I'm looking at building a 20x32' 1.5 story cabin in NE Wisconsin. The site has about 6-8" of sandy top soil with clean dry sand and large rocks at times under that. As the area is on top a slight hill, I expect it will always be fairly well drained. I'm looking at my foundation options. I'm thinking about going with a shallow system as the site is not easily accessed by large equipment and the acreage has a lot of granite boulders which could hamper attempts at digging piers.
The system I first looked at was a series of 2x2x1' holes filled with gravel and tamped. Adjustable concrete piers would be set atop the gravel. I was looking at going with these piers at 4' intervals along the outside walls and down the center support. Here is the place I found this: http://www.coyotecottage.com/cabin/cabinconstruction/foundation.htm
I have looked at pressure treated foundations in the past for a house I built and this idea occured to me. Its basically setting a PT Wood foundation on grade on top of gravel. Here is my thought process:
Dig a trench 1' deep and 2' wide matching the perimeter of the cabin ensuring the trench bottoms are undisturbed soil/sand.
Fill this trench with washed 3/4" limestone gravel, level and compact.
Build two 32'x2' and two 20'x2'walls out of pressure treated (PT) wood. Glue and nail pressured treated 1/2" plywood to 1 or both sides. This box beam would give these walls strength to deal with slight variations in support by the gravel.
Nail a 2x8 PT footer to the bottom of these walls and set them on the gravel.
Set a center beam and support it with short piers on 2x2x1' gravel holes.
Set the floor joists across the walls & the center beam and put occasional angle braces between the bottom of the 2' walls and joists to provide tip over resistance.
I was also planning on placing 2' wide band of foam insulation on the ground around the outside and backfilling slightly. The foam would disrupt frost action, which I would not expect a lot of anyway, and protect the trenches from getting hit with rain water.
I would probably add some large screw in anchors for wind resistance.
I would appreciate any comments on this foundation method.
Thank you,
Jerry
You have described the crawlspace version of the All Weather Wood Foundation.
Here's an overview. http://www.toolbase.org/Technology-Inventory/Foundations/wood-foundations
I have plans for this and a wood framed basement foundation as an option in the 2-story Universal Cottage (http://www.jshow.com/y2k/listings/57.html) plans.
You can find much more history and information on a search of "all weather wood foundation"