Less Likely to Fall Off
The most significant characteristic of a circle is that its radius is constant. Therefore, when a round manhole cover is placed over a manhole, it won't fall to the bottom. If the manhole cover is rectangular, its width is not fixed. Since the diagonal of a rectangle is longer than any side, the cover could potentially fall to the bottom.
Less Likely to Be Damaged
A circle distributes force evenly and can withstand loads from uncertain directions. When a vehicle drives over it, the cover remains flat. Other polygonal shapes with sharp edges are not only prone to warping and causing accidents, but their edges are also more easily worn down.
Easy to Move
Round manhole covers are easy to move. Because they are heavy, large metal blocks, if a cover is damaged or needs replacement, maintenance workers can simply roll it aside.
