Main reason manhole covers made circular, is that their production requires a minimal amount of material. Normal sewer manhole width is 600 mm, that is a circular hatch cover should have a diameter of 600 mm and a square - 600 mm in side, in such amounts square hatch area was 0.36 m2 and 0.25 m2 round all that 28% less.
The round cover is not only smaller in size and thinner, as it is uniformly distributed load, and therefore the strength requirements are reduced.
Rectangular or triangular covers may accidentally fall into the hatch to get them because of the great weight will be problematic, they could damage equipment or injure a person in the fall. Round lid does not fall through the hatch, suitable for its size. Sometimes manhole covers made in the form of Reuleaux triangle, which is also called circular triangle, a triangle also goes into the hole, as it has a constant width.
Round manhole cover is easier to install, it always falls into place. It is not necessary to rotate properly to accurately hit angles as it would with any other form of cap.
Round manhole covers have smaller length of the leg portions than rectangular versions and, therefore, reduces the risk of marriage. Cover with protrusions or recesses on the supporting parts are destroyed faster and clap.
It is arguable that the manhole covers round because so they can be rolled, not carry. In round covers have special support arms, so that the normal roll them still does not work, on the other hand, such a move the cover a few tens of centimeters actually much easier than rectangular.
Round manhole covers almost never subside, but the square is almost always subside. Finally, sewer shafts often have a circular shape, so it is logical for them to cover and make a round.