Deep foundations are structures that transfer building loads to the earth far down from the surface. They often have a depth to width ratio of 4 to 5. These structures are most appropriate in the construction of a high rise building or where the soil is very poor or incapable of supporting a structure at shallow depths.
The design of a deep foundation begins with analyzing subsurface conditions and the anticipated structural loads to be transferred to the ground. The load capacity of the soil and the type of pile to be used are also important factors for determining the most appropriate foundation solution for a given site.
Pile based foundations, which are the most economical option, transfer design loads by direct contact with the adjacent soil mass, relying on pile friction and/or end bearing. They can be used in a wide range of soils, including those that are weak, contaminated or prone to caving.
Driven piles, auger cast piles and drilled shafts are common types of driven pile foundations. Choosing which type of pile is appropriate to the subsurface conditions and anticipated structural loads requires analysis of the soil, rock and groundwater.
Non-driven pile foundations include cast-in-drilled hole (CIDH) concrete piles and alternative footing design piles. The use of these types of foundations is primarily on an experimental basis and are selected when conditions warrant their use.
The primary advantages of CIDH piles are their cost-effectiveness and the fact that they can be placed at any desired depth without disturbing existing utilities or damaging soil. However, cave-ins and groundwater problems may occur during the drilling process, requiring corrective action. A temporary casing, a drilling slurry or a low cement/sand mix can be used to minimize the impact on utilities.
Pier foundations are also referred to as helical piers or screw piles. They are large diameter cylindrical holes with helical fins drilled into the soil to a required depth. Unlike piles, which are designed to transfer loads through the friction of the soil and the bearing, piers just carry the load through the soil.
A pier foundation is preferred when the top layers consist of decomposed rock overlying strata of sound rocks. In this situation, it is more difficult to drive bearing piles into the decomposed rock.
In general, a pier is more expensive to construct than other types of piles, but their construction methods can be much quicker and less labor intensive. In addition, piers can be constructed in open excavations, eliminating the need for underground construction.
The most common type of foundation used for deep structures is the pile foundation. These are usually made of steel, wood, and concrete, although reinforced concrete is a popular choice. Depending on the soil and the expected load, a different number of piles are installed and the depth of these piles is designed accordingly.
