Proper design of footings for earth-sheltered construction requires a knowledge of loads and soil bearing capacity. Increasingly, building officials are requiring that the design be done by a registered professional engineer or architect. Frequently soil tests are needed to establish the bearing capacity that is, the soil load which the soil can support without excessive settlement or failure.
For earth-sheltered residences and light commercial buildings, there are two common types of footings: continuous footings and spread footings. A continuous footing runs the full length of a wall or building with only minor interruptions. A spread footing is square or rectangular and does not run continuously under the wall or column line. When beam-and-column systems are used to support roof loads, spread footings are typically used. Otherwise, the footings will be continuous. In order to plan a proper footing, the designer must evaluate the loads to be supported by that footing, based on code requirements for live load as well as known weights of concrete and other materials. Commonly used values are 150 pounds per cubic foot for concrete and about 120 pounds per cubic foot for earth.
Three principal factors affect the width of a continuous footing: Roof spans longer spans mean larger loads and therefore wider footings; soil cover deeper soil cover means more load, and therefore wider footings; bearing capacity of soil higher bearing capacity means smaller footings. The continuous footing is designed as an inverted cantilever beam, supporting a section of wall 1 foot long. Loads on the footing are computed on a per lineal foot basis. For earth sheltered residences 1 foot is frequently assumed as footing thickness, and this exerts a load of 150 psf on the foundation bed. This 150 psf is subtracted from the soil bearing capacity to obtain a net allowable soil pressure under service load. The area of footing under each lineal foot of wall equals the applied load divided by the net allowable service pressure.