DFW Metroplex Foundation Maintenance Tool

If “doming” (fn) is a serious problem during dry seasons for anybody who’s built on Las Colinas Tufa… and if clay soils all over Texas are an issue… why not just insert a cheap plastic barrier into the soil when the concrete’s being poured?  It’ll move around some as the soil does, but it would keep the moisture under said foundation much more consistent, and the total cost to the builder would probably be less than a hundred dollars.

Even after the fact, you could do this with several days of serious soaker-hosing, combined with a good drenching, and do this with nothing cobbled together from Home Depot or Lowe’s.

This seems like a no-brainer to me.  Is it, or am I stuck on stupid and missing something important?

(fn)– defined to me by my buddy Phelonius as what happens when moisture collects in the soil under center of the foundation while the ground all around the edges are drying, thus producing serious foundation stresses as half of it expands and pushes up on the concrete, and the other half contracts, thus “pulling” it down.

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8 Comments

  1. eowyn

     /  August 31, 2006

    Not that I’m pretending to know what the hell I’m talking about, but couldn’t you accomplish the same thing with a layer of bentonite (i.e. the stuff they use to line irrigation and cattle ponds)?

    Reply
  2. convivialdingo

     /  August 31, 2006

    Here’s what I was told.. though I’ve never had a slab foundation house. It worked well for my pier and beam house (which had a footing around the perimiter)

    The best thing to do for clay soils is to get a soaker hose around the whole house, get a 10 dollar water timer and just water it every day for 5 minutes.

    If you have trees near your foundation (especially mulberries and other river-bottom type trees) then they will absorb a lot of the water and will cause settling. Prune the tree to lower of amount of moister it needs to support the leaf system. Also give a little extra water to the area to maintain the moisture or double the soaker in that area.

    Reply
  3. That will keep the moisture steady as well… I’m just wondering, since clay that’s plastic-wrapped doesn’t dry out, if a layer around the end of a foundation would be a cheap way to keep folks from having trouble in the first place…

    Reply
  4. convivialdingo

     /  September 3, 2006

    I think plastic may be to great – it may cause swelling and softening of the soil underneath to the point where the foundation “slips” if you’re not cautious – especially during the rainy season.

    It might be best to just use mulch next to the foundation or plastic a foot or two off from the foundation to give it some breathing room. YMMV.

    Reply
  5. GOOD mulch might be just the trick.

    Reply
  6. There are, of course, licensed structural engineers who spend their entire careers worrying about foundations. I’ve represented a few in foundation cases. They seem to know what they are talking about. The fact that they don’t recommend this process makes me think it doesn’t cure anything.

    I frankly don’t understand how a water barrier at the slab affects water in the clay underneath the slab. If anything, it could trap more water under the house, making the problem you describe worse. Some sort of french drain under the foundation (if such a thing is possible) would make more sense.

    Reply
  7. Shingle, if you’re a lawyer you should know better than to post an argument from authority in such a shoddy manner. Unless, of course, your having represented SE’s such as the one I hired recently makes you an engineer by osmosis.

    A moisture barrier at the slab may actually turn out to be a rotten idea: I don’t know. That’s why I made this post. What it would protect against is excess moisture migration within the top layer of soil — during heavy rain and during drought. In the first case, it would keep erosion at bay. In the second, it would slow moisture from migrating out of the soil underneath the edges of the slab because the soil just adjacent to that was subject to heavy evaporation during drought. While I don’t see how such a system could possibly increase the doming effect, unless the barrier is four feet deep or something ridiculous like that, it’s theoretically possible.

    Reply
  8. The conditions you are describing is called the drying edge effect.
    Placing a moisture barrier under the slab prevents moisture from seeping through the concrete. Today’s building standard calls for a moisture barrier to be placed under a slab.
    Trees around a foundation are a different, and sometimes larger problem altogether. Large trees can wick water away from the center of the foundation causing a condition called cupping. Cupping is the opposite of doming were the center of the foundation sinks while the edges stay in place. Not only do trees absorb water from beneath the foundation, but as trees grow larger, the root system also grows and can damage the foundation. We have all seen a sidewalk that becomes uneven or broken, which is typically caused from tree roots underneath the concrete. Your foundation will do the same over time. Truthfully trees should not be planted any closer than 15 feet to a foundation, although I know that this is never the case.
    Clay soil in North Texas will only accept 1/16 of an inch of water per hour, so absorbtion is a slow process. It is actually better to water at night when evaportation slows due to lower temperatures. Clay soil will expand and contract a total of about 2 feet. The clay must also expand and contract uniformily or other foundation issues will become evident. Mulch around a foundation will help stop evaporation around the foundation but wood mulch, combined with retained water is a ideal enviroment for wood destroying organisms (termites, carpenter ants, etc.).
    Hope this helps.

    Reply

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