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dvonk 04-09-2008 10:19 PM

Foundation depth for deep soil frost line
Soil frost line here is about 60" (1.5 m), subterranean water level normally is about 90" (2.3 m), soil in my yard is so hard (on the depth of 40-60 cm) that it requires some special technic to dig it (heavy sand clay - I hope that I've used the right term).

What is acceptable depth of foundation (gravel layer and concrete layer)? 40 cm & 20 cm?
Sorry for stupid question - have no experience at all.

Thanks in advance,

Frances 04-10-2008 04:55 AM

Re: Foundation depth for deep soil frost line
Well, I'm not really the best person to give advice on this, but since no one else has answered yet...

The way I understand it, your easiest choice is to make a floating foundation, i.e. one that will remain stable in itself and "float" on top of any soil heaves the frost may cause. I would guess that 40 cm of gravel and 20 cm of re-enforced concrete would be fine. Maybe you could even get away with less gravel... say 30 cm?

So, is anyone going to give Dimitri some informed information on the question? :)

dvonk 04-10-2008 05:08 AM

Re: Foundation depth for deep soil frost line
Thank you!
I was thinking in this direction - at least each 10 cm of digging and packing with gravel take a lot of work :)
I'm just afraid that soil on 35-40 cm is not heavy enough. Will those 30 cm be good enough for drainage?

PS. In Finland I've seen the whole 2-store mansion laying on the sand pillow, covered by styrofoam under concrete slab. All that - above ground level.

dmun 04-10-2008 05:16 AM

Re: Foundation depth for deep soil frost line
I haven't jumped in here because these soil and frost conditions are extreme. I suspect that the usual advice will suffice - pad on well drained crushed rock, or massive footings on bare soil below frost level - but not being a structural engineer I don't know. My usual cite on this is Canadian, they have presumably extreme climate conditions up there. However more than one westerner has come to grief presuming to know things about Mother Russia, just ask Napoleon.

Good luck with your project. I think you need to consult with local builders to get a sense of local practice.

dvonk 04-10-2008 10:52 PM

Re: Foundation depth for deep soil frost line
Thanks a lot!
I've found three or maybe four familiar words in this article :)
It helped me a lot - it proves that 30 cm of gravel is OK. I will encircle gravel pad walls with Typar to ensure that it will not loose it consistence in a while during the spring time.
And probably I will make two additional catchments along outer sides of foundation - the oven will rests under the roof in a corner of the outdoor kitchen, probably it will help to make pad stay more dry (or moisten more even during autumn rainfalls) - to assure the uniform heaving.

Anyhow, the massive footings on bare soil below frost level doesn't make a sense - it means about 6 cubic meters of concrete works (I've undergo that kind of excavation during septic tank installation - no more!).


CanuckJim 04-11-2008 03:55 AM

Re: Foundation depth for deep soil frost line

I live in southern Ontario, Canada, and our code here for frost is four feet, though it rarely goes that deep. However, I had a serious issue with the fact that my land has about 30-40 cms of topsoil over very dense yellow clay. Between the layers, there's constant movement of ground water. The combination is a recipe for frost heave. I excavated all the topsoil and some of the upper layer of clay, then drilled ten 20 cm Sono tube holes as deep as I could to get to around the four foot level (glacial rocks get in the way). The tubes (how many depends on the size of the pad) were filled with concrete and pieces of 1/2 rebar were placed in the wet cement, protruding about 20 cms from the tops. Twenty cms of crushed stone was put in once the forms were built, then a 1/2 inch rebar and mesh grid at the halfway mark of the concrete pour to come next, and the protruding rebar was bent and tied into the grid. The actual pad is 20 cms thick. After five years and a particularly frigid winter this year, I've had no movement at all. Be very careful that the area around the pad is well drained so snow melt does not get under the pad. You can do this with perforated drainage pipe covered with cloth that's buried in crushed rock. Make sure to run it away from the pad. Although some people recommend lining the excavation with insulation board, this is not common practice here, nor is using plastic sheeting, which will rot over time.



dvonk 04-13-2008 10:43 PM

Re: Foundation depth for deep soil frost line
Thank you, Jim!
I've read you reply only today, when I made all my excavation. My situation is more simple, since we do have ground water on a relatively deep level - at autum it was 2.30 m. I will make a drainage, of course.

On the depth of 35 cm the top soil is over.


Ed_ 04-17-2008 09:53 AM

Re: Foundation depth for deep soil frost line

I just ran across the following page this morning. I know it's a bit late for you, but perhaps it will help somebody else (like me!). The diagram at the very bottom is of particular interest for us, I think.

ESB: Frost-Protected Shallow Foundations


dvonk 04-17-2008 11:50 PM

Re: Foundation depth for deep soil frost line
Thank you, Ed!
I've looked through it, and as I've mentioned before, I've seen that kind of foundation (even slab, since it was on the very surface of the ground) in Finland.
I don't think it will work for me very well since the small size of slab. And, to say frankly, I'm can't imagine right now to dig all the gravel out, to dig in foam and so on - that will make the excavation job bigger in times.
I hope that good drainage and floating slab will work.
Thanks again, anyhow - it was informative

jpark1wirrcom 05-28-2008 03:13 PM

Re: Foundation depth for deep soil frost line
ESB: frost-protected foundations only apply to heated structures, where the heat loss migration is trapped by the insulation under and around the foundation.

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