is the best inspection frequency?
High-traffic areas need quarterly inspections, while low-traffic zones
can be inspected annually.
4. Are there standards for manhole cover strength?
Yes, covers follow EN 124 and other regional standards that specify
load-bearing capacity.
5. What should I look for during an inspection?
Check for cracks, displacement, rust, class label clarity, and ensure the
cover is flush and stable.
