Building on the Past
Two broad categories of modern foundation design can be defined, the shallow form and the deep form. Shallow foundations include slabs, stem walls, footings, basements, and other forms which can be created using surface excavation techniques. Deep foundations refer to driven pilings and piers, shafts, caissons, injected borings, and forms which derive stability from subsurface geological formations and soils.
Determining the type and load bearing capacity of the underlying soil and rock is the geotechnical contribution of the design process. A site survey is conducted to identify the geological materials present. This survey may include testing and analysis of shallow surface samples as well as deep core samples taken from drilled borings, and review of data from existing related formations. The resulting geotechnical report will identify compressive and shear load capacities available for the various layers of soils and rock formations present at the proposed construction site. It may also identify settlement potential, moisture swelling, erosion concerns, and freeze/thaw processes. This may result in modifying, moving, or even abandoning a construction project site due to inadequate foundation support capacities of the local geology.
Once a geologically adequate site has been identified the structural design of the foundation can proceed. Distributed and total loads of the proposed structure must be taken into account, along with the potential for seismic activity, floods, high winds, and other live load events that may bear directly on a foundation. For example, a site may be geologically suitable for the dead load of the structure, but the potential for flooding may dictate the need for a raised, pier supported foundation as opposed to an on grade stem wall or slab. Seismic potential may require the use of base isolation, elevated building foundation, or other alternative structural designs to utilize an otherwise geologically attractive construction location.