Shallow PWF Foundation

[redside]

I started a owner build project, but thought it would be better to post a topic here involving a foundation concept that I haven't seen used much.  It is a shallow permanent wood foundation a guy who has a bunch of remote building experience in Alaska showed me.  You basically dig a trench about 24" wide and 12" deep around the perimeter of the building and fill it with gravel.  On this you lay down two 2x12s layered on top of each other.  You can then build a 2x6 pony wall 12" OC up to the height of your floor joist.  You can also sheath the inside of the pony wall, but he didn't think it was necessary.  This foundation "floats" on top of the soil and will heave; however, in his experience his buildings have remained relatively level and no stuck doors/windows have ever been experienced.  This might be a good foundation for me because I am building near a lake (high water table) and the ground has a layer of blue/clay silt that I can excavate to and build on top of.  The 4 pony walls are built stout and essentially act as beams.  Am I crazy for considering this over sinking post or putting up cribbing which can be leveled easier.  Obviously, good site preparation is critical for this foundation.  My concerns are there is no ground anchor to keep the building from moving laterally (except burying it slightly in the gravel trench); the bottom is not tied together except at the wall corners; and of course the movement the structure will experience.  The advantages are I get an enclosed space for storage; it is relatively inexpensive ; easy to construct for a remote build; nothing buried which could rot; and no lateral pressure from soils pushing on the walls since it is above grade; and don't have to worry about digging holes that could fill with water due to water table.

Here are some pictures of the concept (really only difference between this and a permanent wood foundation is it is completely above grade). 

From Southern Forest Production PWF manual:    


A very rough skethup of the look of the building:

A real world example courtesy of the man who presented this idea to me:


If this doesn't work, I am leaning to sinking 3.5" steel piles with a 1/2" steel footing plate welded to the bottom.  I prefer this over wood due to life of steel and ease of installation.

[azgreg]

Here's an owners project using this technique.

http://countryplans.com/smf/index.php?topic=10671.0

[Don_P]

Typically blue clay is saturated. If saturated soil freezes it heaves.

[new land owner]

I did use a PWF on my camp and I have had no issues with it.  I do have very good gravel and have no water issues as it drains very well. I go to my camp most every month and have not had any issue with sticking doors. My camp is in a very cold area in the winter and I have not seen any issues with frost heaves.

[redside]

Checked out the build zagreg posted and what an awesome job.  My soil is not nearly as for drainage.  By building the foundation on gravel above grade I figure there will be no water/rot issues.  Of course, building in a dry location and doing the best job to keep water away from the foundation is critical.  There will no doubt be movement with this system since the foundation is not below the frost line.  By building a "big rectangular box" strong enough to weather the heaves, it is my hope that the building will not be torn apart.  Also, the ground is fairly uniform and the heaving should be fairly uniform.  This is a very remote build so no access to heavy equipment and good back-fill is hard to get nearby.  I think it might work.......just don't know.  It is either this, cribbing, or steel post on a footer.  Any more feedback from anyone that did or heard of something similar?

[Don_P]

How deep is frost line and can you daylight from that elevation? If you can work steel and create a well braced frame that is a good option. Looking at it objectively, a pwf foundation wall is well braced, are you creating a beam, not really.

[akwoodchuck]

I've used permanent wood stemwalls over concrete footers on a number of builds...works great in conjunction with a compacted gravel base, bituthene waterproofing, perimeter drain, porous backfill, etc. etc....your proposal doesn't sound good to me. A "real", code-approved PWF will have a base of crushed, washed stone...if you can't do that, I would go back to the drawing board. First step in any case would be a soils report if you haven't got one yet....

[NathanS]

Is this a vacation place or year round... Alaska? If it's heated all winter all you need is vertical insulation and then at the footing a horizontal wing of insulation depending on climate.

The FPSF shows provisions for unheated structures as well. It sounds like you are excavating the entire crawl space as well.. Go however wide you need for the wings, and the entire excavated area would get 2" XPS laid down before building upward.

http://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf

In my opinion, if building a structure to float with the ice, it's better not to have those stem walls at all. What could happen is they get pushed out of place in the winter, and then not return to where they were... do that for 30 years and you are going to have a really crooked house. If the entire thing floats on the ground you might be better off. I did that with my storage shed and have had no issues, would do it again. If it ever gets too far out of level I can jack it up and shim. Would not do that if I was investing so much money to make it a real house, though.

There was a Canadian building article written in the 60s (link broken now of course) that really went into depth on how frost action actually works. Was a really good read and probably still out there somewhere. Even then they talked about the trio of things that cause real frost heave (ice lensing), which was a continuous supply of water, freezing conditions and expansive soil. If you eliminate one of those you won't get heave.

[ChugiakTinkerer]

If far enough north that permafrost is a concern, then the recommended solution is to elevate the structure above the ground and don't let heat from the building warm the ground at all.  Various foundation types have been studied at the Cold Climate Housing Research Center.

[redside]

Thank you all for the comments. I feel the same as you and logically it doesn't make sense.  I am going below frost line with proper footings and piers.  Just to big of a gamble with a house.  Thank you all.