I’m putting together a bid on a restaurant TI where they want to build a raised concrete floor, and am wondering if some out there with experience in this can give me some advice.  Here is the scenario:

This is ground floor of a multi-story building.  The slab is 3′ thick in some places and heavily reinforced.  There is quite a bit of plumbing involved, and the plumber, after trying unsuccessfully to bury the waste lines in the slab, has laid all the lines (ABS) out on top of the existing slab, supported by blocks of p.t. lumber.  The architect has designed a raised floor system with ramps, etc. that will incorporate the plumbing lines within the 1′ raised portion.  Unfortunately, the architect has not indicated how this raised floor is to be constructed.  My thought was to bring in drain rock to 8″ deep, lay 6 mil plastic over the rock and cover it with a 4″ concrete slab to bring it to the finished floor elevation of 12″ above the existing slab.  My thinking is that the drain rock will be self compacting and, when covered with the membrane, will be a stable base for the new slab.  One of my fellow GCs insists, however, that sand is the way to go, wetted and compacted in lifts.  Seems to me this is a lot of extra labor and $$. 

Anybody have a better suggestion, helpful tip, criticism, etc.?  Thanks.