Drainage cover the following hole is round, because the cylindrical can best withstand the pressure of the surrounding land. Moreover, a sewer hole to set aside space enough for a person adopted. And a man down the ladder to climb down a cross-section is substantially circular. So naturally become round manhole shape. Circle covers just to cover a circular hole.

But sometimes in some square hole you will see a square lid. This cap than the big entrance, surrounded by rails, this normally Drainage cover is made of metal, very heavy. We can imagine, square hole two feet wide, 1 to 1.5 inches wide ledge. In order to fall into the covers, we need to lift one end and then rotated 30 degrees, so that it is not hampered ledge, and then covers a 45 degree angle with the horizon, then shift the center of gravity was enough to make the covers fall. Square covers fall may indeed exist. But unlikely. As long as the person responsible for opening the covers a little training, he would not have made such a mistake. From an engineering point of view, the shape of the covers depends entirely on the shape it is to cover the hole.