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basement with high water table, expansive clay. Bad idea?

carltonh Hatfield


Joined: May 02, 2010
Posts: 15
I need helpful opinions, but I think I just need to tell the whole story.

I live in a northern suburb of Dallas, and went to look at a house in Dallas, which of all things listed as including a basement.  My very experienced realtor, who was a home inspector for 10 years before that, said this was incredibly unusual.  The only other residential Dallas basement he had seen was concrete.

This house was/is a pier and beam home built in 1950 without a basement.  It is on a hill, and at some point the home had been dug out underneath to create a basement basically under the whole house.  It is currently owned as an investment property, with the investors knowing nothing about the how, why, or stability of such structure.

Whatever flooring in the bottom of the basement was there, was removed by the investors, and it is currently a dirt floored basement.  They also removed the basement wall paneling to inspect the wall.  The dirt and earth is being walled out by corrugated steel.  My realtor doesn't think that would be good enough to securely hold up over time.

There are two good reasons that basements and underground housing basically don't exist in Dallas.  We have very expansive clays that easily destroy many well built foundations, sometimes in only a few years.  2nd, we have a very high water table.  I realize that building on a hill can partly minimize this, and this house is on a hill too.  If I could feel secure about the basement, I think I'd love the house.

Feel free to look up the house if you'd like, but the pictures on realtor websites aren't including pictures of the basement.  MLS#: 11420875  It was only recently listed, I think.
Jami McBride
volunteer

Joined: Aug 29, 2009
Posts: 1760
    
    3
Hum... using Mike's http://www.undergroundhousing.com/ methods -  you maybe able to hire someone to help you to put thick plastic against the clay walls, then reinforcement wood planks and last of all vertical polls if needed for additional strength.

But Mike would know better than I what is possible - hopefully he'll jump in here 
carltonh Hatfield


Joined: May 02, 2010
Posts: 15
Here's a question though.  When I went to see it, it had been a few weeks since it rained much, I think.  However, the underground part was still pretty humid, unlike the 95 degree temperature out doors.  It didn't smell moldy.  Maybe a little mildewish.  Is that normal or appropriate for underground shelters?
Muzhik McCoy


Joined: May 26, 2010
Posts: 278
Location: Iowa, border of regions 5 and 6
carltonh wrote:
Here's a question though.  When I went to see it, it had been a few weeks since it rained much, I think.  However, the underground part was still pretty humid, unlike the 95 degree temperature out doors.  It didn't smell moldy.  Maybe a little mildewish.  Is that normal or appropriate for underground shelters?


For something built in the 50's it's normal.  It's also not desirable.  I'd wager it's because there's no vapor or moisture barriers in either the floor or the walls.  My guess is that since the basement was dug out after the house was built, there are no ventilation ducts to or from the basement.  It's quite possible that this was dug out primarily to be a fallout shelter, with the person doing the digging probably not knowing what he was doing.  What, by the way, is holding up the walls on the first floor?  Was the original house built on a slab, and the person just dug underneath the slab?  Is there anything underneath the slab to hold it up in the center, like a pillar?

If you're willing to make the investment (and it will be significant), I think it's possible to make it work.  Based on my admittedly limited experience (i.e., none), my guess is whatever architect you hire to do this will have you do something like this:


  • [li]Dig down at least a foot more into the floor, and a foot out from the existing corrugated steel walls.  Have the new floor and wall sections angled to direct water away from the structure, possibly into a french drain.[/li]

    [li]Line the new outer walls with railroad ties fixed with spikes, so they flex but won't collapse.[/li]

    [li]Cover the floor with gravel, then lay Styrofoam over that.  This will create a flat surface for the new floor.[/li]

    [li]Lay thick plastic over the Styrofoam, then pour the new floor.  Put in vertical rebar and pour the new walls, so there is at least a 6-inch space between the railroad ties and the concrete wall.  [/li]

    [li]Put plastic or some other moisture barrier over the concrete.  (The architect may put more foam on the outside of the concrete first; I don't know, since you're in a warmer zone than I am.)[/li]

    [li]Fill the space with gravel.[/li]



  • Hope this helps.
    Muzhik McCoy


    Joined: May 26, 2010
    Posts: 278
    Location: Iowa, border of regions 5 and 6
    Oh, and while I'm thinking of it:

    If you're going to go through the expense of fixing the basement, make sure to put in some safe-egress window wells.  Not sure of the correct name, but they're the windows that allow you to escape from the basement or allows firefighters to get in.  Up here, if you put a bedroom in the basement, it has to have one of these windows in order to be a "conforming bedroom."
    Jami McBride
    volunteer

    Joined: Aug 29, 2009
    Posts: 1760
        
        3
    No it shouldn't smell, but that's what I'm saying, your seeing it in a raw state now.  If it was finished using Mike's methods and had cross ventilation design into it, well that would be a different story altogether.

    I think the biggest problem that house/basement has working against it is the hill on the uphill side.  All the water that lands on that upside will run down to the house/basement. - yikes!

    If you use Mike's ideas you would need to cut into the hill just above the house (uphill side) removing that soil and cutting the land so it will drain away - like in this image of Paul's -



    But of course your house would look more like the modified version posted below -

    That's your biggest concern, the up hill contour of the land. 

    So the questions become:  Can you excavate the up hill side?  Can you finish off that basement so it is watertight? 


    [Thumbnail for Image1.jpg]

    Old hammy


    Joined: Jun 27, 2010
    Posts: 59
    Location: NW Ontario
    I would have to say that if all that is holding the earth back is some sheet metal, you might potentialy be looking at a death-trap. Clay soils are heavy, especially when saturated with water and will creep inward over time with unbelievable horizontal pressures (like a ratchet). Also, when warm humid air is allowed to enter a relatively cool environment (like an earth pit) it will condense and potentialy lead to mold problems. There are plenty of examples of wicked rot and mold problems in vented crawlspaces. Also, be careful about toxic soil gasses (radon) in a space like that.
    Muzhik McCoy


    Joined: May 26, 2010
    Posts: 278
    Location: Iowa, border of regions 5 and 6
    Old hammy wrote:
    Also, when warm humid air is allowed to enter a relatively cool environment (like an earth pit) it will condense and potentialy lead to mold problems. There are plenty of examples of wicked rot and mold problems in vented crawlspaces. Also, be careful about toxic soil gasses (radon) in a space like that.


    That is why in the last 5 or 6 years, the recommendation has been to seal up any vented crawlspaces, making sure you have a moisture barrier (i.e., thick plastic) over the bottom of your crawlspace  and going up the sides.
    Emerson White


    Joined: May 02, 2010
    Posts: 1206
    Location: Alaska
    If the death trap was dug after the house was built could she just fill it back up with fill dirt? Dallas always has construction going on, I'll bet that a creative person could find someone looking to get rid of clean fill.
     
     
    subject: basement with high water table, expansive clay. Bad idea?
     
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