Underslab void forms 1

[geotekker]

We are involved with an office building project located in an area underlain by highly plastic clay. We have recommended a traditional belled pier and suspended slab approach of dealing with the potential for soil heave. However, the project team is now debating the merits of conventional cardboard void forms vs. the foam type product as the void form. Can anyone with experience with the two different materials comment about the pros and cons?

 

[haynewp]

I didn't know there was a foam type. I have only used the cardboard type, I have read the non-diagonal cell type of cardboard form is preferable to the diagonal cell type. I will see if I can find the paper on it again, if you are interested.

 

[JAE]

What is the compressive stiffness of the foam? If it has some stiffness, then upward swelling of the soils may induce some significant pressures on the slab system.

You have AT LEAST enough stiffness to support the wet weight of concrete so that is probably most of the dead load pushing up on your slab when the soil heaves.

 

[structuresguy]

I hope by foam type you are not referring to GeoFoam. If so, this is definitely not the type of product you want to use with expansive soils. We use it frequently to support odd slab on grade profiles at stages/ramps, etc. It it quite strong (there are several strength grades), and will easily lift a slab on grade. We use it up to 100 psf live loading without any problems.

 

[structuresguy]

Oops, forgot to mention that I would only use the card board void forms in your situation. Have used them successfully in the past, and don't see any reason to change from that.

 

[geotekker]

For those who are interested, the foam product name is VoidPro. The foam supposedly deforms as the subgrade heaves, rather than slowly decomposing and creating the void beneath the slab. Based on test results provided by the manufacturer, it appears that crushing of the foam occurs at about 400 psf (3 psi) at a strain of about 4 percent. One of the manufacturer's main selling points is durability during the construction period when standard cardboard forms can be vulnerable to prolonged rainfall.

 

[structuresguy]

400psf crushing load seems WAY too high for me to expect it to crush under a slab which weighs only 100 psf +/- (for 8" slab). in this case, you would need to design your slab for an uplift pressure of at least 300 psf, maybe even 450 psf to have a SF=1.5 against uplift, or even 600 psf to ensure a safety factor against the actual crushing strength.

This will kill your slab, compared to simple gravity design, I would think.

I just think this is all easily avoided by using traditional void forms.

 

[dik]

For inside construction, the cardboard mat'l works well. You might want to consider caissons for the column loads and intermediate slab loads taken by friction piles.

With highly plastic clay, it is unlikely that you will have heave once the building is enclosed; the soil tends to dessicate and shrink.

I usually use the foam type for grade beams (a product called 'Frost Cushion') Foam can have a crushing strength of several psi and may be problematic with slabs. I've done reports on beams that have heaved when conventional cardboard was used and the void filled with water.

Dik