thanks for the feedback, ya'll. Based on the opinions, I fear that I may have complicated the description of the situation. At the risk of needless reiteration (sorry, no offense), here goes. I know what is in the existing soil and grade. It is about 12" of loamy topsoil, then hard clay forever. We are going to scrape up some more of the same clay, about 100 feet away from within the pond basin, and push it up and build up over this existing grade. In the building area, I expect a thickness of about 2 - 3 feet of this clay fill, compacted with the weight of a D-6. Within 1 - 2 months of placing this fill, I want to set a foundation: either a slab on top of this fill OR drill piers down to the original existing "hard clay forever".
Now, there are a couple of engineering notes that I should add. I have access to plenty of rebar, should I go with the slab...but... I am still leary of a long term, stable bearing surface based on new fill...it's gonna settle, right? Yes, the slab supported by the piers would be best, but even then, when the fill settles I don't want a structure that supports like a concrete parking garage. I lean toward exclusive use of the piers, combined with stout floor framing...kinda like a stilt house. Theo mentioned potential issues with critters underneath. Man, I know that deal. I just sold a vacation place that was built around a 14' x 70' mobile home...my wife called it the Mouse-house (thank God the dollar store sells mouse traps). Anyway, to address that potential headache on this new project, I plan to apply house wrap to the underside of the framing (for insect barrier) and then over that a layer of lite aluminum or galvanized sheeting (critter barrier). Then, standard mobile home skirting around the perimeter. Naturally, the flooring "sandwich" will be stuffed with fibreglas insulation.