time-settlement characteristics of foundation soils. As a

which has been found to give values about as reliable as

any method. This equation assumes qa (kips per square

conservative approach, mats founded on compressible

foot) for a settlement of about 1 inch with a safety factor,

soils should be designed for two limiting conditions:

F ≅ 3. A typical range of values of ks is given in table 3-

assuming a uniform distribution of soil pressure, and

assuming a pressure that varies linearly from a minimum

7.

of zero at the middle to twice the uniform pressure at the

edge. The mat should be designed structurally for

whichever distribution leads to the more severe

a. General. This design procedure provides

conditions.

minimum footing dimensions complying with criteria for

tilting rotations resulting from operational wind loads.

Design of the footing for static load and survival wind

a. Control of groundwater.

Exclude

load conditions will comply with other appropriate

groundwater from the excavation by means of cutoffs,

sections of this manual.

and provide for temporary or permanent pressure relief

b. Design procedure. This design procedure

and dewatering by deep wells or wellpoints as described

is based upon an effective modulus of elasticity of the

in TM 5-818-5/AFM 88-5, Chapter 6.

Specify

foundation.

The effective modulus of elasticity is

piezometers to measure drawdown levels during

determined by field plate load tests as described in

construction. Specify the pumping capacity to achieve

subparagraph d below. The design procedure also

required drawdown during various stages of

requires seismic tests to determine the S-wave velocity

construction, including removal of the temporary system

in a zone beneath the footing at least 1 1/2 times the

at the completion of construction. Consider effects of

maximum size footing required. Field tests on existing

drawdown on adjoining structures.

radar towers have shown that the foundation performs

b. Downdrag. Placement of backfill against

nearly elastically when movements are small. The

basement walls or deep raft foundations constructed in

required size of either a square or a round footing to

resist a specific angle of tilt, α, is determined by the

open excavations results in downdrag forces if weight of

backfill is significant with respect to structural loading.

following:

Estimate the downdrag force on the basis of data in

chapter 14.

3

2

(

)

B = 4320(F)

M

1-M

(square

α

footing)

(10-5)

Es

3

2

D = 6034(F)

M

1-M

(round

(

)

α

footing)

(10-6)

Es

a. The modulus of subgrade reaction can be

determined from a plate load test (para 4-6) using a 1- by

where

1- foot plate.

B, D

= size and diameter of footing,

respectively, feet

ksf = ksl B

(10-1)

F

= factor of safety (generally use 2.0)

M

= applied moment at base of footing

where

about axis of rotation, foot-pounds

ksf

=

the modulus of subgrade reaction for

α

= allowable angle of tilt about axis of

the prototype footing of width B

rotation, angular mils (1 angular mil =

ksl

=

the value of the 1- by 1-foot plate in

0.001 radian)

the plate load test

Es

= effective modulus of elasticity of

The equation above is valid for clays and assumes no

foundation soil, pounds per cubic foot

increase in the modulus with depth, which is incorrect,

The design using equations (10-5) and (10-6) is only

and may give k1, which is too large.

valid if the seismic wave velocity increases with depth.,

For footings or mats on sand:

If the velocity measurements decrease with depth,

special foundation design criteria will be required. The

( B 2B1 )

ksf

=

ksl

+

2

(10-2)

discussion of these criteria is beyond the scope of this

manual.

c. Effective modulus of elasticity of foundation

For a rectangular footing or mat of dimensions of B x

soil (Es). Experience has shown that the design modulus

mB:

of elasticity of in-place soil ranges from 1000 to 500, kips

( 15m5)(10-3)

per square foot. Values less than 1000 kips per square

foot will ordinarily present severe settlement problems

with a limiting value of ksf = 0.667ksl.

and are not satisfactory sites for radar towers. Values in

b. ks may be computed as

excess of 5000 kips per square foot may be encountered

ks = 36qa. (kips per square foot)

in dense gravel or rock, but such values are not used in

(10-4)

design.

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