Residential foundations for expansive clay 2

[DRsquare]

I'm working as a plans examiner in north central Florida. We have expansive clay in some locations, and the comon fix seems to be cast in place concrete piles. The troble is, no one wants to really take the time or expense to obtain soil borings and actually do a rational engineering desigh.

I notice that the FBC and IBC allow the use of WRI design for sog, and PTI post tension slabs. It seems this prescriptive approach would be cheaper and easier. Is there some reason the piles are better or prefered? What are the pros and cons of these designs?
Thanks

 

[JAE]

Slab-on-grades used in expansive clay regions are usually designed as "stiffened" slab-on-grades. These typically are either mild reinforced or post-tensioned slabs which are constructed somewhat like a waffle slab - only with larger spaces. The slab is laid out with monolythic grade beams at 12 to 16 feet on center each way - about 30" in depth depending on the degree of expansiveness. The grade beams are typically reinforced with #5 to #7 bars top and bottom and stirrups.

The post-tensioning method uses the same concrete layout above but pre-stresses the grade beams to limit cracking and provide strength.

The main idea with SOG slabs here is to create a stiff plate that the house can sit on so that when local areas of soil dry out or get wet, the slab has a degree of stiffness to bridge over the irregularity in soil support (both positive and negative bending) and avoid excessive damage to the house. My mentors in south Texas never took to the post-tensioning system, believing that the prestress would be limited by the soil resisting the compression of the gradebeams.

For pier systems, you will spend more money as you are creating a deeper foundation to stand on lower, more stable soil, and then you create a void under the slab structure to allow the soil to expand upward without bearing (and moving) the slab upward. This requires extra cost for the piers and extra cost for the slab which must now structurally span between piers. The spacing of the piers can be set to an optimum with some study, to find the best combination of numbers of piers vs. span of the slab.

The voids can be created either with cardboard carton forms or by actually digging out a crawlspace (with venting and drainage) to separate soil from slab.

Why they don't go for piers too often? Probably cost.

But my house in South Texas, despite the stiffened slab on grade, always had cracks around window corners.

 

[TDAA]

To expand on what JAE said:

The stiffened/ post-tensioned slabs will help control the damage that the movement may create, but it will not limit the movement. The piers on the other hand will move the bearing down below the variation of the depth of wetting. This not only keeps the soils below the piers from moving, but provides some uplift resistance to the expansive soils at the upper portion of the piers. The pier should be reinforced with enough steel to resist the uplift forces.

In your relatively moist climate, the change in seasonal moisture is likely not very deep. Here in CO, it can be in the range of 25 feet when you look at an arid climate switching to an irrigated lawn. Sub-excavation and replacement of expansive soils (up to about 10 feet below the footings depending on swell measured in the lab and bedrock orientation), or piers are the more widely used methods to control the swell. Post-tensioned slabs are sometimes used to reduce the depth of sub-excavation, since they can accommodate a bit more movement.

If it is a concern that there is no investigation, and there are problems due to the soils, start a movement to require soils information. Here in CO, it is required for all structures by law, due to past problems (also for residential in some parts of VA, due to poor construction practices highlighting the expansive soils). I am sure the area geotechs would love you for the ob security that would provide.

 

[sjhgeotech]

I have a similar situation where a builder wants to place a foundation on a saturated clay located at 10 feet below existing grade. What tests should I run to determine the soils sensitivity? Obviously, if the water table ever drops below the depth of the foundation, settlement is going to be an issue. The builder thinks that placing a slab on grade on 3-4 inches of crushed stone is enough to take care of any settlement concerns, as he has done that in the past. I am thinking that a waffle slab may be the best option here.

Thanks!

 

[dgillette]

sjhgeotech -

Start with an oedometer test to determine preconsolidation stress and C-sub-c in the stress range of concern. You may find it's not an issue at all if the clay is and will be overconsolidated. The increase in effective stress would just be 62.4 pcf times the drop in the water table.

However, if there is a chance that the clay would become desiccated by exposure to the atmosphere - not just left above the water table but protected by overlying soils - then it could undergo pretty significant volume change, in which case you would want to compare in-place water content with the shrinkage limit. (Does anybody do shrinkage limit tests anymore? Has someone found a better/safer way than liquid mercury? I haven't run one since about 1979 in Prof. Saada's undergrad soils lab.)

Back to the topic of expansive clays, what about relieving loads on walls? My basement wall is bowed in, I think because of the expansive soil I know exists in the yard. (My porch moves up and down with changes in the weather. I think it's colluvium derived from the Denver formation.) The sheetrock in one room has been damaged and offset 3/8 or 1/2 inch as a result. The movements do not seem to have stopped. I keep coming up with ideas like digging a narrow slot and putting in crushable foam or something like that, but haven't gotten around to trying anything. Ideas? The brick walls bear on a sole plate that has remained more or less in place while the concrete basement wall moved inward. The floor is a floating slab that, curiously, dropped about 1-1/4 inches in a few months about 4 or 5 years ago. The house was about 30 years old at the time. I don't know whether groundwater pumping increased or drought caused the foundation soil to desiccate or what.

Best regards,
DRG

 

[sjhgeotech]

I have a question regarding the use of "poly" fabrics below foundation slabs. When should these fabrics be used and what is their purpose? Should they only be used on clay soils and does the concrete get poured directly on top of them, or does well graded sand or gravel go on top of them?

Thanks for the help!

 

[JAE]

sjhgeotech,

Check out this link:

vapor Barrier

Generally, we use a VB only under floors that will subsequently be covered with a flooring material such as tile, carpet or wood flooring. For warehouse structures we do not use a VB due to slab curling. Some engineers do specify a sand or fine gravel over the VB and this does work, but the contractors don't like it all that well and I've always avoided using sand in my designs - as the sand will pump as workmen walk over it and you get uneven concrete thicknesses.