Should I take a gamble by building my small cabin with an on-grade foundation? Thoughts appreciated!

Hi there!

I'm planning to start building a smallish cabin (12x16, no second floor) on my land as a temporary dwelling, so that we have a way to spend more time up there comfortably while we work on getting the land livable full-time.  (We still need to turn our trail into a drivable driveway, add a well, etc., but it might take us a few years to get all that done.  In the meantime, tent camping whenever we're up there is getting a little old.)

Anyhow, I am hoping I can avoid the trouble of building a cement pier foundation.  We're on the Cumberland Plateau in Tennessee, and our soil is extremely hard clay and full of sandstone rocks.  Digging is a terrible chore.  My hope is that I could get away with a compromise, like digging down 6in or so, putting in a couple inches of sand/gravel, and using precast concrete pavers/cinder blocks/deck blocks, sort of like what this person did:  http://cpiat.com/g/2/be/beam-and-footing-cabin-foundation-types-plastic-base-for-log-concrete-building-on-rock-architecture-cheap-options-cheapest-to-build-how-solid-low-cost-off.jpg

It's my understanding that the main dangers of this are as follows:

1.  Frost heaving.  I can't find the exact frost depth for our area on the Cumberland Plateau in Tennessee (the nearest towns are Monteagle and Sewanee).  I've seen some maps say 6in, others 12in.  Even though our land has no restrictions, I'm reluctant to contact local authorities for an answer, just to avoid the rigamarole.  Does anyone have experience with frost heaving in a relatively temperate climate with a freestanding structure?  My hope is that if it happens, we could use a car jack to prop up an edge of the cabin, and just slip some wood/rocks under the supports to balance things out.

2.  Erosion/slippage.  It seems like another danger is that the cabin might 'walk' off its foundations.  Since the site is pretty darn flat, I'm not as worried about this.  But should I be?

3.  Soil settling under the foundation.  I'm not sure how to deal with this or account for it.  Since our soil is mainly hard clay, does that make this less of a risk?


I know it's a bit of a gamble, but I'd be curious to hear from others who have taken this gamble before.  It would really be a beast to put in piers, so I'm willing to assume some risk, but it would be so sad if the cabin became unstable/uninhabitable after one season.

Thanks in advance for your thoughts!

Best,
Juliet

Around here, lots of small sheds are simply  plopped down onto the ground. If the site is really windy, perhaps they get a cable anchor on each corner. If the building is floating on top of the soil, it doesn't matter whether the frost heaves or not, the building is still floating. If there is a problem with settling, a 12' X 16' building is so light, that an automotive jack could easily lift a corner to allow a shim to be installed..

Edit to add: In my area, a "foundation" triggers building code enforcement. Something that is sitting on the ground is temporary and not regulated.

Hi Juliet, of your three bullet points, I think the only risk is the frost heaving one.  If you had sandy, well draining soils the frost heave would be less of an issue but I believe clay is more prone to frost heaving.

If your cabin is 12x16, you could probably get away with 6 piers.  How hard would it be to dig down a foot in each place?  If your frost depth is 6-12", a foot deep is good enough.  If you happen to hit a big piece of sandstone that you believe is deeper yet (down below the frost and won't be heaved) then you could use that as the base of your pier even if it isn't 12" down.

I'd be willing to skimp on piers, especially if you're ok with monitoring the heave and adjusting with shims.  If it becomes unbearable, you can always jack up the building and dig them out "correctly" later.  It would suck but it might be a reasonable calculated risk.

One side thought...  If you dug down just a few inches and put down gravel and pavers, all you need to do is keep it from freezing there.  Arguably you could surround that pier with some 1-2" thick styrofoam (say a 4' circle with your pier in the middle).  Cover the foam with some dirt and that insulation should keep the ground under the pier from freezing.  The risk with this would be if critters want to dig into the styrofoam or something.  

You could always call your local authorities from a cell phone and if they want your address, just say you bought the place and can't remember.  Or just make up an address.  Odds are they won't ask or care

The frost line is only 3inches so I think that it is fine. http://www.cookeville-tn.gov/codes/adopted-codes/
I say build it on grade and if it moves it will be the entire structure moving as one so it would still be safe.

I am liking these blocks alot, they accept 4by4 lumber and they have a height of 8inches so they will be fine with the 3inch frost line and dont forget this is a temp structure not a 100yr structure.
https://www.homedepot.com/p/11-1-2-in-x-8-in-x-11-1-2-in-Concrete-Block-10550005/100350712

I'm no engineer but I myself would put the footings below the frost depth. It's much easier to dig now than it is later with a cabin in the way. I think the biggest problem that you may encounter from frost heaving is in a worst case scenario busted windows, but at a minimum you'll have doors and windows that can become very difficult to open and close if frost heaving has racked the structure, and then settled again in the spring to a different point. As far as erosion, just make sure you have gutters of some sort and get that rain runoff away from the foundation, perhaps into a rainwater collection system

Here's a link to codes in Cookeville. I imagine the footing codes on the plateau aren't going to vary too far from these. http://www.cookeville-tn.gov/codes/adopted-codes/

Just my opinion here from our experience with heavy, saturated clay soil that heaves with deep winter frost.

A 12 X 16 ft. structure is small enough, even with the weight of framing/siding/roof etc., to put on timber "skids".  You may not have a tractor (or suitable vehicle) to move the structure now, but you can always come upon one, either borrowed from a neighbor or having them move it for you.  We have countless buildings of similar size that sit permanently on skids.....and have the comfort of knowing they can be moved anywhere else on the property rather easily.  (I may be wrong, but I think Paul W. has many structures not far off that size on skids as well.) You also don't have to worry about frost heaving issues or any other things that causes tilt to your building:  When that happens, just get one of the beefier hydraulic or screw jacks to raise the building slightly and shim until you get the "levelness"  that you desire.  The one drawback of such a building in our area is the threat of tornadoes.....which could easily send the building tumbling across the county.  And if we were to use it for winter dwelling, the cold temperatures in northern Minnesota would me having to insulate the perimeter with hay bales or some such item since the floor would get mighty cold.  Otherwise, I would consider wooden skids as one possibility for the foundation of your cabin.  Good luck!

Thanks for this excellent food for thought, everyone!

The frost line where I live is much deeper than yours is but I do have a fair amount of building experience and wit say this. If you just build on grade you may or may not have an issue, I kind of doubt it because of the small size. The other thing I have done is a pole building with treated posts sunk 3 ft in the ground. a gas powered auger is a big plus if the ground is hard.  

I am an Civil Engineer
I would look at building it on skids, that way heaving will not be an issue, and you would be able to move it if you wanted.
The skid could be old steel trusses, girders etc and may be built off site and bought up to be bolted together.
If the ground cannot be levelled for any reason, just pack it as required.

I built one 12x16 I used treated 8x8's as skids with the ends tapered so it would sledge easily.
I skinned both sides of my floor (1/4" below 1" on top) to minimize rodents moving with each move, if you use plywood trusses as floor joists its easy to screw through the bottom chord to secure the skids to the floor prior to insulating and laying down the sub-floor.
When I gave it away we loaded it with a Caterpillar front end loader on a flat bed trailer, space your skids 2.5 feet in from the edge to comfortably sit on a standard trailer. If you don't go mad with interior partitions, use a tin shower, an on demand water heater, and minimize the fastened in place fixtures, the rig will stay light enough to move without undue strain.

I would have to build it to last a hundred years. Temporary sometimes becomes permanent. You can always use more storage space or convert it into an animal shelter. I once converted an ancient chicken house into a nice garage with very little effort because it had been built so well.

Ken, there is no reason not to build it to last 100years.
You just use materials that suit, its that simple.