Deep-ish Residential Foundation

[XR250]

I am working on a one-story slab-on-grade residence in Indiana where the frost line is reportedly 5 ft. I am not sealing the job – just working with a local Arch.in NC to get his details reasonable. He is showing a conventional 24” wide footing with a 5 ft., 12” CMU wall. Is that typically how it is done or are there other more economical options such as filling the trench with washed stone and placing a footing on top of that? Seems they would have to widen out the trench a fair amount to get a Mason “safely” in the hole. Load-wise, I only need an 18” wide or even a 12” wide footing. The slab needs to be isolated from the foundation wall for acoustical reasons so a monolithic slab is not an option.
I practice in central NC where we do not have these issues.

Thanks.

 

[CANPRO]

In my area it is standard 4ft frost line. Large majority of residential foundations are poured concrete strip footing with a poured concrete frost wall. The other common option here is a thickened slab edge (apparently not an option for you) with insulation for frost protection.

You probably could excavate and replace the top 5' with washed stone, but by the time you dig down to the frost line, you may as well install a conventional footing/frost wall and backfill with the same material you removed. If 5' is standard frost line in that area, the foundation contractors shouldn't have any issue with what is being proposed.

 

[Canuck65]

Digging a trench, pouring a footing and then building either a cast-in-place concrete or masonry block foundation/frost wall is very common in northern climates where seasonal frost occurs. My advice is, don't cheap out on the footings, if something goes wrong it is a major, major fix. Not worth the very small amount of money the client would save if you specified a 12" wide footing. In my area, 20" wide is usually the default minimum even though code minimum allows less. Bring the foundation wall above finished grade elevation by at least 6" to keep any wood framing elevated.

I would not fill the trench with washed stone. It either would have to be wrapped in geotextile ($$$) or if not over time the fines would migrate in and eventually it might start to heave. Geotechs in my area will reject placing footings on clear stone granular because of A) fines migration and B) settlement due to punching of the stone into the supporting soil.

 

[moltenmetal]

By the time you're down even 4 feet, the extra cost of going to 6 ft and adding a basement is fairly small relative to the benefit you get- hence many (most) homes here in Ontario have basements rather than crawl spaces.

 

[charliealphabravo]

Footing width is a function of both bearing capacity and settlement. In a prescriptive design like for a residential foundation, the wider footing might reduce settlement even if it isn't needed for load. In the absence of load and soil data a 20" or 24" wide strip footing is not unusual for 1 or 2 story residential construction.

 

[XR250]

Thanks everyone.

Geotechs in my area will reject placing footings on clear stone granular because of A) fines migration and B) settlement due to punching of the stone into the supporting soil.

I have always questioned Geotechs about issue "B" as they do this all the time down here in really crappy soils. It seems to fall on deaf ears.

 

[JAE]

If you use masonry stem walls on top of the footings - be sure to fully grout the blocks below grade.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[XR250]

If you use masonry stem walls on top of the footings - be sure to fully grout the blocks below grade.

The arch. has switched to concrete stem walls but point well taken - thanks.

 

[XR250]

One other question..
For spread footings, what is typically done - augured holes?

Thanks

 

[jayrod12]

Where we are, no, dug down, pier on pad, backfilled.

 

[Canuck65]

Depends on the load. For exterior deck piers with small loads, sometimes an augured hole with a sonotube, or perhaps a sonotube + bigfoot form would be used.

For interior spread footings, they are most often excavated and poured so that the top of the concrete cast-in-place footing is just beneath the slab-on-grade. Typically for an interior footing, no frost protection necessary as long as it is heated space. If it is a spread footing around the exterior, then excavate to frost depth and then pour a spread footing and concrete pier up to grade.

 

[XR250]

Thanks