Thickened Edge Slab - Insulation

[CANeng11]

For an unheated garage built on a thickened edge in Canada we typically spec insulation under the garage slab (and out a few feet horizontally all around as well). We are basing our design off of a design guide called Frost Protected Shallow Foundations put out by the National Association of Home Builders. The theory is that if you insulated under the slab, the cold air wont be able to freeze the ground below the slab due to the heat in the ground. Also, according to part 9.12.2.2 of the National Building Code of Canada, the minimum depth of foundation containing no heated space must be below the depth of frost for silt and clay unless designed for lesser depths by a registered professional.

So I am wondering if this is standard practice for these slabs? I was talking to a contractor and I was told that they have never seen insulation required under a slab as we have spec'd. The garage is just large enough that it needs engineering, so its not very big, but I don't see how frost is being dealt with without the insulation in place.

 

[r13]

Well selected backfill material with adequate thickness can eliminate deep penetration of the freezing temperature. It also cuts off the capillary path for migration of the water, if the ground water is kept below the backfill.

 

[CANeng11]

How is that thickness determined? They say they typically use 8" of crushed gravel.

 

[CANeng11]

I found a reference that states "Replacing all the frost prone soil in the freezing zone often is impractical in northern regions, however. Frost in these areas may penetrate more than 4 feet. The cost of removing and disposing of that much soil and replacing it with gravel or crushed stone usually is too great. Even so, replacing some of the frost susceptible soil reduces frost heave."

It doesn't really give any information on how deep would be acceptable though... Here is the source: Article

 

[jayrod12]

I believe you can get around it using clause 9.12.2.2.6).b)

 

[CANeng11]

I don't think that applies to this garage since it is larger than 55m2

 

[r13]

I admitted that it is cost prohibitive the farther north, because the fill has to get down to the local frost penetration depth to be effective. Suggest to check on the electrical utilities for their outdoor equipment foundations. I remember GE has standards for extreme cold regions.

 

[jayrod12]

Well then that's the answer to the contractor and owner. Sorry but your garage is too big to find a way around it. Unless you could make the other clause there work about local soil conditions. What kind of soil are we talking about, I'm not overly familiar with Alberta's subgrade.

 

[GeoEnvGuy]

I typically see them strip off the organics leaving the clay crust, backfilling with a 2 feet thickness to run the utilities and get back to grade, insulation around the perimeter only. That is for a heated structure and heated industrial garage.

 

[CarlB]

The requirements you state are similar to code requirements in Alaska, that is foundations for unheated foundations are to extend to below design frost depth, unless soil in that zone is determined to be non-frost susceptible, or if ground is prevented from freezing through use of insulation.
ASCE 32 is the standard for determining insulation scheme, which is likely similar to the guide you refer to. Below is figure from that guide for unheated building. Dimensions for insulation are in a table, based on Freezing Index for the site.

 

[oldestguy]

Hey guys forget abut this stuff with a certain depth of freezing. In our area of Wisconsin they say 4 feet. Well here is a story that might upset your apple carts. In La Crosse Wisconsin there was a fire at a store and they blamed it on a gas pipe under the road getting cold from freezing and shrinking. Then a weak joint opened and gas came out. Ground being frozen trapped the gas and it went into a building and did its job. Gas pipe was about 4 feet down. This was before Christmas. So I was asked to help explain it, since it was so early in the cold season. Geology there is clean sand and gravel to deep depths (near Mississippi River). So I did a hand auger boring down under a sidewalk there. Measured temp as I went down. Down at 8 feet depth it was below freezing. Didn't go deeper to see temps. So certainly the gas pipe was in below freezing temps. Why so deep? Remember as ground freezes the water there has to freeze and in doing so it gives off calories. The higher the moisture content, the lesser depth of freezing. Not much moisture in clean sand, so deep freezing goes down. So forget those so called depths of freezing unless you know soil moisture conditions. Explains why the shallowest depth of freezing is in a lake. 100% water. So you undercut the frost susceptible soil and replace with clean stuff. THEN YOU WILL HAVE LOWER DEPTH OF FREEZING. ANY FROST SUSCEPTIBLE STILL DOWN THERE WILL FREEZE. Good Luck!!

 

[CarlB]

The ASCE 32 insulations designs are based on thermal modeling, and are intended to prevent any frost penetration beyond the thin NFS layer below the insulation. The recommended footing depths/insulation schemes are not a function of local frost depths, they are a function of design Annual Freezing Index.

Your mention of ice related to a gas line reminded me of a project where several feet of frost was discovered around a gas main in late summer, which was being crossed by a water service. The frost was due to the lowered temperature of the gas itself, as it was near a gas pressure reducing station.