Silverstrata has a point re building on sand. The Rio-Antirrio bridge across the Gulf of Corinth is built on a level gravel/sand base. There are no piles through this as the Gulf is increasing in width by about an inch a year and therefore the bridge has to expand and move to accommodate this. The area is prone to earthquakes and since it's opening the area has suffered a quite severe earthquake with no damage to the bridge. The sand would have to be contained by a sturdy perimeter wall. In areas of very high load it would be possible to use rock etc for the island formation. Therefore by careful planning a mixed sand and rock island could be built. Areas between the runways could be sand based as well as possibly some of the terminal buildings (depending on there design). Whilst areas of higher loading could be built on rock. It would be expensive but with good design it would be possible.