Each year in the United States, shrinking or swelling soils inflict more than $2 billion damage on houses, roads, pipelines, and other structures and buildings.


Differential heave, rather than total movement, generally causes the worst structural damage. Such movements are usually caused by nonuniform changes in soil moisture. Swelling of soil usually results from expansion due to wetting of certain clay minerals. The mineral responsible for most swelling is montmorillonite, often called bentonite.


In some places, slabs on grade are post-tensioned to prevent damage when soils expand beneath them. In Denver's "hot" areas of expanding soil, roughly 80 percent of the homes are supported on concrete caissons about 10 feet apart built to a depth prescribed by a soils expert to keep them free of expansion troubles. The caissons are frequently belled or notched at the base for better stability. A reinforced concrete grade beam or foundation wall is supported on these caissons to create the dwelling foundation. An essential feature of this type of construction is the floating floor slab. Complete isolation of the floor from bearing walls, columns, nonbearing partitions, stairs, and utilities allows the slab to move with minimum damage to the structural integrity of the building.


A 3- to 6-inch space must be left between the foundation soil and the grade beam or bearing wall to concentrate the weight of the building on the piers and keep the soil from pressing upward on the grade beam. Corrugated cardboard void forms save time and labor in creating this open space. They are placed on the ground between caissons under walls or beams to establish the space between concrete and soil.