I'm not sure if this is the best place to post this, but I'd appreciate your comments.

I'm planning to set a drop-in airtub (36 x 66X18), but when the rough plumbing was done the drain pipe (2" abs) only has 2.5" of concrete over the pipe. The concrete is rebar reinforced (drilled into old slab) but nonetheless only 2.5 inches of concrete is covering the pipes. The drain is off to one side so the majority of the slab is 4" thick.

I am also planning to set the tub in Sand/Concrete mix and am wondering if I should raise the height of the tub podium and add another 1.5" of concrete under the tub and then do the mud base, or if the 2.5" slab will support the tub.