Clay & Soil Bearing Capacity

Bearing Capacity Factors

• The weight, or load, of a house rests on its foundation, which must rest on soil that has the capability to withstand stress factors that could cause critical structural damage. These stress factors are known as shear failure, relating to rifts in the soil caused by ground movement that will force the foundation to shift. The rated bearing capacity of soil includes both the ultimate bearing capacity and the allowable bearing capacity. The ultimate rating refers to the soil’s ability to withstand catastrophic movement, such as earthquakes or severe shifting. The allowable rating describes the maximum foundation stress allowed due to general ground movement.

Clay Soil

• Clay soil is rated in construction as being an expansive soil, meaning that clay soil expands when water is introduced. Additionally, this type of soil will contract when it dries out. These factors have a significant effect on the stability of clay soil because soil expansion and contraction lead to ground movement. This is a slow process that can go unnoticed until signs of structural damage occur, such as interior wall cracks or doors that won’t open or close properly.

Clay Soil in the U.S.

• According to the American Society of Civil Engineers, at least one quarter of homes in the U.S. incur structural damage caused by expansive soil. The areas that contain soil of the highest expansion values are in the central U.S., from the Dakotas to Texas and Louisiana. Moderate soil expansion is found from Oklahoma eastward through the Great Lakes region and the Atlantic Seaboard, as well as scattered areas of the Midwest and Pacific Northwest.

Clay Soil Bearing Capacity

• Clay soil expands and contracts because of its cohesiveness. It has a plastic-like consistency, especially when wet, that prevents the soil from softening into a normal muddy consistency, thus making its bearing capacity problematic. Clay as a cohesive soil is rated by its bearing capacity from very soft, which can bear a maximum of 500 pounds per square foot, to hard, which can bear a maximum of over 8,000 pounds per square foot.