Slab on clay soil

[jshwhite]

Hello, long time reader, first time posting here. Really appreciate all the great information here. I have a question which has probably been discussed already but I can't seem to find anything specific to my situation.

I'm building a 32x56 pole barn, planning to pour a 4" slab. Southern Missouri - heavy clay soil. I prepped the site well and scraped it all down to virgin soil. It was pretty dry at that time. I framed it up, spread my gravel base (from about 10" to 2" thickness, as the grade was low on one side) and then had to go out of town for a month. During that month, it's rained like crazy.

So I have it pretty well ready for concrete, but now the clay subsoil is completely saturated with water. Is that going to be an issue if I pour the slab now and then the clay shrinks as it dries out? If so, how long do I need to wait for it to dry out?

Thoughts? Thank you!

 

[Jim greengo]

Hello, long time reader, first time posting here. Really appreciate all the great information here. I have a question which has probably been discussed already but I can't seem to find anything specific to my situation.

I'm building a 32x56 pole barn, planning to pour a 4" slab. Southern Missouri - heavy clay soil. I prepped the site well and scraped it all down to virgin soil. It was pretty dry at that time. I framed it up, spread my gravel base (from about 10" to 2" thickness, as the grade was low on one side) and then had to go out of town for a month. During that month, it's rained like crazy.

So I have it pretty well ready for concrete, but now the clay subsoil is completely saturated with water. Is that going to be an issue if I pour the slab now and then the clay shrinks as it dries out? If so, how long do I need to wait for it to dry out?

Thoughts? Thank you!

Let it dry.

 

[readhead]

Probably best to let it dry out and it should never be wet again.

 

[jshwhite]

Thanks. I guess my question is how dry is dry? Do you think 2-3 weeks without rain would be sufficient? It would be really dry come July/August, but I don't really want to wait that long.

 

[ConCretin]

Is the clay in your area classified as expansive? This would be important to know. If the clay sub base is covered with 2 to 10 inches of gravel, how do you know the clay is saturated? many types of clay are more or less impervious to water. Issues like this can be hard to comment without local knowledge but around here, we're not waiting around weeks for things to dry out.

 

[Firebrick43]

If your in the olive drab area of southern misery then your probably good.

It’s always a good idea to have footing drains to daylight or a tile to slow rot of the poles and keep moisture from coming up thru the slab. Remember to put stegowrap under the slab

 

[LopezBart]

I had similar issues with a swimming pool forming in the area cleared of topsoil when we had a wet winter during the initial land prep. A footing drain (just visible in last photo) that drops steadily to daylight has kept the water from accumulating.

 

[jshwhite]

If your in the olive drab area of southern misery then your probably good.

It’s always a good idea to have footing drains to daylight or a tile to slow rot of the poles and keep moisture from coming up thru the slab. Remember to put stegowrap under the slab

Super interesting. Thanks for sharing. That's correct - I'm in the brownish/drab area. I did a bit more research and I think that's correct that our soil isn't highly expansive. It does hold water like crazy and runoff like crazy if it can. I know just from observation that it does shrink a bit mid summer when it gets really dry, but perhaps that's not considered enough to be a concern?

Definitely will be doing a footing drain. As regards the stegowrap, I was just planning to pour the slab straight on the gravel. But I'm open to being convinced otherwise. What are the reasons/pros/cons?

 

[ConCretin]

As regards the stegowrap, I was just planning to pour the slab straight on the gravel. But I'm open to being convinced otherwise. What are the reasons/pros/cons?

A vapor barrier such as Stegowrap blocks water vapor from coming through the slab. While it's generally a good idea to limit moisture intrusion into interior spaces, the primary reason for installing a vapor barrier is to protect the bond of adhered floor coverings such as epoxy. If you plan to coat your floor or want to preserve the option of doing it in the future, a vapor barrier is a prudent investment.

Many try to get by with 6 or 10 mil poly but these products break down over time and are subject to damage during construction. A better choice is a 15 mil product such as Stego. In addition to being more effective at blocking water vapor, it's much tougher and resistant to damage.

 

[ericm]

Super interesting. Thanks for sharing. That's correct - I'm in the brownish/drab area. I did a bit more research and I think that's correct that our soil isn't highly expansive. It does hold water like crazy and runoff like crazy if it can. I know just from observation that it does shrink a bit mid summer when it gets really dry, but perhaps that's not considered enough to be a concern?

Definitely will be doing a footing drain. As regards the stegowrap, I was just planning to pour the slab straight on the gravel. But I'm open to being convinced otherwise. What are the reasons/pros/cons?

Your county may have an on line GIS system that shows the soil type and if they consider it expansive. If not the planning dept likely has the data and can look it up for you.

 

[Copymutt]

I can’t comment as I don't know your freeze depth, your design: mono, slab on grade, stem wall w/ footer?

 

[ConCretin]

I can’t comment as I don't know your freeze depth, your design: mono, slab on grade, stem wall w/ footer?

He told us it's a 4" slab in a pole barn in Southern Missouri in his first post. Comment away.

 

[SBAG]

I’m in north central MO, but most clay in MO is definitely not expansive. Dig a hole? Have a pond. People bring in clay as fill for building here.

 

[Copymutt]

Slab only on grade it is. Footing drain? What footing?

 

[jshwhite]

Sorry I got busy and didn't get around to replying. I did a bit of grading on site and it was wet only about an inch or two down. So it seems very true it's not an expansive soil. Good to know! I poured the slab yesterday and everything went well. Thanks everyone for the input. Lots to learn here.