Drought condition impact on new concrete slab 1

[netknight]

We are experiencing a drought condition in Dallas that has left the area without rain for 60+ days. I am about start construction on a large home that will be on a "engineered" post tensioned slab on piers. The soil is black clay and I am concerned about the possibility of heaving once the water returns to the soil. Comments?

 

[Ron]

A valid concern. In your area, post-tensioned slabs are typically designed to withstand normal seasonal variations in moisture content of the near-surface/subsurface clays. What you are experiencing at this time would not be considered a "normal" seasonal variation and you could expect greater movement on re-hydration of the clays. You might look into artificial re-hydration (wetting the area to force some degree of return to normalcy), assuming the drought is somewhat temporary.

The heave potential of these clays is significant!

 

[Ron]

One additional thought...
You might check with a local geotechnical engineer to see if there are any soil/ground improvement techniques being used on the residential scale (i.e. lime treatment) These can be cost effective for commercial construction but not sure about residential.

Good luck.

 

[Jim6758]

Well, so much for the drought! It has been raining steadily in the DFW area since Oct..
If your grade beams were on piers, I hope you used void boxes underneath them. Some of the electrochemical products seem to work well on some of the fat clays, but not all. If your slab was fully structurally supported rather than soil supported, you probably don't have too much to worry about providing you used void boxes there as well.

 

[netknight]

Yes, we have gone from one extreme to the other. It has put my project at least 3 months behind with rain and mud.

We havn't seen any signs of stress or problems with the foundation during this extreme re-hydration of the soil. I'm guessing that if there were issues, we would have seen it by now.

The slab is post tensioned with beams that are on piers. I'm not a structural engineer, so I'm not sure what a void box is or what it function it provides. I'll have to contact the engineering company that designed the slab to find out if void boxes were used. Can you explain what I should ask?

 

[Taro]

One way of dealing with expansive soil problems with concrete structures is to essentially pour the concrete structure on cardboard boxes. After the concrete has set, the forms rot away and a void is left between the concrete and the soil. Then the clay is free to swell or shrink without impacting the structure above. As long as you design the piles for the drag forces from the soil, you're OK.