Moist sandy gravel between frost-proof monolithic slab and insulation. Is it going to freeze and expand?

Question

I’m planning on pouring a monolithic slab with insulation beneath to prevent frost heave of the ground below. It’s a somewhat common technique, there’s just not many folks I can find in Massachusetts with experience.

The contractor that came out first filled the bottom of the forms with a very sandy gravel mix, maybe 60/40 sand/gravel. The gravel was kinda moist, and then water was used to compact it, and then some water got into it when it rained the other day. This is in the layer that, see above, is called "non frost-susceptible fill." On top of this fill goes the vapor barrier, and then the concrete. The moisture under the vapor barrier is between foam insulation and plastic sheets: the moisture is trapped there for life.

I’m a little worried that with all the effort I’m putting in to mitigate my slab breaking in half, that water is going to freeze and break my slab in half. Contractor's already been fired for a myriad of other offenses, but should I go ahead and replace the gravel he's got down with pea gravel?

Answer 1

Quoting from "Common Questions and Answers about Frost-Protected Shallow Foundations" on page 29 of the Federal HUD guide on constructing frost proof shallow foundations dated June, 1994:

Question No. 1: How does insulation stop frost heave from occurring?

Frost heave can only occur when all of the following three conditions are preset:

1) the soil is frost susceptible (large silt fraction),

2) sufficient moisture is available (soil is above approximately 80 percent saturation), and

3) sub-freezing temperatures are penetrating the soil. Removing one of these factors will negate the possibility of fromst damage.

Insulation as required in this design guide will prevent underlying soil from freezing (an inch of polystyrene insulation, R4.5, has an equivalent R-Value of about 4 feet of soil on average). The use of insulation is particularly effective on a building foundation for several reasons. First, heat loss is minimized while storing and directing heat into the foundation soil -- not out through the vertical face of the foundation wall. Second, horizontal insulation projecting outward will shed moisture away from the foundation further minimizing the risk of frost damage. Finally, because of the insulation, the frost line will rise as it approaches the foundation.

Since frost heave forces act perpendicular to the frost line, heave forces, if present, will act in a horizontal direction and not upwards.

So you are OK as long as your non-susceptible fill does not contain much silt.