Our first job that dealt with expansive soils (or prairie soils, as they are commonly known) was in Denver. Since then, have used cardboard “carton forms” several times over.
Carton or “void” forms are designed to deteriorate and leave empty space under slabs and/or grade beams. Using these forms allows room for the expanding soils to swell when wet and shrink when dry. Why? In some parts of the country, soils are more susceptible to wet/dry cycles. If the void area was not in place, wet soil could heave the concrete and potentially cause structural movement damage or cracks in concrete foundations, floor slabs-on-grade or walls.
We used these carton forms recently on a drilled pier and grade beam foundation job. Our piers were drilled, then grade beams were formed and poured to span pier to pier for support of the building structure. Beyond the forms, we installed “soil retainers” vertically on each side of the carton form to keep the soil from filling the void area after deterioration of the void forms. These are most commonly plastic or some non-deteriorating product, as they need to remain in place. I our case we used plastic.
A couple of things to remember:
- Forms are usually cardboard, with or without a wax coating or injected wax.
- Design thicknesses depends on soil conditions as evaluated by geotechnical and structural engineers.
