Scientific answer -Round manhole cover cannot fall through from its circular opening, whereas a square manhole cover could fall in if it were inserted diagonally in the hole. Circular covers don't need to be rotated or precisely aligned when placing them on the opening. A round manhole cover is easily moved and rolled
Re: Round manholes
by sushil verma on Nov 29, 2014 07:53 AM
Another reason is with circular manholes minimum material is required as compared to other shapes if area of man hole is constant.
Hence we do see square or rectangular manholes for shallower depths and round ones should be preferred for depths above 1.5 m to provide stability to the structure underground and for larger working area. Hope this explains.
1: the decision of whether round or react angle depends on the depth. Round shape is preferred for depths greater than 1.5 m round is more stable. Actually the shape is like a drum or dhol below the ground thus giving a larger area for the person inside to move at the waist level when he is standing 2: Round manholes are easier to clean as there aren't edges to hold dirt.