There have been recent reports in the news of exploding manhole covers. This post covers some of the common causes and options for prevention.
Cause: A combination of flammable gases and a spark cause an expansion of air volume that lifts the manhole cover in the air.
Prevention:
- Slotted manhole covers that allow pressure relief. (This will not work for sanitary sewers as it allows storm water to enter the sanitary sewers).
 - Inspection of wiring in areas below ground to assure the wires are in good shape. This is often done using infrared cameras.
 - Early detection of flammable gasses or liquids in the underground utility system.
 - Proper disposal of flammable liquids as hazardous waste.
 - Avoid using sparks or flames near manhole covers.
 
How it Works describes the circumstances for an exploding manhole.
The science behind exploding manhole covers - 2 min. 
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