Expert: Dan Griffin - 7/10/2008

Question
QUESTION: I am building a walkout basement. First off, can I build my footings on compacted gravel or is undisturbed soil the best bet? I am planning to put in piles as well. Every 8 feet, 8" by 8 feet. Is that sufficient?

ANSWER: Footings are normally installed on undisturbed soil. Piles are a term for driven concrete, steel,  or wood pilings.  YOu are referring to a home made version of a pier.

Brent, I cannot answer your question.  All footing designs are based on the total load of the structure and the weight bearing capacity of your soil.  Code will normally allow 1,000 lbs per SF without testing the soil unless you are in a swampy or obviously questionable soil.  Why are you considering piers?

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QUESTION: I've been told to build on the gravel and just pack it to near 100%. Is this a mistake.

As for the piles, I am in Canada and where I am houses normally have sat deeper in the ground to avoid frost. It is suggested that anything less than 8 feet will not be above the frost line. The piles are supposed to help the footings resist frost heaves and to stabilize the house because there will be dirt (pressure) against 2 walls and almost none against the other 2.

Answer
Brent, I don't feel I am the right person to advise you on deep frost heave issues.  Frost heave is real, but here frost depth is 18".  We do have some major expansive clay issues and one of the design methods used here is piers with grade beams.  The grade beams bear totally on the piers and we use carton forms that create a 4" void under the beam to allow for soil movement.

I don't like not being able to answer your question, but I do not feel confident advising you.