dear frnds all of ur explinatins are praise worthy bt the main reason fr a circular cover of mainholes is that every where the drainage system or the passage ways fr certain purposes are made in round shapes and to cover their surfaces one have to provide such protection which should be durable and convenient,
Thats why circular shapes fit it more feasibly coz circular shapes lids or covers never fall into the circular holes because of its geomatery.

So simple and logical.

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