I am interested in learning more about what frost does to footings and have questions. Here in central Minnesota we are required to have the bottom of footings 42" down, undoubtedly the frost gets deeper at times.

How much frost has to get under a footing to heave it?

The short answer is yes, any frost getting under a foundation can heave it.

In actual practice, heated structures (built to proper frost depth) rarely get heaved because of the heat transmitted.

Frost heave has more to it than just freezing temperatures. Other factors are the presence of water and certain soil types. I'm not sure what else.

You may want to investigate shallow frost protected footings.(see http://www.toolbase.org/Technology-Inventory/Foundations/frost-protected-shallow-foundations) Widely used in Scandinavia, these have horizontal insulation layers just below ground level. Thus the frost does not get to the footings. Insulation is much cheaper than deep footings. These share some of the basic principles of heat conduction with passive annual heat systems; you can learn about it here; http://earthshelters.com/. Just as concrete stores thermal energy, so does the earth. The heat is conducted a certain distance over time. during the summer the soil warms up. By providing a horizontal protective layer, the cold winter's freezing effects don't reach the footings.

I was just going to suggest the same thing, SFP footing. Check the IRC, they are shown in there. For your area, it looks like your thickened slab edge would need to be somewhere around 16" thick, and vertical insulation r values of around 9. you also need horizontal insulation going out about 24-30", and it needs to be protected.
It is a great approach for walkout basements, otherwise you need to step your footings down 4' to have ground coverage. Adds a lot of $. You can pour a monolithic slab and do your footings all at the same time.

I am also curious about FPSF.  It seems it would be an effecient (material and most likely time wise) way to construct homes with a lower level but with an attached garage.  In our climate, the frost line sits a 6 feet therefore it makes for some pretty deep foundations walls and footings.  I cannot understand why this practice is not utilized more.  One has to consider that more foam is needed however foam is relatively innexpensive and in most norther climates here the frost line is fairly deep, there are less problems with insect burrowing into the foam.  Is there something logical or structural that I am missing?

I wonder about the long term durability of FPSF. I've seen what carpenter ants can do to XPS; the answer for foundations is to add a stucco coating which continues below grade. Why do proponents of FPSF think the XPS will escape ant or termite damage forever? What about movement of adjacent soils inlcuding settling on the XPS?

I do not have the IRC beside me, however I do know a certain level of protection is required for the horizontal insulation once it reaches 24" out from the slab edge. really cold climates it gets more tricky with the horizontal lengths, as well as the corner protection.  I think it is great for basement with a sloping lot and a walk out.  Termite and ants are not a big problem in our northern areas, although they can be an issue. I suppose a treated foam could always be used. I think Dow makes a version of XPS. 
THe foam should be horizontally placed on compacted soils, so settling should be a minimal issue. 
One thing to keep in mind, this is meant for use to support CONDITIONED spaces.  And the catch here is the earth below needs to be heated enough from the slab that is stays above freezing.  This goes against large amounts of below slab insulation. This prevents the ground from warming enough. I believe 1.5" XPS is the maximum insulation you can have below your slab with a heated space (60-something degrees yearly min) if I remember correctly.

Posted By Bob I on 25 Jan 2011 04:33 PM
I wonder about the long term durability of FPSF. I've seen what carpenter ants can do to XPS; the answer for foundations is to add a stucco coating which continues below grade. Why do proponents of FPSF think the XPS will escape ant or termite damage forever? What about movement of adjacent soils inlcuding settling on the XPS?

Borate-loaded EPS fixes the ant & termite risk to FSPS.  Copper Z-flashing over the stemwall foam can also do a real number on termite factors once sufficient copper has leached down the face of it (I'm not sure if that keeps ants off too.)

EPS more brittle than XPS but has otherwise good below-grade performance once you've mitigated the insect risk.   A Rastra style EPS bead-crete  or EPSCement non-structural insulating concrete with fiber reinforcement in the mix might also be a good choice for both stem wall insulation and lateral insulation.  It might be hard to guarantee the R-value in an uncontrolled DIY mix but it's not like the recommended Rs in FSPS mean much once your over R8.   They claim something like R1.5-ish per inch of thickness with this type of mix, but I haven't seen ASTM C 518 test data online.  (This is a popular material amongst some DIYers of a certain age, who remember the '60s fondly, if through a bit of a haze...)

We have done some frost protected shallow footings. But I am more interested the what happens when frost does get under the footings. Is frost an unstoppable force, that any amount of frost will do damage? Is the weight on the footings a factor? So many structures are built here in the winter, with basement footings not protected.

Is the weight on the footings a factor?

Not really. Very difficult to stop the expansion of ice. The force is measured in hundreds of megaPascals - like 20,000 or 30,000 psi IIRC.

There is a process. First, you need a freeze, then you need water to "add" to the freeze layer, then another, usually deeper freeze and so on. The more the process is allowed to go on, the more the amount of heave.

We've been using insulated raft foundations for up to 10 years now in Ireland, we always put a drain all around the raft to keep it dry and we usually use 8 inches of 1-2inch stone with no fines under the insulation to create a capillary break. You can see some of the construction details we use here http://viking-house.co.uk/passive-slab-insulated-foundations.html

I presume that is the "150-200 compacted 18-35mm hardcore"?

Those slabs are pretty well insulated, so does that mean frost (and the resultant heave) can be allowed to get under that without ill effect? Is there a mechanical benefit to the raft other than the "capillary break"?

Well there's no water around the foundations because of the drain and the capillary break beneath the slab. No water, No Ice!

Deeper. Maybe in the undisturbed soil?

As you know, some soil types can draw water from some distance or underneath. I'm guessing, but you must have put some of these up in areas that are on the line between deep seasonal frost and permafrost.

These foundation systems have been used all over Scandinavia for the last 30-40 years, that's all they use, you could use wing insulation all around the foundations as an extra precaution.

So many structures are built here in the winter, with basement footings not protected.

Well, there you have it. I think the incidence of frost heave in heated buildings is nearly zero. Accepted building methods, including drainage strategies, insulation and, of course, putting footings below the "frost line" work against frost heave.

Are you worried about footings in an unheated open construction that is in process during extreme cold temperatures?

During construction is most of my concern. For example, my own place is a slab on grade with 4' frost walls on 10" thick footings. No heat, no snow, but plenty wet. How deep is this frost, deep I bet, cold here in Minnesota.

We did a concrete floor system, and the shoring is still in place (on frozen ground). Will the small pads under the shoring lift the concrete floor system? No evidence of it.

What about with a frost wall with an insulated slab and ICF frost walls. The building heat is not helping much to keep the frost away. How about in a cold year? What about a non climate controlled building?

If you are on bed rock nothing to worry about for eave. This is why you see winter construction with no protection on the footings.

In past winter I built myself a shop and had no heat for the first winter and tyvec in the overhead doors.
The shop is on a shallow footing with eave protection on the outside. On the inside I had layed 4' of 2'' eps around perimeter to eliminate frost. I removed a couple sheets to see the result and could to my surprise, a huge difference.

Forgot to mention no concrete floor was poured at the time.