Questions on frost line

[weedsportpete]

I thought I'd ask... I've seen references to frost lines and frost heave quite often here, but I'd like to get some specific information on the subject to fill in the gaps.. I'd appreciate any information.

When someone says the frost line is 4 feet down (ours is 42"), does that mean the entire support thingy going in the ground must be below 4 feet? Or does at least part of it have to be below the frost line? What part of it? Is the portion determined by weight? What if you are putting in a post - obviously most of it will be above the frost line, so how much must be below the frost line? (Lets say I have an 8 foot post, and the frost line is 4 feet down - does it have to go 5 feet down?? /forums/images/graemlins/confused.gif

The other questions on the subject have to do with the ground around the area you are digging out - if you are digging for a foundation / pier / support, and you dig down to or below the frost line, do your thing and then fill in around the area with dirt, haven't you just disturbed the area enough so that the frost line will now have moved farther down, because the dirt above is not settled anymore? Is that ok, or will it be ok in a few years?

Thanks, Pete

 

[Eric_Phillips]

What you are looking to do is to keep any water from freezing below your support and pushing it up. As long as the bottom of your post/peir/foundation is deep enough so that water cannot freeze below it you are fine. I have never research how the frost line is determined. I would think there is some slight variablity in that line over a given area. I would always leave myself a little insurance of about 4-6 inches if possible. This will also take care of any settling issues you are asking about.

By over analyzing the issue I think you would have more problems with a concrete heat sink going down below the frost line than any settling of the disturbed soil. But from what I can tell it doesn't matter what the material is you are using for support concrete, wood, etc. the frost line is all the same.

My two cents.
Eric

 

[schole]

Excellent questions, Pete.

I hope you get a geologist or some expert to answer you -- I am not.

In Thunder Bay (way north of Duluth MN), if we were really worried about frost going down a wall or footing, we would lay out some hard styrofoam (the blue stuff) running at an angle out from the foundation at anywhere below grade to avoid any shovels from landscaping etc. Water lines would get a layer of the stuff for extra insurance. Even the highway repair crews would be seen laying a thick layer under the gravel base (quite a few feet down) if there was a particularly notorious frost heave area as a last resort.

Obviously, this is no substitute to actually digging down a good 5 or 6 feet. This is not always possible if you run into a solid rock base at 4 feet -- the insulation helps at this depth.

shawn

 

[Eric_Phillips]

Shawn,
I don't understand what you are trying to do. You have a heat source under the ground, it is why we have valcanoes and hotsprings, so why are you insulating under footings and water pipes? I would think the styrofoam could possibly lower the frost line and hold water that could freeze.

Eric

 

[woodlot]

I understand the foam on the SIDE of the foundation and OVER the water lines. Why UNDER a road? (unless it is a heated roadway w/ hydronic heat, PEX within the concrete.) Maybe there is good reason for this and I'm always trying to learn a thing or two. please explain, thanks.

 

[Junkman]

The frost line is an imaginary line where in theory, the frost will not penetrate below during the winter season. This line has been known to vary depending on the type of weather the winter gives up. If there is a lot of snow, the snow covering will keep the ground warmer, hence the frost level will not go as deep. If we get a very cold winter with little snow, then the frost line is going to be deeper into the ground. When I built my home, I excavated for the driveway down 12" and then put in about 48" of gravel to bring the gavel driveway up to the new grade. After 10 years, I decided to have the driveway paved and that paving, being a base coat lasted till this spring. This summer, I had the top coat of asphalt installed, and they had to remove some "heaving" under the base. In each instance the heaving was caused by very large boulder that had been heaved toward the surface by freezing under them and pushing them up. The last 3 winters have been particularly cold in New England so the frost got deeper into the ground. The frost line here is considered to be at 42". Usually when you are building, you go about 12" - 18" below what is normally considered the frost line. The larger the structure, the deeper the foundation is dug. I remember when I was working in the construction field during a summer between semesters, the office building being constructed was going to be a 4 story building. The foundation footing for that building was as deep as the building was going to be tall. I don't know what the exact reasoning for this, but I know that it is commonplace. Remember the excavation hole for the Twin Towers of the NY Trade Center Buildings? How deep did they go before they hit bedrock? Once the hole for the foundation is backfilled after the wall or pier is completed, the ground will settle around it quite quickly. The frost will not travel any further down alongside the wall than in the ground in the surrounding area. If the backfill material is fluffy, you might get some additional freezing prematurely, but nothing that will cause problems. The freeze / thaw cycle will actually help to compact the soil. Most times, the soil is compacted in sections as it is being placed. They put in a foot of soil, compact and then continue filling an additional 12" and compact again. This is how it was done on the office building. On home building, usually you don't see this happening, but you will see rapid settling of the backfill material as the home is built. Many times they will actually mound the fill higher at the foundation wall knowing that settling will be happening naturally. When I backfill, I usually "water" the backfill as I am filling the hole. Makes a soupy mess if you step into it, but it does avoid getting any settling voids. I also backfill with stone and stone dust mixture for tight compaction and drainage. On my home foundation, I installed perimeter drains on both sides of the footing and the backfill allows any water that comes close to the foundation wall to quickly seep down to those drains and away from the foundation. My home is built on a mound and the curtain (perimeter) drain empties about 100' away at a point that is lower than the foundation footing.

 

[schole]

Sorry to sound so confusing. I just know this works great and am not 100% certain of the physics, but will try to explain my understanding of it.

The insulation under the ground seems to act like a blanket of snow on top. It saved our water lines from freezing in areas where we couldn't otherwise get them down deep enough (eg bedrock). There is no substitute for getting things down deep, but the insulation over the lines can buy you the equivalent of x# of extra feet deep of frost protection. No formulas to give you (any engineer's out there?).

As Junkman mentioned in his note, ground heaves are often related to large boulders being pushed up over the course of a freeze thaw cycle. Some areas of highway up north were notorious for never lasting more than one winter or so without having a really nasty heave in the spring -- this is with MANY feet of gravel base, excellent drainage, etc, etc. The road would only stay put after a layer of insulation was placed under the gravel base. It seemed that certain rock-cuts (ie road notched into a hill of bedrock) which were exposed to the harshest weather faired the worst (any Northern road-builders out there?).

As mentioned above, the frost will go down much farther when the snow cover is light (melted, removed, packed, blown away, etc). One common place is under a dryer vent. Melt the snow with a couple loads of laudry and you see the bare ground. Some folks put a deflector to keep the snow from melting off (often melts anyways). A piece of 1" thick styrofoam lying under the ground will prevent the frost from getting down so far in that area (especially important if water or septic lines are below).

I guess the assumption is that the native soil temperature is generally warmer than the falling air temps in fall and winter. Block the cold a little, and the warmth from below is enough to keep water lines flowing and boulders from heaving.

Again, there is no substitute for getting things down deep, getting proper drainage, and doing all the other things the expert builders will tell you.

Hope this helps.

Shawn

 

[weedsportpete]

Thank you for your replies. That does straighten out my confusion. Either I go the insulation route or I make sure the bottom of the pole or footing is a few inches below the frost line, so that water can't freeze underneath it.

I think I need to think more about the insulation method be fore I totally understand it.

 

[Eric_Phillips]

Pete,
I am not sure what your project is but if you are setting wooden posts you can wrap the bottom of them with barbed wire. When I worked on a ranch in WY a couple of summers we would wrap the wooden posts so they would not work themselves out. When containing cattle you not only have the frost lifting your fence posts but also the cattle. With wire around the bases they don't come out even when you want them to.

Eric

 

[Farwell]

Also a good way and place to get rid of some old barbed wire.