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Dual foundation?

aaronbeckworth | Posted in General Questions on

Can anyone direct me to a detail for a dual foundation, piers abutting a stem wall/slab on grade?

Our situation:
We are located in a small mining town in SW New Mexico, CZ4B. The original 1920’s structure was built on piers on a sloping site. A poorly built addition will be removed/torn down as no consideration was given to run off and erosion of the sloping site.

Our idea:
Begin by removing the addition. Pour a stem wall to a height matching the subfloor of the original structure. Then backfill and compact to form a level terrace for the addition. Stone retaining walls would continue the terrace from the far ends of the stem wall across the site and tie into the garden terraces. We would end up with a terraced building site and a house half build on piers and half on a slab on grade. The original structure on piers will not be enclosed with any type of skirting.

Thanks,
Aaron

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Replies

  1. Expert Member
    Peter Yost | | #1

    HI Aaron -

    I am not sure what sort of detail connecting the two foundation systems you need. I am assuming that the crawlspace will be uphill of the piers. If that is the case, you just treat the footing perimeter drainage system of the crawlspace foundation as independent of the pier system, presumably draining your crawlspace drainage system to daylight in such a way that it does not end up loading any piers.

    Does that make sense? If I am missing something might take a sketch to add to your text description.

    Best - Peter

  2. Expert Member
    Dana Dorsett | | #2

    When dealing with foundation issues on potentially unstable erosive soils on a slope have an engineer make the call on how to deal with it. This sort of thing isn't as amenable to "design by web forum" as other construction problems. Stablizing a slope adequately isn't always straightforward as installing some stone retaining walls and draining them well.

  3. aaronbeckworth | | #3

    Dana,
    Ultimately I will be working with a design professional on this project, and would have that person seek the advise of an engineer if need be. At this point I am just thinking about different possibilities.

    Peter,
    I think I may have caused confusion by using the term ‘stem wall’. Maybe the attached drawing will help explain what I am thinking. I do appreciate the responses.

  4. Expert Member
    MALCOLM TAYLOR | | #4

    Aaron,

    What you have drawn will work fine. If you want, you could let the (pt) ledger into the form so the concrete supports it. The thing to watch out for is a situation where the intersection of the old wood frame and new concrete floor systems meet where it isn't a wall (like a doorway without a threshold, an arch, or a large room that spans both the existing and addition). There will be differential movement at that joint and depending on what flooring is used it may cause problems. I'd draw a few quick sections, like the one you have done, in a few places on the plans to make sure it works everywhere.

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