TM 5-818-1 / AFM 88-3, Chap.7
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
10-7.
Foundations for radar towers.
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.
10-5.
Special requirements for mat foundations.
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
10-6. Modulus of subgrade reaction for footings
3
2
D = 6034(F)
M
1-M
(round
and mats.
(
)
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.
10-5
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