Put in a foot of non frost susceptable soils before placing slab.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

Search for "frost protected foundation"; many references at this site amd elsewhere. Assuming the garage is heated, you need only to frost protect the perimeter, which can be addressed using subgrade insulation.

find out what the frost depth is, and then make sure your perimeter foundation extends to at least this depth. Also, find out from the local building official what the required minimum footing depth is.  With the type of soil you describe I recommend making sure that your perimeter footing goes at least to the frost line.

I also agree that you need a layer of non-frost susceptible material under your slab.

What bjb suggests is not necessary if the building is heated and you insulate the slab edge properly--that is, rigid insulation should extend down the outside face of the thickened slab, then turn 90 degrees and extend away from the building a sufficient distance to prevent the soil below the thickened slab from freezing.

DaveAtkins

Dave,
If the garage is heated you could probably get way with an insulated foundation.  I'm not comfortable with that approach however, and if it was my garage I would want the footing down to the frost line.  If the garage is heated, but at some latter point the decision is made to stop heating it, you have a problem.  This assumes a residential garage, not a commercial garage that would be normally heated.  I live in a cold area, so I guess that I am biased against using an insulated foundation above the frost line. In my area, footings almost always go down to the frost line.  

If you have to turn the insulation 90 degrees and extend it beyond the building line, you incur additional costs for excavation, backfill and compaction.  With a thickened slab at the edge and the type of soils described, I believe that the thickened part could be constructed with a trench pour, and then the earthwork costs would be minimized. Granted, having a deeper footing increases your concrete cost, but in my opinion you get a better finished product.  That's just my 2 cents.

In Massachusetts and most New England states you have to have a perimeter foundation wall below the frost line. Near Boston that is four feet. If heated there are specific requirements for foundation wall insulation. You can use the thickened slab only on small sheds. Talk to your local bldg inspector.

See "Design Guide For Frost-Protected Shallow Foundations" (free .pdf download) at this link
http://www.huduser.org/publications/destech/desguide.html

www.SlideRuleEra.net

I guess all the stand alone garages in Minnesota have frost heave? Because they have a thickened edge, 12-16 inches, a couple #4's and 3000 psi concrete.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

I bet that some do.  It's also a function of the type of soil.  If they are sitting on frost susceptible material, they will heave if not insulated.  How many of these buildings get a proper soils report, of have involvement with a knowledgeable soils engineer to determine if the native soils are acceptable?  Another point to consider is the consequences if heaving does happen.  If it's a relatively minor garage that is not attached to anything else, you can probably live with it.  If there are any utilities going into it through the ground you could have a problem though.  In my area, it is a requirement that footings have to be at least 4 feet below finish grade at the perimeter of the building. One municipality around here also requires that all interior footings be set 4 feet below grade too.