soil pulls away from slab when dry. Is this a problem?

[Hobby_Man22]

I live in the south and we have a lot of clay and when we have a drought, the dirt can pull away from the base of the foundation as much as 1/2" People put soaker hoses around the perimeter of their foundation. Is this necessary?

 

[jrsavoie]

It's why I backfill with 1' of clean rock along a foundation.

 

[ConCretin]

The gap you are observing isn't necessarily a problem in and of itself but it is an indication of soil conditions that are potentially destructive. It sounds like you have expansive soils, which just means they expand and contract with varying moisture levels. These types of soils can tear a structure apart if the movement is severe enough. I suspect your neighbors are attempting to maintain moisture levels to eliminate the movement.

There are construction methods that stand up to expansive soils, which could eliminate the need to artificially maintain moisture levels. Post tensioned (edit) slabs have special reinforcing to combat the movement of expansive soils. There are other methods as well.

I think the first thing I'd do is identify the type of slab I had and depending on the answer, I'd consult with a local geo-tech firm for additional information on my soil conditions and what if anything needs to be done.

 

[Hobby_Man22]

It's got 12" footers around the perimeter and thru the middle. They may be 24" I'm not sure. So makes me wonder what problems it would even cause.

 

[ConCretin]

It's got 12" footers around the perimeter and thru the middle. They may be 24" I'm not sure. So makes me wonder what problems it would even cause.

I tend to agree.

 

[dutchgray]

The local clay ground in my area has been the cause of many cracked houses, in the days when a footing was 9" of concrete in a trench only about 2" deep. They almost always break when it dries out over a long dry summer.

Nowadays 3' deep concrete fill in a 18" wide trench is about as small as they get round here.
Dad did one once for a bungalow, 3' wide, 10' deep concrete with trench boards (insulation sheets) to separate the clay from the concrete, walls were only 8' on it

The ground around your footing drying out and pulling back is probably not a problem, its when the ground under your footing dries out and shrinks back that the real damage can be done, especially if the drying out is uneven across the building.

You don't want big trees near a building if the ground is clay either.

 

[Showkey]

Just Google “texas expansive clay soil foundation damage”. Yields hours of reading………

Worked in large shop building in Dallas ………the floor shifted so the garage door was 10” out of level.

Its not about thickness of the concrete…….it’s soil.

Daughters home in Corpus had $40k repair before her purchase. Including damaged foundation, plumbing and brick work. Walking around Corpus the roads, alleys, driveways and side walks have huge crack, large breakups, 4” trip points, extreme out of level ………much worse than freeze thaw damage in the north.

 

[FredWanaker]

I know of a whole housing tract in San Diego County that had to get new slabs, foundation wall repair and remedial changes to compensate for expansive soils. The slabs looked like someone had taken a jack hammer to them.

 

[Copymutt]

If your footers are up to local code, you should not have to worry. Here the expansive clay is dug out two feet below footer and replaced and compacted. Another aspect of the dried out soil is loss of ground via ground rods. Keep em watered once a month or so.

 

[strutaeng]

Yeah, definitely depends on where you are in the State...even within that map, there are areas that are far worse than others.

Where are you BTW? RGV? I think @LLWillysfan meant post-tensioned slab? The performance of P-T slab on grade on really expansive soils, unfortunately, based on my experience hasn't been great, but on paper it works. Typical construction of slab-on-grade here is a slab with turn-down grade beam, cast in the same operation (not a footer, per say, but it serves the same purpose.)

Anything within the upper "active zone" can and will move...it's kinda complicated. You need a geotechnical boring/report to know this. In residential, this is usually not done much, so no way of telling...I'd say just do what your neighbors are doing. It's standard practice to do the water at the perimeter if you see what you describe. Maybe talk to them and see if they have any issues.

 

[bradpac]

I'm in the same soil here in Texas. It will pull away an inch or so from the foundation. I've heard some people swear by watering their foundation and other say it will damage it. I do not water mine, no issues so far. My trees are doing more damage than the soil. Our house post-tension cables and so far it's held up well, there is an addition to the house that does not and it does have some cracks, but it still pretty level. My new garage has 30" deep beams all the way around and a cross through the middle with a 6" thick slab. Hopefully that will keep it from cracking, although it might get out of level at some point.

 

[ConCretin]

I think @LLWillysfan meant post-tensioned slab?

Yup. Thanks.

 

[niget2002]

I'm in the same soil here in Texas. It will pull away an inch or so from the foundation. I've heard some people swear by watering their foundation and other say it will damage it.

The real recommendation is to water the soil around the house... People take the easy way and run a drip line around the foundation. If they wanted to do it correctly, they'd have the drip line 6-12" away from the foundation. But that'd require picking up before they mow, and most people are too lazy.

Around me, if you're watering the yard enough to keep the grass green, then the soil shouldn't be getting dry enough to pull away.

 

[jrsavoie]

The real recommendation is to water the soil around the house... People take the easy way and run a drip line around the foundation. If they wanted to do it correctly, they'd have the drip line 6-12" away from the foundation. But that'd require picking up before they mow, and most people are too lazy.

Around me, if you're watering the yard enough to keep the grass green, then the soil shouldn't be getting dry enough to pull away.

To me , the real recommendation is to put a foot of clean rock under the slab around the outside.

 

[545_days]

A foot of clean rock does nothing in expansive clay. A few years ago during a drought, I wondered how deep the 2” to 3” cracks in ground went. I ran a 1” wide tape measure thirteen feet deep before the crack narrowed enough to catch the 1/4” wide lip at the end of the blade.

Expansive clay soil can wreck your foundation.

 

[K'ledgeBldr]

Showkey has the right info!
As I read the posts I kept thinking “where in TX?” I know all about that “Black Gumbo”!

And if you’re in an area of the Black Gumbo, you may need to install an irrigation system around the foundation. In some areas of TX it’s a standard item and/or requirement.

 

[Hobby_Man22]

A friend of mine

Showkey has the right info!
As I read the posts I kept thinking “where in TX?” I know all about that “Black Gumbo”!

And if you’re in an area of the Black Gumbo, you may need to install an irrigation system around the foundation. In some areas of TX it’s a standard item and/or requirement.

A friend of mine says it's do the moisture gets soaked under the slab so you're less likely to have the cracking beneath the foundation since technically the dirt 30ft towards the middle of the foundation never really gets wet.

 

[Hobby_Man22]

A friend of mine

A friend of mine says it's do the moisture gets soaked under the slab so you're less likely to have the cracking beneath the foundation since technically the dirt 30ft towards the middle of the foundation never really gets wet.

I'm not sure how much merit his theory holds though.

 

[rjacobs]

My sprinkler system has permitter spray heads... to help control this... And so the main rotors dont have to soak the house.

 

[niget2002]

My sprinkler system has permitter spray heads... to help control this... And so the main rotors dont have to soak the house.

Same.

 

[K'ledgeBldr]

I'm not sure how much merit his theory holds though.

Very little.
My sister’s house-Dallas area (has owned 25+Yrs) and has had no problems with the foundation because of the irrigation system.

Remember that drought about 10yrs ago or so?

 

[jkuro]

The DFW area had alot of poor construction over the years. Slab on grade homes with no footings or foundation to speak of. The builders got away with murder. These are the problem homes. Tree roots easily grow under the homes. When in a drought, the soil shrinks. A lot of these homes foundations are compromised and need repair, very costly. Soakers hoses help in a drought but minimal. Let the buyer beware. Posts or piers and beams under the slab are done to stabilize the home if you can afford it.

 

[beemerphile]

A foot of clean rock does nothing in expansive clay. A few years ago during a drought, I wondered how deep the 2” to 3” cracks in ground went. I ran a 1” wide tape measure thirteen feet deep before the crack narrowed enough to catch the 1/4” wide lip at the end of the blade.

Expansive clay soil can wreck your foundation.

I built a plant expansion in Corsicana, TX in the mid-1970's and we had to drill through to stable soil to install bell bottom piers for a furnace foundation and a tall slip-formed process building. The soil change happened at about 30 feet. The soil changed color as well at that point so it was easy to know when you were deep enough. Crazy dirt in that part of Texas.

 

[dcg9381]

It's probably a soil thing. Here, we're largely built on limestone. We did full foundation engineering and where we were not on limestone, we drilled piers.

Our shop actually has a graywater distribution system that goes along the foundation. Can you capture HVAC condensation and distribute it in a similar fashion?

 

[Hobby_Man22]

It's all clay it' shrinks and expands

 

[Lt CHEG]

I’m a chemical engineer with an MS in explosives engineering, so I’m not a structural engineer, BUT I oversee environmental remediation of superfund sites and deal with structural issues as it relates to soil, etc. often. If you have expansive clay soil then the shear strength of your structural members is absolutely dependent on the moisture content of the soil. I can’t opine whether or not irrigating the soil around your foundation is the best solution, but I would say that keeping your soil at a fairly constant, steady state should help the shifting of structural members. Good luck!

 

[PoorUB]

In my area we have heavy clay soil that moves around with changes in moisture. Concrete block foundations in this area fail. In fact the city will not allow block foundations in new construction.
A few years ago during a bad dry spell I had 2" openings between the foundation wall and the soil. I could shine a flashlight and look all the way to the footing six feet down! I started running water and a soaker hose and closed it up. I don't know if it made a difference to wet it down, but I figured it couldn't hurt.

I know of a guy that about 15 years ago started building a huge expensive home. We had the heating and plumbing contract for $125,000. The contractors poured the foundations and basement walls and then the soil started moving around. It was so bad it lifted the concrete footing and basement wall a few inches in one spot. The house never got built as the soil conditions were so unstable.

 

[Hobby_Man22]

Makes me wonder if it's best to pour a foundation in a drought. Every gate and or door always has clearance issues in a drought then realign once it starts raining again.

 

[PoorUB]

Makes me wonder if it's best to pour a foundation in a drought. Every gate and or door always has clearance issues in a drought then realign once it starts raining again.

It will not matter as soil conditions change.

My garage door opener needed adjustment a few times a year as the floor would rise and fall enough to affect it. I finally found a spring loaded opener link that allows about a inch of movement in the floor and has stopped the adjustments.

Also the man door might need adjusting, like in rip it out and rehang it! I finally found a happy medium and in a dry summer I may have to run the garden hose near the corner of the garage for a day to soak up the ground, which in turn raises the slab in that corner and the door works again.

 

[budget76]

Sister in law in TX just learned that apparently people in TX water their foundation. How'd she learn that? by having the whole slab crack through the house, tile floor ruined etc. No idea what the situation is with insurance or such

never knew it was a thing. apparently it is. Cookie cutter housing complex might have something to do with it

 

[Hobby_Man22]

Sister in law in TX just learned that apparently people in TX water their foundation. How'd she learn that? by having the whole slab crack through the house, tile floor ruined etc. No idea what the situation is with insurance or such

never knew it was a thing. apparently it is. Cookie cutter housing complex might have something to do with it

The part I don't get is the soil underneath it. Was I supposed to place a soaker hose down before they poured the foundation? Unless they're just talking about losing support on the sides and the middle of the slab not getting water not being an issue.

 

[budget76]

The part I don't get is the soil underneath it. Was I supposed to place a soaker hose down before they poured the foundation? Unless they're just talking about losing support on the sides and the middle of the slab not getting water not being an issue.

i don't understand it either, haven't researched at all though. Have not seen any pictures from their place, just word its going on.

 

[niget2002]

The part I don't get is the soil underneath it. Was I supposed to place a soaker hose down before they poured the foundation? Unless they're just talking about losing support on the sides and the middle of the slab not getting water not being an issue.

The second part of your statement is correct. It's the soil pulling away from the foundation no longer providing lateral support.

Your house is heavy, so it's not common, but if the soil pulls too far away, it does increase the risk that some of the foundation can move. Typically the whole house doesn't move, so the foundation snaps.

My soil around my pool equipment has dried out enough this year that it caused one of the PVC pipes to break. Granted, I think the person that did the install used sub-par materials, but nonetheless, I had to repair it.

As an aside... The ground has shrunk so much around my fence line that the bottom of my chainlink fence is no longer buried and has over a 1" gap between it and the ground. I've been having to install metal strips across the bottom of the fence so my Pyrenese can't stretch the chainlink fence up and squeeze under it.