Pouring large slab on red clay.

[Dyne127]

Hello! Me and my father are looking for some advice on pouring a 7200 sqft slab on top of red clay. We are looking to purchase an indoor tennis court that was built with clay. (Red clay.) We have gotten many mixed opinions on this matter and we are trying to figure out whether or not to pull the clay out first. The clay is only 4" thick, and we are looking to pour a 6" slab on top. Will a few inches of gravel and a ton of rebar be OK on top of the clay? The court was certainly compacted heavily over the years. I have heard from some masons that the clay can cause the slab to crack and sink down the road. This is going to be a very expensive pour and we want to do it right. Any input is appreciated. Thanks!

 

[BigH]

What is beneath the "red clay"? Do you have a local engineer involved or are you doing this "on your own." The question that begs, in my view, is that to pull out the red clay might be a good insurance policy - what will be more expensive - pulling out the clay (presuming you have good material beneath and place the slab following good engineering and construction practices) and have no problems or leave it in with a risk of problems that will require potentially expensive remedial action along with lost revenue because your business is not open.

 

[Dyne127]

There is regular soil beneath the clay.
I agree it is a better idea to rip up the clay. Yes, we have professionals lined up to do this. I'm just trying to get another view on this matter being that I have gotten contradicting opinions. It looks like we are going to rip up the clay. I appreciate your input. Thank you.

 

[cvg]

there is no way I would leave the clay there, especially since it is just 4 inches thick

 

[hokie66]

I don't really have an opinion as to the wisdom of leaving the clay, but does anybody know what potential problems the clay could cause? I don't.

 

[BigH]

. . . and what is "regular soil"? Personally, I would take it out but are you in an area where the "regular soil" might heave if it gets wet or shrink if the moisture reduces? That is regular soil in many parts of Texas and elsewhere.

 

[cvg]

this is an interior slab, water intrusion is likely not a big concern. however, if you have a plumbing leak or for some reason the groundwater level rises, the clay will expand and move the slab. this could cause cracking and may affect walls, doors and windows. however, if the red clay is on top of gray clay, than there may be no point removing it. In that case, than you might want a post tensioned structural slab

http://www.jlconline.com/architectural-mesh/post-tensioned-slabs.aspx

 

[bb29510]

we pour on red sand clay every day, sometime with vapor barrier sometime without, nothing wrong with it. its the only dirt we have around here, if its compacted and not wet, no problem

 

[TXStructural]

You need to know if is it expansive clay (fat clay) or the kind of clay used as fill because it is not expansive. As said, the slab is indoors, so assuming you cont have moisture problems now, I would not expect service problems in the future. That said, if there is open soil around the building exterior that will allow water to seep or soak under the slab, you need to have a geotechnical engineer take a look at it. Where I am (Dallas), we have native soils that swell such that over time, the edges of a slab like this may fluctuate on the order of inches per year with seasonal rainfall. Over time, I have seen the slab under a 700,000 sq ft building where the perimeter continuously dropped or interior rose uniformly 10 inches over 20+ years.

 

[Ron]

"Clay" tennis courts are usually not clay in the sense we think of them as engineering materials we either have to modify or deal with in situ. Most "clay" for courts is ground from something....shale, brick, etc. It does not particularly have classic clay properties as we know them in engineering.

As cvg noted, this is an interior clay court and thus not exposed to rainfall. Since the "clay" used for the court is generally not expansive anyway, I would put this at a low risk for movement of the slab. Having said that, the permeability of the "clay" is relatively low and is susceptible to capillarity, so if the water table is within a few feet of the bottom of the material, I would remove it since the concrete will be a capillary barrier and water imbibed by the material will get trapped there, thus lowering its stability as a compacted base material.

Have a simple Atterberg limits test run on the material by a local lab. The cost is nominal (usually less than $100 US) and you will find that the material is likely non-plastic or very low plasticity.

Considering the small amount of material, it is easier to take it out and feel comfortable about it than worry otherwise.