Let’s start with the definition of manhole cover. It is nothing more than the cover that is placed over manholes, through which one may inspect, when necessary, what is underground (sewerage distribution networks, cable routing, aqueducts, etc.).
Manhole covers come in different shapes and materials, but when it comes to choosing which type to use, one must take into account mechanical strength, durability and the net clearance of the opening.
When we talk about mechanical strength, we are referring to the class that characterises each manhole cover. The choice depends on the area of use, which is divided into 6 groups according to the EN 124 reference standard. Each group is assigned a resistance class that must be complied with:
With regard to durability, one must consider many aspects, including the material of the manhole, the area in which it is to be installed, the temperatures and fluctuations to which it is subjected, and the frequency of traffic.
The clear opening is the diameter of the largest circle that can be inscribed in the net cross-section of the frame, basically the net clearance of the manhole. This value is normally unique for all manhole cover manufacturers. The element which is not subject to particular specifications is the outside dimension of the cover and above all the outside dimension of the frame. Therefore, each manufacturer may have different thicknesses and outside dimensions of the cover, making it impossible to interchange the covers.