**TM 5-818-1 / AFM 88-3, Chap. 7**

**CHAPTER 10**

**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

settlement analyses.

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,

changes occur.

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.

The

settlement of footings on cohesionless soils is generally

**10-3.**

**Footing design.**

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,

the

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.

requirements of static equilibrium. Examples of the

(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

**10-4.**

**Mat foundations.**

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

safety of

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

**10-1**