Because one of the major problems with my original proposal for an airport in lake Michigan was that it would sink just like the one in Japan, and because it would be difficult to find dirt to fill in the airport's underwater volume, a friend of mine and I have contrived the following idea...
The airport island would be mostly hollow. A vertical-walled steel box would sit inside the island, taking up most of it's interntal volume with about twenty feet of soil on top and five on the bottom, and then the lakebed. The interior of the "box" would be eighty-five feet tall. Every hundred feet mirrored-pyramid steel beams would support the upper layer (a triangular pyramid of steel beams going half-way up, mirrored by an upside down one on top). The result would be that the entire weight of the airport's top mass would be transferred through these beams to the lake bed, but the air in place of water would make the overall weight considerably lighter, meaning that the island not only wouldn't sink, but that it would virtually float. Heavy curtains of soil on top of and around the sides of the well-secured air box, plus it's own weight, would ensure that this wouldn't actually be possible, and the final dimmensions of the container would depend largely on that risk.
The "box" (called a Massive Underground Load-lightening Atrium, or "MULA") would be built in phases:
1) A seawall would be constructed around the perimiter of the airport
2) The area within the seawall would be drained
3) The steel floor of the box would be constructed
4) The six-pronged mirrored-pyramids would be welded into place
5) The roof would be welded into place
6) The side seawalls would be built up (thickened)
7) The upper layer of soil would be placed
8) Construction of the airport's facilities would proceed as normal
So... I need all the thoughts you can give me on why this wouldn't work or how to make it better. Thanks!