For inanimate, tough objects, manhole covers seem to find themselves in a lot of trouble. Or trouble finds them. They can explode, be stolen, or just weigh as much as 500 pounds, which makes moving them for repairs a project in itself. Manhole covers have been around since the time of the Roman Empire, but they are still a work in progress. Current designs for manhole covers are focussed on using lightweight, composite materials that are strong enough to take the stresses demanded by the rough job.
Throughout the year, summer and winter, different types of pressures and stresses create dangerous conditions underground. Smoking manhole covers are a common sight in urban areas, but it’s when they begin sparking that it’s time to watch out and get out of the way. Exploding manhole covers are not just urban legend but real life accidents waiting to happen.
Exploding manhole covers are not just an urban legend
In the winter, the salt used to keep roads clear of ice and snow mixes with the snow as it melts and drips into the manholes. This can combine with underground sewer gases to produce a highly explosive and dangerous mixture. Sometimes the dripping water can fall directly on an underground cable or electric wire, wearing away the protective coating. The exposed wire provides the spark leading to a dangerous explosion.
In summer, heavy electricity use due to air conditioning can stress underground electricity cables and wires, leading to sparking and even explosions. “Manhole events”, as utility companies call them, could be anything from smoke from a short circuit to a dangerous explosion, casing injures and even fatalities. The Consolidated Edison Company reported 3,369 “manhole events” in New York City in 2014. Of these, 32 were serious explosions.
Who’d want to steal a manhole cover?
A surprising number of mishaps can befall manhole covers. As well as explosions, they are at risk of being stolen. Manhole cover thieves are a very enterprising lot, and operate not only in the U.S., but all around the world. The effort of stealing a manhole cover that could weigh as much as 500 pounds is worthwhile, because they are sold at high prices for scrap metal. In the Indian city of Calcutta, well organized thieves stole more than 10,000 manhole covers within two months. They also stole the replacement concrete covers, for the iron rebar inside them.
Thefts can cost municipalities and local governments, who have the responsibility for providing replacement manhole covers. Covers made from composite materials eliminate this problem, since the material has zero scrap value.
It’s a tough job, but someone’s got to do it
Engineers are now designing composite manhole covers which weigh considerably less than the traditional cast iron ones. They also last much longer, typically for 30 years or more. They also significantly reduce the need for maintenance and replacements over the 30-year period.
Manhole covers made from composite materials are both lighter and stronger than metal or concrete ones. They weigh 63 to 85% less than cast iron manhole covers, which have a normal weight range of 100 to 200 pounds, or even more. Though lightweight, composite manhole covers have fully traffic rated versions that meet and even exceed current US DOT / highway weight limits. They can handle up to 30% more weight loading than the legal limits.
Composite manhole covers have many advantages
Fibre-reinforced plastics or polymer (FRP) is a durable and strong composite material widely used in the aerospace, automotive, marine and construction industries. FRP manhole covers are lightweight but more durable than traditional manhole covers.
FRP covers are also much more convenient, being easier to remove for repairs and regular maintenance work. In facilities like drainage networks, sewage systems, water treatment plants where regular access to sewage systems is necessary, they are a much better alternative than traditional metal covers.