Cold climate house foundation question

Hi ,
I am getting ready to build a new bunk house on my ranch I just bought . I have noticed that quite a few homes under construction ,in the area seem to have the foundation area of their homes dug out about 3 feet into the ground . Than the foundation is poured and then they bring the dirt back in and place it around the outside of the foundation to bring it back up to grade level . Is this for insulation or freezing protection? Why don't the contractors just dig the concrete footing down to the 3 foot level so they don't have to move so much dirt ? I could understand it if a basement was going under the house but this way only leaves about 3 or so feet of head clearance in the crawl space .Any other ideas for cold weather building would be very much appreciated .
Big Al

In general, the bottom of the footing has to be below the frost line, or the level to which ground freezes. Around these parts, thats 4' below the surface. I have read about using styrofoam insulation to essentially 'lift' the frost line such as for slabs.

Plus, the bottom of the footing has to be below the topsoil and on undisturbed ground. You really don't want your basement below the water table, either, because that causes all kinds of problems.

The builder could dig all the way down, or just down below the topsoil and 'fill up' as near as I can understand it doesn't matter. Of course, part of excavation is getting rid of the material you dig out. So it could be to save cost (displosal costs can be quite high), to help with drainage (especially in an otherwise flat area you want to make sure the water flows away from the house), to stay above the water table (no leaky basement), or just plain old local tradition. For example, in many places foundations are always block, around here block is rarely used and foundations are poured.

Cleaning out also makes installation and removal of forms easier.

Egon

A crawl space has the advantage of costing less than a basement (less excavation and less cement), and still provides a wooden floor with space underneath for heat ducts, plumbing, wiring, etc. Wooden floor joists can't sit directly on the ground. Poured slab floors need heavy insulation underneath in cold climates or they lose a lot of heat in the winter. And basements are sometimes cost prohibitive.

The reason they are usually 3 feet or so deep is that 18 inches is just too darn uncomfortable to have to crawl through on your back to thaw a frozen pipe. You will also probably find that the depth of the crawl space will be determined by local ordinance.

Steve