It dawned on me recently that since I moved here, I haven’t seen degradable void forms specified or shown on any concrete foundation details… I wonder why. I thought to myself, “is this because the soils here are different?”
So, first, I reminded myself exactly why we used void forms, at least in Minnesota (particularly under concrete stoops and sidewalks). This statement from “SureVoid” sums it up pretty well: “The void form material, lying under structural concrete construction, gradually absorbs ground moisture and loses its strength after the concrete has set, creating a space into which soil can expand without causing damage.”
Okay. Does soil in the NOLA area expand?
Here’s what I found…
Pink = Over 50 percent of these areas are underlain by soils with abundant clays of high swelling potential.
Blue = Less than 50 percent of these areas are underlain by soils with clays of high swelling potential.
Orange = Over 50 percent of these areas are underlain by soils with abundant clays of slight to moderate swelling potential.
Green = Less than 50 percent of these areas are underlain by soils with abundant clays of slight to moderate swelling potential.
Brown = These areas are underlain by soils with little to no clays with swelling potential.
Yellow = Data insufficient to indicate the clay content or the swelling potential of soils.
The map above is based upon “Swelling Clays Map of the Conterminous United States” by W. Olive, A. Chleborad, D. Frahme, J. Shlocker, R. Schneider and R Schuster. It was published in 1989 as Map I-1940 in the USGS Miscellaneous Investigations Series. Land areas were assigned to map soil categories based upon the type of bedrock that exists beneath them as shown on a geologic map. In most areas, where soils are produced “in situ,” this method of assignment was reasonable. However, some areas are underlain by soils which have been transported by wind, water or ice. The map soil categories would not apply for these locations.
lospecs.com 
Interesting. Soil here is the same. my soil is almost totally sand. I have put cement on the ground without any problem
LikeLike
I’m thinking that MN not having a large “pink” area doesn’t mean the soil doesn’t heave (freeze/thaw)… which probably isn’t a problem in your neck of the woods!
LikeLike
Lynn, just found your blog and this entry. We use void form in MN under entry/exit stoop slabs to provide a separation between the soil and the concrete which accommodates any expansion of the soil caused by freezing during winter that may push the slab up and keep the door from opening.
LikeLike
Thanks for the comment! and yes, I recall that it probably had more to do with freeze/thaw cycles. Hope you read my other blog entries!
LikeLike
Quick update on this entry: I have since learned that local job sites, at least on the “South shore” (of Lake Ponchartrain), are too damp to use void forms… implication being that the forms would start to disintegrate before concrete is poured.
LikeLike