FOUNDATION TYPES. The foundation design will probably incorporate one
or more of the following four basic types.
Individual Spread Footings. Spread footings are usually the most
economical type of foundation. Their design, depending upon the soil bearing
capacity and the predicted settlement characteristics, considers one of the
following two situations:
(1) The footings will be proportioned for equal bearing pressure
under total dead and live load. Ordinarily, this procedure is adequate.
(2) Should the soil foundation investigation and settlement
characteristics so indicate, individual footings may be proportioned for
uniform pressure under dead load only so as to have uniform initial and time
dependent settlements under dead load. Differential settlements are
determined by evaluation of the live loading conditions. The supporting
structure, the chamber, and its components should be designed for the
movements and deflections which take place.
b. Mat or Raft Foundation. A mat or raft provides structural rigidity.
A mat foundation will impose the minimum bearing pressures on underlying
soils and will bridge isolated zones of weak or compressible materials.
Pile Foundation. Piles may be friction, end-bearing, or, as most
frequently the case, supported by a combination of skin friction and
end-bearing. Pile foundations are used to carry loads through compressible
soils or soils of low bearing capacity to deeper strata of higher bearing
(1) Evaluate the effect of the impact and vibration resulting from
driving the piles.
(2) When seismic loading is a consideration in the design, evaluate
the lateral force on the piles and pile caps, batter the piles in at least
three directions, and adequately connect piles to the pile cap.
(3) The differential settlement between pile groups may become
significant if the length of piling differs substantially from one group to
the other, or if the material upon which they bear differs substantially.
(4) The piles can be overloaded if negative friction, resulting from
settlement of soft or loose overburden, has not been accounted for in the
(5) The predicted load capacity of the piles and their settlement
characteristics should be substantiated by conducting load tests on a test
d. Caisson Foundations. Design considerations for caisson foundations
are similar to both spread footings and pile foundations in many respects.
The primary difference between caissons and spread footings is that caissons
are normally founded at a greater depth because they are useful in