The first cover below the hole is round, because the cylindrical can withstand the pressure of the surrounding land. Moreover, the access to the drain means to leave enough space for a person to pass, and a person who climbs down the ladder is essentially round. So the circle is the shape of the drain.
Some square holes will also see the square of the lid. This covers the entrance than large, there are around. Usually the lid is made of metal, very heavy. We can imagine that a square hole two feet wide, 1 to 1.5 inches wide. In order to fall into the covers, the need to lift one end, and then rotate 30 degrees, so that it will not be rung hiders, and then covers a 45 degree angle with the horizon, the focus at this time was sufficient for the transfer of the covers off. And the circle has no such concerns.
From an engineering point of view, the shape of a manhole cover to cover it completely depends on the shape of the opening.
In order to withstand the load from the uncertain direction. Just think, if the sewer cover is square or other with the shape of the edges and corners, when there is the impact of the wheel load on it, the edges and corners of the site is very easy to damage. The inspection well of the sewer is made into a circle, and it is also out of the force to consider that the arch structure can effectively resist the earth pressure.
However, as the area of the circle in the circle of equal perimeter is the largest, the material used is the most. So, from an economic point of view, some places have been designed using other shapes of manhole cover.
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