Dear Colibrita,

1)      A square cover 1 metre x 1 metre has an area of 1 square metre.

A circular cover of 1 metre diameter has an area of .5 metre x .5 metre. X 3 and 1/7, which is slightly more than � of a square metre.

So a circular cover is smaller, lighter and cheaper.  It is easier to lift.

2)      A square cover has a frame with sides 4 x 1 metre = 4 metres.

A circular cover has a frame with a side 1 metre x 3 and 1/7, which is 3 and 1/7 metres long.

So a circular cover has a smaller frame than a square cover, making it smaller, lighter and cheaper.

The cement fillet around the frame is smaller, it needs less grease to seal the frame, and the possibility of sewer gases escaping is reduced.

3)      As someone else has remarked, a circular cover overlaps the frame by a small amount, so the circular cover, turned on its side, cannot fall into the hole of the frame.  However, a square manhole frame, 1 metre x 1 metre, has a hypotenuse of approximately 1.4, (thanks to Pythagoras for the formula).  The 1 metre x 1 metre cover can easily fall through this opening into the manhole below.

4)      A manhole cover is usually made of cast iron.  A circular casting is easier to pour, and there are fewer failures, because the molten metal does not have to flow into the awkward corners of a square casting.

5)      If a heavy load, a moving lorry for example, rests on a square manhole cover in the road, the weight can be unevenly distributed, causing cracking.  This is because of the asymmetrical square shape.  Because the circular cover is symmetrical, the weight is more evenly distributed, and this problem is reduced.

6)      A building labourer can �roll� a circular cover into position, but not a square cover.

Les