When the load bearing capacity of the soil at the building site is high and the soil compressibility is low (the soil is dense and will not compress) then settlement is not a great concern or problem. Under these favorable soil conditions a shallow foundation is the most economical foundation selection. Often shallow foundation construction is selected for smaller commercial and residential buildings. When the soil conditions are not favorable, deep foundations are the alternative selection. Unfavorable soil conditions include soil that is loose and/or moves under heavy loads or soil that will compress significantly under load conditions.
A deep foundation will provide the needed load bearing capacity and will limit the movement and settlement of the building structure. Deep foundation systems can include driven piles of timber, steel, or concrete as well as poured piers and drilled shafts. Driven piles are common in coastal areas which have higher settlement tendencies and unfavorable soil conditions. A Slab on Grade foundation is one of the most common types of foundations.
It is relatively simple, quick, and inexpensive to construct. It is a single layer of concrete that is poured to a thickness of approximately four inches for residential use. The concrete can be poured on compacted soil or a bed of crushed gravel to improve drainage. And steel rebar or reinforcing rods are placed in the slab prior to the pour to improve the overall strength.
Usually the edges are designed and poured to a greater thickness as well as linearweight bearing areas in the interior of the structure. The interior linear areas and edges can be eight to twelve inches thick. This foundation is suitable in geographic areas that do not freeze often. Pile foundations, also called pier and beam, are also one of the most common types of foundations. Piles are steel, pressure treated wood, or concrete columns that are either driven into the soil or placed in excavated holes. The piles provide support by transferring weight or load to the underground soil by way of skin friction and/or direct contact with stable underground strata.
The piles extend downward until a stable soil layer or rock is reached. The piles will also extend above the soil line and will be arranged in a linear fashion so that a beam may be placed on top of two or more piles. The walls and floors of the structure are thus supported by the beams resting on top of the piles. This foundation method is suitable in areas where the soil is unstable, coastal areas, and sites with steep slopes.
Foundation Walls is another one of the basic types of foundations. This method is found in structures with basements. It uses the basement walls as the foundation walls for transferring the weight of the building to the ground. Usually the walls are made from poured concrete or hollow concrete blocks filled with concrete. Both methods are reinforced with steel rebar.