Alternative foundation

[dabstar]

Hi all,

I am starting a build here in Copenhagen, Denmark. I have seen a guy do an alternative foundation and was hoping to get some feedback on it. See the image below from a danish blog:



Would this be a good alternative to a concrete foundation slab? I believe this guy dug 18 inches into the ground which is half of the "normal" danish rule of thumb that you need to be 36 inches underground to be frost-free.

This approach seems easier to me, but I haven't seen it elsewhere. Is it structurally safe? Any other thoughts and comments?

Thanks in advance,
David

[dmun]

I can tell you what the conventional wisdom is, which you probably already know. Build on a slab on well drained crushed rock, and you're unlikely to have frost heave issues. (no water accumulation, no frost) If you're building a two story masonry chimney, you'll need proper footings, below the frost line, and protruding six inches (150mm) beyond the exterior of the structure. Anything halfway in between, like the thing you've illustrated, or some tubes of concrete sticking down from the slab, is likely to be self defeating. Such subsoil structures just give the ice lenses something to grab on to.

The "frost protected shallow foundation" is another approach. This uses insulation to use ground heat to protect your structure. I don't know any oven builders who have tried this.

[Neil2]

"This approach seems easier to me, but I haven't seen it elsewhere. "

I doesn't seem easier to me. There is nothing simpler to build than a reinforced concrete slab. The slab is structurally a better choice since it will "float" as one unit if there is any settlement or heaving. Differential movement with the type of foundation you have shown will likely lead to cracking or even failure of your suspended slab.

I disagree somewhat with dmun re the concrete pier approach. Piers set down below the frost line with even a bit of belling at the bottom will not be jacked by frost heaving. I have had no movement at all with mine and mine is built on frost sucepetable soils.

[dmun]

Piers set down below the frost line with even a bit of belling at the bottom will not be jacked by frost heaving.

We are in agreement about the bell at the bottom of the pier. That makes a good, stable footing. Most folks, however, just dig down, put in the sonotube, and fill it up.

They even make a form for doing this properly:



However, this is ancillary to the original question. There are only a few special situations where anything more than a slab on well drained crushed stone in needed. It's the easiest, and the best.

[dabstar]

Thanks for your reply. I have decided to go with a traditional foundation slab. Excavation for it started this weekend. See more at http://www.fornobravo.com/forum/f8/d...ild-12774.html (Davids WFO build)