Surely, more than once, you have noticed, while walking down the street that the vast majority of the manhole covers that you have passed in front of, were round. Possibly you had never asked yourself the reason; in the current post we are finally going to discover what is behind that mystery, whose answer has a lot to do with math.
A manhole cover is used to protect the access in an inspection chamber; these are often found in sewage and public utility systems. The manhole cover must be robust, to withstand the passage of people and vehicles, while, at the same time, it must allow workers access for inspection and maintenance.
The mathematical reason that can explain this shape is very simple: prevent the manhole cover from falling down the hole. Some of these covers can weigh up to 90 Kg. The material of construction of the manhole covers is determined by the location and type of traffic to which it will be submitted. There are galvanized steel, ductile cast iron, stainless steel, aluminum and even concrete. However, the continued theft of this type of manhole covers for the subsequent sale by scrap in the black market has forced municipalities to change them by other materials «less appetizing» for these thieves, like the composite or polypropylene.
If we bear in mind the high weight that manhole covers reach, which periodically for maintenance reasons an operator must raise to access the system. Do you imagine the problem that involves sliding the cover two or three meters down the inspection chamber damaging the concrete or the installed manhole steps having to raise those 90 kg back to the surface?
Let’s see the mathematical reason behind it:
If we call the width of a plane figure at the distance between two opposite points of the edges, it is true that the circle is of constant width, since the distance we can find between two opposite points of its edge circumference) is always the same, the diameter. This implies, in our case, that if we put the lid in the hole of the sewer, it is impossible that the lid enters through the hole.
If the manhole cover were square or rectangular there would not be a constant width since the diameter between opposing vertices is greater than the distance that exists between contiguous vertices. The scientist George Freederick, in the magazine Livescience illustrated with a simple image this concept.
Although the vast majority of the covers are round, there are also square or rectangular covers. These usually protect small chambers used to receive, bond and distribute pipes or underground pipes. They are used in sanitation, drinking water and irrigation networks, being able to house the keys of cut of buried nets. They are also used in electricity distribution networks and other wired services, such as telecommunications. Usually these chambers are of a very reduced height, reason why if it falls or slips the cover will not have problem in recovering it.
If you want to know more in depth the severe problem that exists with the theft of sewer covers, such as the recent theft of 180 log covers happened in Gijón (Spain), visit our last post, where we also talk about FUNDILOCK, the unique and patented solution of IVERNA to prevent the theft of manhole covers and the theft of copper and cable that is inside.