Foundation Types and Their Uses

Foundations can be described by their geometry (e.g., depth, dimensions and relationship to other footings).  A shallow foundation, as opposed to a deep foundation can be made for an individual load (i.e., isolated) or to support multiple loads (e.g., strip, raft or mat).  This is the most common, particularly here in NH and MA, where near surface soils typically have satisfactory bearing capacity.

For building structures, the shape of an isolated footing is typically square or rectangle, and a column is supported near its middle; for instance, a basement lally column.  A strip footing is installed under a wall, and as its name implies, it is relatively narrow and long.  Shallow isolated and strip footings should be installed at frost depth, which is driven by the project’s location and governed by the local authority having jurisdiction.

Raft or mat foundations spread across broad areas and support loads both from columns and walls.  These foundations reduce the likelihood of differential settling between loads, which is settling of the structure at different rates in different locations. Raft foundations are comprised of engineer designed structural concrete with reinforcing steel and are substantially stronger than a basement floor slab.  The project geotechnical engineer would determine whether the raft foundation could be installed above frost depth and specify its design requirements.

Deep foundations, such as piles, drilled shafts or caissons, carry loads to more supportive soil or rock below unsuitable or low capacity soils.  They can also be used to prevent uplift under heavy winds or earthquakes, and/or to limit differential settlement.  A steel pile is typically an I-beam or pipe; which is hammered, vibrated and/or rotated into the ground.  Piles can also be concrete or lumber.  The pile can support loads via friction along its length and/or at its bottom end.

Drilled shafts or caisson foundations can carry higher loads to depth than piles.  There are a number of advanced methods and materials of construction. The more basic is a drilled shaft that is installed using an auger, which is then filled with concrete with or without reinforcing steel.  When caissons bear on soil, the bottom is shaped like a bell to spread the load over a wider area.  Caissons can bear on or in bedrock, be pre-fabricated and placed into an excavation or used in underwater construction.

With so many foundation types to analyze and design, contact Team Engineering to help you select the most practical and economical foundation for your next building project!