Shrink Swell soil

[warburtonplayer]

Does anyone know what the specifications are on pouring a concrete pad on shrink swell soil.., How deep should the pad be how thick?

[RedDirt]

I won't be able to answer your question on how thick but I do know when exposing expansive soil for your pad you'll want to keep it moist until the concrete is placed. You don't want it to dry out and show cracks. If the soil dries out too much and shrinks before concrete is placed problems will abound when it expands in the wet season.

On a large home excavation that was exposed for about 8 months I had to continually balance the soils engineer's recommendation to make it wetter and the concrete crew's request to keep it dryer. We'd water down the 7000sqtf excavation every day "after work" with a 1 1/2" fire hose for one to one and a half hours! The goal was to have the surface be just a little mushy (slippery to walk on and mud stick to your boots) the following work morning.

As far as the needed thickness, you'll need to supply more information to get a reasonable reply from someone. Required thickness is calculated from the loads placed upon the slab, foundation, etc. A builder designs from the bottom up but a structural engineer needs to design from the top down. He needs to know the loads placed on the slab before he can calculate the strength needed to resist those loads.

What is the pad's use? It there a building on it or is this just a open patio or is it a driveway?

[schmism]

Originally Posted by RedDirt

As far as the needed thickness, you'll need to supply more information to get a reasonable reply from someone. Required thickness is calculated from the loads placed upon the slab, foundation, etc. A builder designs from the bottom up but a structural engineer needs to design from the top down. He needs to know the loads placed on the slab before he can calculate the strength needed to resist those loads.

What is the pad's use? It there a building on it or is this just a open patio or is it a driveway?

actually when it comes to foundation, you need to work from the ground up. I have to know how much the soil can support (ground PSI) before i can determine how wide my footings need to be. for a given load 20,000lbs, if i have 5000psi soil, i need 40 sq inches.... if i have 2000psi soil i need 100 sq inches... untill you get a soils report from your soils engineer (soil tests) you wont know how big a footing/thick a pad youll need.

[RedDirt]

Originally Posted by schmism

actually when it comes to foundation, you need to work from the ground up. I have to know how much the soil can support (ground PSI) before i can determine how wide my footings need to be. for a given load 20,000lbs, if i have 5000psi soil, i need 40 sq inches.... if i have 2000psi soil i need 100 sq inches... untill you get a soils report from your soils engineer (soil tests) you wont know how big a footing/thick a pad youll need.

Sure, I agree. But you still have to know what that "given load" is for a proper design. That's what I was referring to in the "top>down" design statement.

[dynasim]

It is my understanding that self-supported mild steel reinforced slabs on expansive(clay) soils will crack. It doesn't really matter how thick it is(within reason). I have never personally seen a badly cracked slab from this, though.

In Houston, where I once lived, the soils were very expansive, and there were many warning about keeping you yard evenly moist, etc.

Almost all of the older slab homes there that we looked at had some sort of cracking. The newer homes either used piers(deep bell bottomed holes filled with concrete), or post-tensioned pre-stressed concrete. Done correctely, both work. Done poorly, and your just stubbing your toe.

Chris

[marimus]

In Australia, they use this system of foam blocks on reactive soils:

Waffle Pod

That is just one manufacturer, there are a few more, but it illustrates what they are doing to build a "floating slab" that moves with the soil, without your house breaking itself appart.

When you are traveling from Toowoomba to Dalby, you hit a plain at the bottom of the range crossing, and the highway (which was built straight and flat) wiggles left and right, up and down depending on the soil moisture.

The power lines running along side the highway are also known to move, and sure make for some interesting photos (you would swear the crew that put them in was constantly drunk)

[RedDirt]

Maimus,

Your towns have such great sounding names! Toogoolawah, Toowoomba, makes me want to beat a drum and howl at the moon.

In expansive soils we do similar here in CA too except the slabs are placed on triangle shaped tubes of cardboard called crush forms. The house is supported on piles and the crush forms allow a void beneath the concrete so the slab is not cracked when the soil expands.

It has been several years since I've done one of these but could look up the old specs for the product name if anyone is interested.

[oliver28472]

If this is for your home you should get an engineered slab. I worked for an agency in TX that financed many homes. All the slabs were engineered for shrink/swell soils. They were called monolithic slabs. Trenches were dug around the outside perimeter. In those trenches were a system of rebar stirrups wired to horizontal rebar which made beams. May have been in the center too. Also had rebar meshed in from one side to the other. Always had plastic vapor barriers and plumbing put in prior to pouring concrete.