Disturbing as little soil as possible
Pier foundations are unlike more conventional concrete footings and walls in that they support structural loads at a number of distinct points, not continuously. Pier foundations can be as simple as concrete-filled cardboard tubes dropped into hand-dug holes. But more complex foundations incorporating very deep piers or helical screws can support much greater loads.
To some green builders, one advantage of pier foundations is that they cause minimal disruption to the soil environment. Excavation can be performed with a shovel, and existing roots and soil organisms remain mostly undisturbed. At the end of the building’s useful life, the site will be easier to restore to a natural state than a site with a full basement.
Design starts in the soil
The number and size of the piers depends on how much weight the underlying soil can hold. The standard design load in the International Residential Code assumes a 40-pound live load; dead load is commonly calculated at between 10 and 15 pounds per square foot, depending on the materials used for construction.
Hard-packed gravel may be able to handle as much as 3,000 psf while soil with high clay content or lots of organic material will support much less weight. Recommendations of the IRC are summarized in a table (referenced in the code as 401.4.1). In situations where the bearing capacity of the soil is questionable, it may be necessary to call in a soils engineer for help.
The most basic foundation consists of a series of concrete posts bearing on concrete footings installed below the frost line.
Pier foundations, however, can be far more complex. On steep or unstable sites, pier and grade-beam foundations are an option when conventional foundations won’t work. These consist of a series of concrete piers, extending up to 20 ft. into…