I am in the process of building a small (14×14) log cabin in the woods on a pier foundation:
After marking the perimeter, I began excavating the soil and dug to below the frost line (36"). The plan is to have 18" above the soil and 3 feet below. The soil is a mix of sand and clay. When I returned the next day, the hole had filled with water:
To save money, I intended to make the piers out of 90% broken concrete chunks (urbanite), mixed with new concrete to hold the broken chunks in place. Now, I'm considering pouring bags of concrete in the hole until I get the right consistency, then plopping chunks in there until I fill the form.
Is this a bad idea? Is it a problem that my pier blocks will be perpetually exposed to water? Do I need to move the location of the cabin to some place higher?
Note that I intend this to be "off grid": no plumbing, no HVAC (other than a wood stove), and maybe electricity run off a generator.
Thanks in advance!
Best Answer
You have several issues: 1) Hold your building up, 2) keep your building stable (lateral stability) 3) freeze/thaw,
1) I would be careful to calculate the number of piers required to support such a heavy load as a log cabin. The piers have a small bearing area (perhaps as small as 12”x12”) and your load is tremendous.
The benefit of a continuous footing is that it can span over a soft spot if necessary. With a pier foundation, each pier had better be on stable soil.
2) Urbanite (reuse of old concrete) is not appropriate for structural foundations. Tossing a bunch of concrete chunks into a form is not going to give you any reliable strength. Concrete mixes have carefully proportioned amounts of materials. “Random” is not safe.
Concrete foundations resist various loads and you need to know what those loads are and provide the correct strength concrete.
Lateral loads are a major concern for your “elevated” house. You didn’t mention rebar. ANY lateral load would be resisted with carefully placed reinforcing steel. Dumping used concrete into the forms does not allow rebar to be located properly.
3) Of major concern is having chunks of old concrete exposed on the edges of the piers allowing water to penetrate the piers. Then, when it freezes the pier will be broken apart.