TM 5-818-1 / AFM 88-3, Chap. 7
SPREAD FOOTINGS AND MAT FOUNDATIONS
beneath individual footings from the influence charts
10-1. General. When required footings cover more
presented in chapter 5. The footing size should be
than half the area beneath a structure, it is often
selected on the basis of the maximum bearing pressure
desirable to enlarge and combine the footings to cover
as a first trial.
Depending on the nature of soil
the entire area. This type of foundation is called a raft or
conditions, it may or may not be possible to proportion
mat foundation and may be cheaper than individual
footings to equalize settlements. The possibility of
footings because of reduced forming costs and simpler
reducing differential settlements by proportioning footing
excavation procedures. A mat foundation also may be
areas can be determined only on the basis of successive
used to resist hydrostatic pressures or to bridge over
If the differential settlements
small, soft spots in the soil, provided the mat is
between footings are excessive, change the layout of
adequately reinforced. Although mat foundations are
the foundation, employ a mat foundation, or use piles.
more difficult and more costly to design than individual
(2) If foundation soils are nonuniform in a
spread footings, they can be used effectively.
horizontal direction, the settlement analysis should be
made for the largest footing, assuming that it will be
10-2. Adequate foundation depth. The foundation
founded on the most unfavorable soils disclosed by the
should be placed below the frost line (chap 18) because
borings and for the smallest adjacent footing. Structural
of volume changes that occur during freezing and
design is facilitated if results of settlement analyses are
thawing, and also below a depth where seasonal volume
presented in charts (fig 10-1) which relate settlement,
The minimum depth below which
footing size, bearing pressures, and column loads.
seasonal volume changes do not occur is usually 4 feet,
Proper footing sizes can be readily determined from such
but it varies with location. If foundation soils consist of
charts when the allowable settlement is known. After a
swelling clays, the depth may be considerably greater,
footing size has been selected, compute the factor of
as described in TM 5-818-7. On sloping ground, the
safety with respect to bearing capacity for dead load plus
foundation should be placed at a depth such that it will
maximum live load condition.
not be affected by erosion.
c. Footings on cohesionless soils.
settlement of footings on cohesionless soils is generally
small and will take place mostly during construction. A
a. Allowable bearing pressures. Procedures
procedure for proportioning footings on sands to restrict
for determining allowable bearing pressures are
the differential settlement to within tolerable limits for
presented in chapter 6. In many instances,
most structures is given in figure 10-2.
allowable bearing pressure will be governed by the
d. Foundation pressures. Assume a planar
allowable settlement. Criteria for determining allowable
distribution of foundation pressure for the structural
settlement are discussed in chapter 5. The maximum
analysis of a footing. This assumption is generally
bearing pressure causing settlement consists of dead
conservative. For eccentrically loaded footings, the
load plus normal live load for clays, and dead load plus
distribution of the bearing pressure should be determined
maximum live loads for sands. Subsoil profiles should
by equating the downward load to the total upward
be examined carefully to determine soil strata
bearing pressure and equating the moments of these
contributing to settlement.
forces about the center line in accordance with
b. Footings on cohesive soils.
(1) If most of the settlement is anticipated
bearing pressure distribution beneath footings are shown
to occur in strata beneath the footings to a depth equal to
in figure 10-3.
the distance between footings, a settlement analysis
should be made assuming the footings are independent
of each other. Compute settlements for the maximum
a. Stability. The bearing pressure on mat
bearing pressure and for lesser values. An example of
foundations should be selected to provide a factor of
such an analysis is shown in figure 10-1. If significant
settlements can occur in strata below a depth equal to
the distance between footings, the settlement analysis
should consider all footings to determine the settlement
at selected footings. Determine the vertical stresses