Fill under concrete stoop

[vmirat]

We have a project designed by others that has concrete stoops at entrances. The engineer intends to dowel the stoop to the building and then use uncompacted fill under the stoop slab to protect against frost heave. His reasoning is that the fill will eventually consolidated due to water infiltration and leave a void which provides space for frost heave.

I disagreed with this approach and suggested we use 10 to 12 inches of compacted open graded base course instead of the uncompacted fill. Their engineer said that this would not prevent frost heave.

I am using the same rationale as you would for concrete pavement design on frost susceptible soils. I can't see how the uncompacted fill will remain uncompacted after you pour the wet concrete.

Am I missing something here? Is this common practice?

 

[SperlingPE]

Not typical in my area. Formdeck to form the underside of the slab with a void between the underside and the soil. I have also seen insulation used beneath the slab, but have never spec'd this myself.

 

[DaveAtkins]

If you provide foundation walls and footings below the frost level around the stoop, the frost heave problem goes away. Otherwise I think it would be wise to use uncompacted fill under the stoop slab, which would be less likely to cause uplift than compacted fill.

DaveAtkins

 

[dasman]

You basically have two options -- eliminate the soil (ie., create a void) or eliminate the water to below the frost line.

Creating a 12" pad of granular doesn't necessarily help if your frost line is 36" and you have clay/silt below -- the clay/silt will heave and just take the granular with it.

As for the uncompacted fill to create a void -- while I understand the concept, I don't necessarily agree with it. You're relying on water infiltration to compact the fill and create the void? How long is that going to take -- a month, a year, a couple of years? For us, it's not uncommon to see 2"+ of heave -- I'd hate to wait an indeterminate time for the fill to consolidate and provide that much of a void.

In our area, formdeck for the slab (to frost walls extending below grade) and a 6" void on the underside of the slab is the standard.