Questions on frost line

[nasdaqsam]

Pete,

A lot of what you are asking is dependent on exactly what you are doing. If you are putting in footers for a house foundation for example, the insulation thing may not be allowed by code. That method is used as an all-else-fails solution. Your local codes officer will tell you what you can and can not do. Normally it is not a choice thing. In our neck of the woods you do not get a choice. You "have" to go below frost unless all other methods have been exhausted and then only if you can get a licensed engineer to stamp it, which from experience I can tell you they do not like to do. New York State recently went to a National building Code which is quite different than the old NYS code book.

What is your reasoning for asking the question? We may be better able to answer your inquiry.

Take Care

 

[weedsportpete]

I'm really concerned with fence posts; I have no plans for building any more structures aside from run-in shelters. But I've always wondered about the posts I put in the first two years, and how quickly they will start to lean or come up because I didn't know what I was doing. Except for one time when I was able to get an excavator to push poles int he ground, I've dug holes with a 3ph phd, down three feet and put in 2 & 1/2 bags of concrete. Aside from wondering why I can't find 9' posts, I think about the frost line that is farther down than three feet.

 

[jim_wilson]

Heat rises - cold sinks, if you put insulation under the ground you in theory prevent the frost from reaching as far into the ground. The air temperature during the winter is never absolute so if you can trap heat in the ground you can in theory prevent the frost from reaching as far down as it might otherwise. Part of the reason that the ground freezes is that the heat that it might normally retain is lost to the colder winter air.

 

[Dargo]

Where I live, our frost line is not nearly as deep. I think our "code" is 18 to 24". Anyway, a common problem around here is the outer corners of the concrete pads in pole barns breaking after a cold winter. The frost line actually gets under the outer parts (but not the inner parts) of the concrete pad and busts the concrete as it lifts. This happens even with 12" thick 5000 psi steel reinforced pads.

The company who did the engineering of the floor in my barn called for a 24" wide 36" deep trench to be dug around the perimeter of my barn. The outside wall of the 24" wide trench had 2" thick insulation placed against it and the remainder of the trench was filled with pea gravel to within 6" of the surface. The engineer said that this method assured that the frost line woud not get under the corners or outer edges of my concrete pad and heave them up. It did cost more, but it made sense and the guy not only has a degree in this area, but has an impressive and lengthy resume of designs that are not damaged by cold temperatures.

The other option would have beent to place the insulation on the outside of the 24" wide and 36" deep trench, and then fill the remainder with concrete along with a grid of rebar held in place inside the concrete layer. I thought that was a bit excessive even for my tastes. I followed his first formula and poured my 10" 5000 psi floor that is reinforced with rebar and the 6 gauge wire matting. Since I did not want control joints in my 60' X 60' floor (long story), that is what the engineer called for. I do have drains and plumbing under the floor. It has cracked in several places. This was expected. However, it has not moved nor have the cracks widened, changed in levels or even gotten big enough to even fill with the thinnest of fillers. I'm happy.

Sorry for the long story, but that is how I was really introduced to "frost heave". All of my utility lines are at or below 36" deep, so in my area, there was no concern for frost. The only hard part was to stay go 48" below the bottom of a creek I had to cross with utilities coming back to my house. That was done with the creek flowing, and before the advent of the new wonderful boring machines. I hated doing that!