Why are the covers of sewers mostly round?

The roundness of the sewer cover is mainly determined by the circular characteristics. Since any diameter of the circular cover is not smaller than the circle in which it is located, the cover of the sewer will not fall from the wellhead in any case. Any side of the square cover is shorter than the diagonal of the square opening where it is placed, so the cover may fall obliquely when it is upright. Therefore, in order to avoid the situation that the cover of the sewer falls off, people design the wellhead and cover of the sewer into a circle.