[Greenbuilding] basement help

[BirdWalk]

On the next house I would like a basement - a DRY basement.  Also on the
wish list is to have a larger than the house basement - sort of a fall out
shelter - tornado shelter - root cellar deal.  Since this would be out
beyond the house I need ideas on structure of the roof for strength to hold
up a few feet of dirt and real importantly not leak.  I figure a minor slop
will keep 'water' from standing on the roof.  Drain tile al the way around
the basement walls running off to a lower than the basement ravine. What I
want is suggestions for the roofed area and dryness.  I understand tar will
break down over time.  what about hydraulic cement?  Other sealers?  How
much gravel underneath the slab? Tiles under the slab?  Water barrier under
the slab under the gravel? What about potentially heating the 'living space'
floor.  We had considered 4 inch drain pipe partially into the gravel and
partially into the concrete for warmed air circulation.  Thoughts?