John Harper wrote:

I've often thought that a good way to deal with runway congestion
at places like Heathrow where expansion is just about impossible
would be double-decker runways.

That would be a *very* interesting engineering exercise. The ramps at Hartsfield are
6' thick concrete slabs. I don't know how much thicker the runways are. I'm also
certain that they would have to be significantly thicker if they weren't supported by
the ground underneath them. Concrete weighs over 3,500 pounds per cubic yard. The
upper runway will be an unsupported clear span several hundred feet wide and about a
mile long. I'm not sure there is any material on Earth that could handle that job. I
think it safe to say that the upper runway wouldn't be concrete, though.

George Patterson
This marriage is off to a shaky start. The groom just asked the band to
play "Your cheatin' heart", and the bride just requested "Don't come home
a'drinkin' with lovin' on your mind".