Should I water my house foundation to keep it from cracking, and if so, what is the science behind how that helps?

August 20, 2008 7:12 AM

Should I water my house foundation to keep it from cracking, and if so, what is the science behind how that helps?

About a year ago, I bought a 55 year old house in Austin, TX. The soil below it is clay, and during this summer's drought, what were hairline in the masonry have become wide enough to stick a penny in. Additionally, the brand new hardwood floor has developed a gap that runs down the center of the house. After freaking the hell out, I did some research, and repeatedly came across the assertion that a homeowner is downright negligent if they don't install a drip system around their house which will run pretty much 24/7.

1) Is this actually going to do anything but be exceptionally wasteful and run up my water bill?

2) Why would this help or not help?

I understand that fluctuations in moisture cause expansions and contractions, and that the idea behind watering around the foundation is to maintain consistent moisture under the slab. BUT, my whole neighborhood is a sea of expanding/contracting soil, is watering really going to keep my slab afloat on some life raft of stability? Isn't there still going to a big difference between the moisture at the perimeter of the slab and the interior, creating an even bigger problem if there are weaknesses running through the middle? Are there any studies that prove or disprove this?



BTW- I am going to consult a structural engineer as well.

posted by worstkidever to home & garden (11 comments total)

It's doubtful that this would help. Clay does expand when moist and contracts when dry. The soil underneath the house will always retain some moisture even when the adjacent clayey soil dries out but the pull from that soil will not be enough to wrench apart the foundation slab.

You might want to look at the possibility of injecting grout along the outer edges of the slab around the entire house which should shore it up against any more settling. That shouldn't be too expensive and will probably be cheaper in the long run than the drip method.

A small crack in the slab shouldn't be too much of a worry but you want to monitor it to see if it does grow. If you have to replace the hardwood floor, install it as a floating floor rather than fixed. That way if there is any future settling, it will not show up in the middle of the floor
posted by JJ86 at 7:59 AM on August 20, 2008

This is a common thing in south Texas. During a drought the soil will shift and it will break water pipes, foundations and anything else in the ground. You will have many fewer problems if you use a drip hose.
posted by sanka at 8:42 AM on August 20, 2008

You actually might want to consult a geotechnical engineer. Texas has something called expansive clays, which (suprise!) expand when wet. So, depending on the typical conditions of the clay under your slab you'll either want to keep it moist or dry, whichever is easier. Most likely that will be moist, but you'll need a monitoring system so you know what the conditions are and a distribution system so that everything is watered equally. Too much in one spot and not enough in another can lead to differential settlement or heaving, which will fuck your slab, proper-like.

Mudjacking, which is what JJ86 described, will probably not help if you have expansive clays. Your options are generally to install a foundation watering system, or dig up the clay and replace with fill.
posted by electroboy at 8:59 AM on August 20, 2008

a geotech consult will provide recommendations for proper slab construction (in your case, should offer recommendations for maintenance or remediation). I've personally never heard of a watering system despite having similar soil heave issues here in arizona, but maybe they're just less common here. More often, non-expansive fill soil will be brought in, and/or a post-tensioned slab will be used so the slab relies less on even support from the soil. Even in new homes, though, moderate shifting and cracking are expected.
posted by Chris4d at 12:09 PM on August 20, 2008

I should clarify, mudjacking is appropriate for filling voids and moving your slab back into it's original position, but it will not prevent ongoing heave/settlement issues. Also the post tensioned slab is appropriate for new construction, but as far as I know, there's no way to retrofit.
posted by electroboy at 1:14 PM on August 20, 2008

My sister's friend had trouble with her house's foundation in Rancho Murieta, CA. I called a local Geotechnical engineer, and I had two guys call me back, and both talked to me for twenty minutes and gave me some advice over the phone. Give one in your area a call. They will know your soils, and can tell you ball park costs.
posted by Monday at 3:14 PM on August 20, 2008

I wasn't talking about mudjacking. As you mentioned, electroboy, it is for filling voids by use of a hose hooked up to a grout pump. Grout injection is a more modern technique which any geotechnical engineer will be familiar with and requires very different tools than mudjacking. Basically, it uses drilling and injection apparatus to stabilize the supporting soils so they do not move.

If the foundation slab had been built directly on crushed stone as is normally done there would be no friction issues as there could be with a slab directly on clays. Clays, by their nature expand with moisture content. An engineer should be able to tell with a site visit if this is the actual case or if there are settlement issues. Settlement issues are another animal entirely. Hazarding guesses over the phone is not recommended.
posted by JJ86 at 8:02 PM on August 20, 2008

Jet grouting is going to be tough too. It works really well with soils with lot of voids, but not as well with a solid mass of clay. Even more difficult will be trying to grout in the interior of the building footprint. Will most likely involve tearing up the floor to do any borings, if it's even possible.

Most of the suggestions above are good ones, but they're complicated by the fact that there's a house right where we need to work. If the geotech quotes you an insane amount to do the work in situ, as a last resort you could have the house temporarily moved, then moved back once the work is complete. But again, that's worst case scenario.
posted by electroboy at 5:53 AM on August 21, 2008

electroboy said: Jet grouting is going to be tough too. It works really well with soils with lot of voids, but not as well with a solid mass of clay. Even more difficult will be trying to grout in the interior of the building footprint. Will most likely involve tearing up the floor to do any borings, if it's even possible.

It obviously depends on the severity of the scenario. According to the seminars I have attended and the Hayward Baker literature I have jet grouting will work with denser soils such as clays. As far as doing the part of the slab in the center of the building, I wonder why you think that would be necessary. A small house isn't going to require much ground stabilization to underpin the slab. Usually all it would take on a normal-sized house is underpinning at the four corners. We're not talking about a skyscraper on a mat foundation here. Underpinning the slab to prevent migration pretty much is the only real, economic option in a worst case scenario.

If the only migration is as the OP says, "...wide enough to stick a penny in", then that hardly qualifies as being much of a problem. The underlying clays can only contract so much, they don't contract endlessly. Moving the house is just plain silly in any scenario.
posted by JJ86 at 9:01 AM on August 21, 2008

Moving the house is just plain silly in any scenario.

Well, that was a joke, but I'm not going to get into a pissing match over the best method, since none of us has seen the house, or have any experience with expansive clays. They're pretty much a southwest thing, and the OP needs the opinion of a local engineer with experience in that area.

So, to make a very long story short. Talk to a local engineer.
posted by electroboy at 9:25 AM on August 21, 2008

So I have met with my first foundation professional. I learned that the perimeter of the foundation is measured base on how much higher or lower it is than the center. This is because the center of the foundation stays dry underneath, unless you have leaky pipes. So usually it is only the perimeter of the foundation that is prone to shifting. This is the reason that watering the edges of the foundation has stabilizing impact on the whole, because the interior is already stable. He seems to think that my problem can be solved by getting the water off of the house, and by installing a drip system on a timer for the dry seasons. We'll see what the next guy says.
posted by worstkidever at 2:16 PM on August 25, 2008