Footings for small cabin re:frost heave

[looksatstars]

I am looking to build a small 600 sq ft cabin on a lake lot.
I would like to incorporate a timber frame structural skeleton enclosed by strawbales for the walls.
The lot is in alberta, canada where the winters can be severely cold.
It will not be a heated space throughout the winter so I worry about frost heaving.
I am looking for a good idea for the basement foundation.
I first thought of concrete piles with a grade beam. The timber posts could rest on the concrete piles.
Then I read that piles should be a minimum of 10 feet. I don't feel this method would be economical.
Then I looked into a slab on grade. But cannot find a design that will work with protection against the frost.
Has anyone had any experience with a void form under a shallow footing?
Any ideas would be helpful.

 

[rochplayer]

For frost heave to occur, three things are needed - 1. Freezing temperatures, 2. Water source, 3. Frost susceptible soil. Take away any one of the three, no frost heave occurs.

I assume all three are present at your building site. Frost heave is going to be a problem. What is the frost depth in your cabin area? The US Army Corps of Engineers has some freeware for calculating frost depth, but the climate input data is based on specific weather stations. Their is probably data for North Dakota and other Northern U.S. plains states. It is called ModBerg - do a Google search for it.

An approach for unheated structures is to use a combination of excavation of frost-susceptible soil and replacement with non-frost susceptible (NFS) and rigid board stock, extending the section 4 or more feet outside of the foundation footprint. The intent is to raise the freezing front up into the NFS backfill by optimizing insulation and backfill thickness. That eliminates #3 in the frost heave formula above.

The pile method you mention won't work unless they extended a sufficient distance below the freezing front to resist uplift due to frost jacking.

 

[looksatstars]

The frost depth in our area is known to be 4 feet, though it rarely hits that depth unless there is traffic on top (ie roadway)

The soil is a stiff clay, so I am assuming that even though it is consolidated it still will have alot of water retention as most clays do.
I was looking at the solution you are talking about with the rigid board stock but read that is exclusively for heated structures.
I was thinking of using a non-frost susceptable soil such as peagravel but I think the clay would heave and push enough on the pea gravel to make the slab move.
I dont think it would be a huge problem if the soil would all heave consistently because I don't think it would be that much. But I would be worried that the south side would have less heave than the tree covered north side of the home and cause alot uneven displacement.
As you say about the pile method, I would be worried that the frost would grab a hold of the pile and use the friction pile against the foundation. I would think a sono tube would help against this but I don't think it would solve it.
Thanks for the input.

 

[rochplayer]

4-ft is not all that much, relatively speaking. BTW, the frost depth is deeper on streets, not due to traffic, but due to them being kept clear of snow, which is an insulating layer.

For the insulating board, the heated/unheated application depends on where you place it.

If frost depth is only 4-ft, could you simply bring in 4-ft of clean (<4% passing 0.075mm [No.200] sieve) sand and gravel and build a pad. Make the pad is wider than your slab. Incorporating the insulating board stock into the pad slows the heat loss from the pad, so you don't need as thick of a heat-sink. The hot Alberta summers will drive a lot of heat into the ground and the board stock will help trap it in - but build your pad only after ALL of the frost is out of the ground, so you don't trap the frost in the ground, where later thawing can cause settlement.

You don't need pea gravel to have a NFS soil, you just need a soil without fines to provide wicking of groundwater to feed ice less growth.

Remember, frost heave can only occur if freezing temperatures occur within a frost-susceptible soil. If you make the frost penetration (e.g. the 4-ft depth) occur in the NFS soil, then frost heave will NOT occur in the soil beneath, because that deeper soil will remain thawed.

Frost heave is a thermodynamics problem that is dependent upon the moisture content of the soil and the latent heat of fussion of soil water.

I recommend anyone practicing geotechnical engineering in the northern tier states in the U.S. and in Canada (or other countries with significant freezing temperatures) take a frozen ground engineering university course or short course.

 

[jike]

If replacing 4 feet of existing soil with clean NFS sand and gravel in a clay soil, won't you be creating an impervious bowl that will simply fill up with surface and ground water? I assume the pad is perhaps 6" above surrounding grade and 3'-6" into the ground.

Will putting a clay cap on top of the NFS fill help to prevent surface water from filling up the bowl?

 

[rochplayer]

jike, I agree - bring in the fill, rather than dig down the entire 4-ft. Sorry I was not clearer about building the pad.

 

[looksatstars]

I think the big dig and backfill option is not economical for us though.

I would be cheaper to just trench out a footing around the perimeter.

 

[CarlB]

If you're looking for "economical" and "manual labor", how about a "post and pad" foundation? This is common in remote Alaskan villages; it accommodates deep frost and thawing permafrost - but can often be accompanied by considerable seasonal movement. It offers an accessible foundation, and they're often designed to be adjustable. You could minimize the movement by placing the "pad" a few feet below grade, with some NFS material below it, and insulation board over and around the pad to limit the frost penetration below the pad.

 

[looksatstars]

thanks to all for the good ideas.

CarlB: what you recommending is exactly what I am looking for. With a quick search, the spacing and depth is what I was looking for.
Here's an exermpt for anyone who is interested:

Pier or Post-and-Pad Foundations
This type of foundation is commonly used to support older Appalachian homes. Modern piers generally consist of cylindrical cast-in-place concrete extending below the superstructure so as to transfer the building load deep below the surface. Piers may be built with a variety of materials, such as treated timber, and need not extend deeper than below the frost line. For supporting power, deep, cast-in-place piers may rely in part on the contact between the soil and the sides of the shaft for frictional resistance, but most piers depend chiefly on end bearing. To increase end-bearing capacity, small spread footings, or "pads" may be provided. Thus, the width of the pier need only be sufficient to sustain the column load from the structure. This arrangement is economical for light structures, especially when posts of treated timber or built-up masonry are combined with cast-in-place, reinforced concrete pads.
In contrast to perimeter wall footings or slab foundations, piers tend to concentrate the structural load. Thus, soil with a relatively high bearing capacity (such as those soils commonly produced by consolidated mine spoils) must be at hand. Alternatively, piers may be built deep to reach firm bearing and mobilize shaft friction, but cost increases quickly with pier depth. Piers deeper than eight feet below grade are seldom practical for residential structures. Except in areas of settlement, piers have no particular advantage over the more common perimeter wall foundation.

A major advantage of pier foundations in areas of settlement lies in the accessibility of the foundation elements. If the pier undergoes settlement, an appropriately designed foundation- superstructure connection can be adjusted. Normally, this "design for adjustment" is not the case. Foundations having built-in provision for releveling are called adjustable foundations and are discussed below.

An important cost element in construction using pier or post-and-pad foundations is the need to provide support members (generally beams or girders) spanning the distance between piers.

In the case of perimeter wall foundations, the walls serve this purpose around the exterior of the structure. Interior beams do not have to support large loads; for one and two-story houses they usually consist of economical built-up wooden beams supported by columns at 8 to 12 foot intervals. Perimeter beams, however, must be designed for the larger loads imposed by the roof and exterior walls. If this requires steel girders, material and material handling costs greatly increase. Thus, if pier foundations are to be used, economy may dictate single-story frame construction with lightweight veneer such as aluminum siding. For settlement-prone areas, an additional benefit of such construction is flexibility.

 

[Slowzuki]

Here in New Brunswick, Canada, this is what we use for this application:

http://www.foottube.com/

It is a tapered plastic tube and pad combined. It works excellent, many camps and even homes are built on them.

Some engineers will spec some light reinforcement be placing inside before filling with cement if much will be sticking out above ground.