NBCC - Can I get away with foundation above frost penetration?

[Serhiy2]

Good evening,

I'm working on a small residential building (cabin - Part 9) designing structural part of an addition. Building is located on the lake shore with soil being sand and existing foundation is concrete pads installed at grade level. I am intending to use the same foundation for the addition. I'm pretty sure that authorities will not mind me sealing foundation drawings with foundation installed above frost penetration depth but in case they do, I would like to have my defense ready. Frost penetration level in my part of Canada (Manitoba) is about 6~7 ft.

In my understanding the only 2 conditions that allow foundation to be installed above the frost penetration depth are the following (as per clause 4.2.4.4. sentence 2):
2) The bearing surface of a foundation need not be below the level of potential
damage from frost where the foundation:
a) is designed against frost action, or
b) overlies material not susceptible to frost action.

To be honest, I don't quite understand what is implied under case a: does it relate to measures which would prevent frost action such as controlling soil water content etc.? Case b is understandable - this is most likely my case as I have sand as bearing soil. One problem is that as per Canadian Foundation Engineering Manual, my soil can still be susceptible to frost jacking if its fine particles content is high. So in case someone questions my design, I would need to do sieve analysis to prove that soil is not frost susceptible or something like that I imagine.

I'm just wondering if there are other routes I can use to justify having foundation at grade level.

Thank you

 

[oldestguy]

For your case a, I think this means you insulate the ground as with Styrofoam sheets.

For case b. an approximate rule is that the percentage passing the number 200 sieve does not exceed 5%. Even then if you have a little more of the minus 200 material the heave action only gradually gets worse with more of those fines. An exceopthion is that heavy clays are so impervious that water doesn't move fast enough to be a problem.

 

[GeoEnvGuy]

I work in Ontario and where the water table is shallow over saturated clays we recommend thickened edge slab on grade, raft foundation.

The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.

 

[darthsoilsguy2]

to expand on oldestguy's comment about the foam.... There is a lot of discussion about Frost Protection of Shallow Foundations in "Structural Engineering and other technical topics" forum if you want keyword search there. Also googling "ASCE 32-01" will get you some good hits to review about this topic.

 

[OldBldgGuy]

I do shallow insulated and uninsulated foundations all the time in Ontario, and I think your case should be dealt with by 2 a). If frost heave doesn't hurt the building, then it's designed against frost action. It doesn't mean you design to prevent frost action, it means you design to deal with frost action. Obviously if you want to heat this cabin and you have plumbing connected to drains in the ground, frost heave creates problems that you must deal with differently.

 

[chicopee]

Wouldn't it be a better solution to build the cabin on stilts? Then insulate under the flooring.

 

[msquared48]

Pin pile.

Mike McCann, PE, SE (WA)