I would add to what was said before in that if you start at grade and cover it with earth, you would have fewer problems with water levels. See Owens house here:
http://www.instructables.com/id/How-to-Build-an-Earthbag-Dome/
You can see he started on the ground, and went up. This is way different than building a hole in the ground and having to deal with a shallow water table. All other water problems apply. Earth wicks moisture both up and down and percolating rain run-off goes down. There is no air down underground to dry things out so they stay moist a long time and are added to at the next precipitation event.
R
__._,_.___
