2

I am in the process of building a small (14x14) log cabin in the woods on a pier foundation:

enter image description here

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:

enter image description here

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!

Improve this question
  • 2
    Any concrete foundation is perpetually exposed to water. This can actually make it stronger by ensuring adequate curing moisture. Your question about concrete mix is unclear. Ground concrete as 90% of the recipe doesn't sound good. Now if you meant 90% ground concrete and 10% portland cement, maybe. I'm not sure I get your second idea. – isherwood Oct 31 '18 at 14:46
  • Pump or bail the water out of the hole just before pouring thoroughly mixed concrete into it. Trying to mix in the hole is not a good idea – Kris Nov 1 '18 at 12:51
6

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.

Improve this answer
  • My piers are 24"x24". Do you think that's a problem? Also, I know the source of the urbanite: it was an old sidewalk. Does that have the appropriate properties? As for #3: how would that be different than just having regular concrete exposed? Thanks for the help! – dfife Nov 1 '18 at 14:52
  • @dfife So to determine if you have enough piers, we add all the Live Load to the Dead Load and then divide by the soil bearing capacity. I’ll do an example: 1) Floor Live Load = 40 psf x 14’ x 14’ = 7,840 lbs, plus 2) Roof Live Load = 35 psf snow load (I’m guessing because I don’t know your location) x 18’ x 18’ (2’ overhang all sides) = 11,300 lbs. , plus 3) Weight of building: 25,000 lbs. (a guess). TOTAL = 7,840 + 11,300 + 25,000 = 45,000 +- Assuming your soil can resist 2,000 psf, you’ll need 22.5 sq. ft. of piers. If each pier is 4 square feet, you’ll need about 6 piers...that’s doable. – Lee Sam Nov 1 '18 at 16:59
  • @dfife The reason piers “work” is because they have rebar. Rebar is needed for the tension stress in each pier. The number of piers we just calculated is just to hold the building up. You need rebar for normal lateral stress...otherwise the piers will crack and be worthless. – Lee Sam Nov 1 '18 at 17:08
  • That's very helpful. I did the calculations. My local roof live load = 25psf (for 8,100 lbs). For the weight of the building, I used a log calculator. Each log is estimated to weigh about 600 lbs x 13 rows per side x 4 sides = 34,060, for a total weight of 50,000 lbs. I looked up my soil composition on websoilsurvey and it says I have sandy loam (which matches what I think I'm seeing), for a psi of 2k-3k (if I understand things correctly). Assuming the worst, for 9 piers I need 2.7 square feet, so I should be fine. Did I miss anything? Also... – dfife Nov 3 '18 at 17:21
  • ...I just read that sidewalks have a psi of 3500, while foundations have a psi of 3000. So wouldn't urbanite from a sidewalk be stronger than for a foundation? I hate to keep pressing the issue, but cost is definitely a factor with this build. Thanks again, @Lee Sam! – dfife Nov 3 '18 at 17:24
3

I don't think you'll find anyone that recommends mixing concrete in the hole - you can't really be sure of a good even mix this way - but people do it for fences and other posts all the time and it seems to work OK.

With 90% urbanite (recycled scrap concrete chunks) I might want to be a little more careful, an unevenly mixed section could weaken the post a bit. The percentage seems very high. But this looks pretty overbuilt so I wouldn't worry too much about it. Is that actually 9 piers for a 14'x14' cabin? That's not far from being a slab...

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.