It is not uncommon that manhole explosion occurs nowadays. Manhole covers are dislodged from the frames which is associated with a release of energy. Manhole explosion occurs mostly owing to the ignition and combustion of flammable gas. Sources of flammable gas include the followings:
(i) Natural gas as a result of leakage of nearby gas line;
(ii) In sewer manholes, it is rich in methane which tends to accumulate
inside manhole;
(iii) Gas generated by degradation of cable insulation.
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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.
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Gases are one of the most common reasons. This is a very common problem and engineers should take care of it while designing manholes and ducts.
The IEEE Industry Applications Magazine Sept 2014 published an article explaining the causes of manhole explosions, written by eng. Estellito Junior, an explosion prevention expert.