As planning continues for pond and building site development for 2006, this request for input relates to the building site at the edge of the proposed pondsite. The entire pondsite is surrounded by timber. The building site will be a notch into the timber at one shoreline on a decent uphill slope, see drawing and pix on this web page: website page We are going to construct a small cabin, likely 16' x 24', within this site and off to one edge. I note in the website that my plan is to use 9 piers of concrete as a foundation, placed near the rear of the site where the fill soil is thin enough for the piers to bear on undisturbed. Also, I would like to have this cabin elevated to allow a crawl for storage underneath. NOW THE QUESTION: Quite frankly, this really should be a plain old concrete slab (I can live without crawlspace storage). I would reinforce it with heavy mesh. My fear, tho, is that the fill soil will not be firm enough to minimize settlement issues. The fill will be a combo of a good clay base topped with topsoil.
a) normal equipment compaction of clay and topsoil will be fine?
b) in the slab zone, no topsoil and compacted clay will be fine?
c) forget it and stick with the bearing piers?

Brettski:

I believe CW would say put down a slab over concrete footers going down to nice, solid, virgin subsoil. This would obviously cost more than just a slab or your concrete piers approach, but would have the benefit of giving a vermin-proof floor for your cabin or, if you raise the cabin floor up, crawl space (so you never have to have a skunk teach you your floor is not impervious to methyl mercaptan!).

Perhaps if the topsoil were scraped off, the subsoil could be compacted sufficiently to put the slab on it directly with the likes of a vibrating shep's foot roller (they certainly do wonders for dam cores).

Unless someone here who is a LOT more confident about this provides input, you should really get the opinion of a good soil (engineer) or trustworthy local builder with local soil knowledge.

I would go with the piers, I am building a small house on the upside edge of my pond and am using them. I had a soils engineer out and he drilled a test hole that placed solid soil at 20' so I had a pile driver sink concrete pilings/piers and am putting the footing/foundation them now. Water runs through my soil almost as if the soil were not there, I would say like a river but, it's more like thousands of tiny streams. Dig any where on the property and within a foot or less I will hit water. In a couple of months that will move to 2'.
In your case the piers would also allow you to adjust for any soil shifting. It has always been my experience that clay expands when wet and again when dry, doesn't sound logical but, that's my experience.. The raised floor will cool you in both summer and winter. A slab is not always impervious to water either. The water will some times be drawn through the slab, hydro static action I think, and cause the surface to deteriorate.
If you decide on a slab look into a "mat foundation" or raft foundation.
www.usace.army.mil/inet/usace-docs/armytm/tm5-818-1/ch-10.pdf#search=’mat%20foundation’
Probably more than you need there but it might help. I built one of these myself and it was not difficult just expensive because of the rebar cost.

Brettski, A soil engineer (I work with a department full of them) would not give a professional opinion without at least one test boring. It doesn't take much to cause settlement (thin layers of peat, fat clays, and other materials can be a problem). Get a couple quotes and figure out if you are better off over designing the foundation or having the testing done.

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.

Either way if done well would work however, for long term use of this building as a living quarters, i would advise the piers. Main reason being is of course settlement, with the piers you would be able to use jacks to make any adjustments to the level of the building, it will also allow you to pumb thru the floor at a later time vs. tearing up the slab to put in a drain line or whatever. As far as critters go, I'd rather not be at their level "on the ground" also at a later time if you wanted to enclose it you could make good use of some of that timber you're cutting, all you need is a portable sawmill and there ya go. Once again, as I said earlier this would make for a better long term plan. :-)