In an earth-sheltered house, earth loads on the roof are significant about 100 to 120 pounds per square foot for each foot depth of earth cover. Because the earth loads and thus the structural strength that is required are larger, a careful soil study and individual structural design are generally needed for earth-sheltered construction. Detailed soil analysis and individualized design may also help to produce a more economical structure in some instances.
Soil can be visualized as a collection of particles interspersed with open spaces called voids. These voids may contain air, water or water vapor in varying amounts and combinations. If a soil has an even distribution of grain sizes throughout the sample, it is said to be well graded. If most of the solid particles are of the same size, it is termed poorly graded. In soil, as in other material under load, predictable stresses and strains are created. In much the same way that the intensity of a flashlight fades out into the darkness, foundation loads are dispersed into the soil. At a depth below the footing equal to twice the footing width, practically all the loads have been "absorbed."
With this in mind, soil borings to determine bearing capacity should be taken where the footings are to be located and should extend below the proposed footing to a depth at least equal to twice the footing width, usually 5 to 8 feet. Soil borings should be taken for all earth-sheltered homes where footing loads are expected to be over 5000 pounds per lineal foot. The density and bearing capacity of a soil depends on the gradation of soil particles. If the soil consists of fine, medium and coarse particles all mixed together, fewer voids will be present and the soil will be denser and stronger. However, if a soil consists of gravel, one size of sand, or is a nonuniform mixture of very coarse gravel and very fine sand with the intermediate sizes lacking, the soil will not be as dense or as strong.