Is part of a footing 3' 1" below grade OK?

[ugstuff]

I'm a auto casting designers so this isn't my area.

We have a newly built bungalow with a walkout at the back in the Niagara, Ontario area. Our grade at the side drops approximately 1' over 5'6". The 6" x 18" footing the basement slab rest on, ends about 8' from the back of the house where it then steps down about 6'. While searching for my weepers (another story) I find the footing just before it steps down is only 3'1" below grade not 4' minimum as stated on my drawing. This gives me an area where at least a 5' length of my footing is not below 4'. The city inspector says this is OK. Is something wrong here? Oh yeh, the weepers end about 8' from the back of the house too.

 

[BigH]

First - you should ask the builder why he didn't follow the plans. This is contractual between you and he.

Secondly, the building inspector is probably correct in his assessment that the footer is okay at slightly more than 3ft with respect to frost heave problems and the like even if the "code" says 4 ft. Can't remember off hand what I used in Toronto - its been 10 years since I practiced there. I'd say the BI would be quite knowledgeable about the practical local conditions. You can always do a bit of landscaping and add a half metre or so.

If you still feel uncomfortable,find a local geotechnical engineering firm in the Niagara region (there are a couple) and ask them about the necessary depth of embedment for frost - for a minute chat, they'd probably just let you know.

 

[LeoSC]

The Ontario Building Code, 1997, Section 9, Table 9.12.2.2, Minimum Depths of Foundation, indicates that the minimum depth of foundation for a foundation containing no heated space is as follows:

For foundations with good soil drainage,
on rock - no limit
on coarse grained soils (e.g. gravels, sands) - no limit
on silt - below depth of frost penetration
on clay - ditto, but not less than 1.2 m (3'-11")

For foundations with poor soil drainage,
on rock - no limit
on coarse grained soils (e.g. gravels, sands) - below depth of frost penetration.
on silt - below depth of frost penetration
on clay - ditto, but not less than 1.2 m (3'-11")

Depth of frost penetration in Niagara Falls: I could not find a specific number, but found that in Whitby it is 1.0 m (3'-4")(see

http://www.town.whitby.on.ca/intro/location.html),

so in Niagara Falls is likely the same or less.

In summary, BigH is correct in saying that the building inspector is probably correct in his assessment.

 

[BigH]

LeoSC - Thanks for your note. It is good to see that foundations of coarse grained soils is separated from the others. I had a job in Terrace Bay, Ontario. The depth for frost penetration there is 9 ft or so. We put the footings about 4 ft down. We were on non-frost susceptible sands and the water table was something like 15 ft. down. Sands wouldn't heave and the water wouldn't rise up to "add" ice potential. Unlike, a tailings dam in Alberta that was built with material later described as sandy ice!!