Foundation above Frost line

[kxa]

The house I am investigating its foundation to see if it can take the load of a second floor addition is about 2 feet above the frost line. By code, the bottom of the footing has to be at least 3’ down. I am thinking of using helical piers but the owner is concerned with the cost. The house is near a bulkhead and the material is (SM) with about N=3 down to 7 feet blow grade. Ground water is tidal. At low tide, water level is about 4.5’ below grade.

Also, they are adding a one car garage attached to the side of this house. I am thinking of recommending a 6” concrete slab w/ wire mesh and the wall footing designed as grade beam 3’ down supported by helical piers.

Any help or suggestion is appreciated.

 

[ishvaaag]

If the existing home has proven a successful design, I would study if to go without the anchors, maybe it is feasible especially if the foundation is a whole plan slab or generously sized; in that case I would try to mimick the existing foundation and the new, in which case I would likely tie one to the other after some study, not forgetting to moderately roll compact the base of the new foundation.

If you underpin, then underpin old and new.

 

[BigH]

What soil types are you dealing with? Are they frost susceptable? Codes, codes, codes - they are general - and one must consider, in my view, the actual site conditions and make an engineered decision based on facts, experience and common sense.

 

[oldestguy]

How does the existing house look for behaving under present loads and its history? Chances are adding some load can be tolerated.

Next about the "code". In my general area they talk about "frost depth" to 4 feet, but no one actually does any scientific determination, usually. However, in your case, for 3 ft. frost depth,why not approach the building inspector with this idea.

Dig down one foot around the outside, place a 2" thick sheet of the pink, closed cell insulation, running it up the wall to ground surface also. In this case, extend it out three feet and use one foot up the wall (from a 4 ft. wide sheet). We do that a lot here and have no problems with shallow footings or slabs on grade.

 

[jhoulette]

I agree with oldestguy, prtect the foundation as needed from frost. You may be surprise how little protection is needed. Below is a link to a guide how to design a Frost Protected Shallow Foundation.

http://www.toolbase.org/PDF/DesignGuides/revisedFPSFguide.pdf

But the helical pier is a good idea, just don't mix a shallow footing foundation with a deep foundation system, they won't wove the same.

Jim Houlette PE
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http://www.evstudio.us

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