TM 5-852-4/AFM 88-19, Chap. 4
snow-free horizontal ground surface; for bare ground, for
vertical movement may or may not be detrimental. If the
example, the surface freezing index would be taken as
heave is differential between footings supporting the
0.7 of the air freezing index and the chart would be
tower, the tower will tip and/or the structure will be
entered with zero wall thickness. If it is necessary for a
unevenly stressed. For a radar or communication tower,
wall or retaining structure to be in contact with frost
loss of orientation may be critical. If the tower is guyed,
susceptible soil over all or part of its height under
the guys and/or the guy anchors may be overstressed.
conditions where freezing direction is vertical rather than
Some may become slack.
Differential footing
lateral, some modification of frost uplift may be provided
settlements may occur during thaw-weakening in spring.
by battering the face of the structure as much as
If the tower is on a slope, progressive downslope
possible. Heaving soil will then tend to break contact
movement may occur with successive cycles of freeze-
with the wall as it is lifted and thus limit the area of
thaw.
c. Granular material may be used as
adfreeze contact. It should not be assumed that this will
eliminate uplift forces. Anchorage against the uplift
illustrated by figures 4-89 and 4-91 to control or even
forces should be provided by such means as extending
eliminate detrimental vertical movement. The simplest
the batter well down below the zone of frost penetration.
approach, as shown in figures 4-89a,b, is to support the
and/or by using an adequately widespread base. In any
tower on a granular mat placed on the surface. Because
case, sufficient reinforcing steel must be incorporated in
of the intensity of the winter cold, it is usually impractical
the concrete to sustain tensile forces developed therein
in arctic and subarctic regions to attempt to make the
without cracking of the concrete in tension by extension
mat thick enough to completely prevent frost penetration
of the reinforcement.
or heave in the underlying frost-susceptible material,
c. In lieu of placing the base of the wall deep
particularly when the mat is naturally well-drained, as in
enough in the ground so that freezing cannot penetrate
figures 4-89a,b, though this may be possible in some
under it into frost-susceptible soils, it may sometimes be
seasonal frost areas.
However, as described in
feasible to support the structure on piles just above the
paragraph 2-5, the magnitude of frost heave may be
ground, "daylighting" the base sufficiently to provide
substantially reduced by a relatively modest surcharge,
room for upward expansion of the heaving soil.
consisting of the weight of the gravel plus the load from
However, risk is then present that this space may be
the structure. For some situations, the thickness of
eliminated by settlement, or by deposition of material
gravel may therefore merely need to be made sufficient
within the gap by water or wind, and be unable to
to reduce frost heave to an acceptable level, assuming
function when needed. This is particularly true if the
the design is not sensitive to possible differential effects.
d. In the type of design shown in figure 4-89b,
structure is a bridge pier subject to movement and
deposition of material by stream flow.
the footings are placed at the natural ground surface
d. Figure 4-87c shows a type of design which
instead of on the mat.
If the mat densities and
minimizes many of the problems inherent in wing or box
thicknesses are the same, the potential for heave
type bridge abutments. Although it may require special
reduction by surcharge will be the same in figure 4-89a
attention to slope stability and erosion control and it
and b. However, safety against overturning of the self-
requires a longer supported span, it reduces the frost
supporting tower will be greater in figure 4-89b because
design problems of retaining structures to a minimum
of the load of the mat on the footings. There is greater
and offers much in simplicity.
possibility in figure 4-89b that the soil under the footing
e. Stability of walls and retaining structures may be
may be overstressed during spring thaw weakening. To
computed using earth pressure analytical techniques as
reduce this possibility, as well as to provide a working
5
presented in TM 5-818-1 .
surface and to make sure that footings will not get "hung
up" and fail to settle completely back to original position
4-11. Tower foundations.
on thaw, it is desirable to specify a shallow granular pad
a. Towers
for
transmission
lines,
immediately below the footing as shown in figure 4-89b.
communication antennas, cableways or other purposes
More steel is required in the figure 4-89b scheme than in
are commonly either self-supporting as shown
the figure 4-89a design, placement of gravel around the
schematically in the left hand diagrams of figures 4-89
structural members requires special care, and protection
and 4-90 or guyed as indicated in the right hand
of the buried steel against corrosion is more
diagrams of figure 4-89 and the right hand portions of
complicated; this scheme is thus more expensive.
figure 4-90a,b. When bank-run gravel is available,
e. If in the figure 4-89b design the footings were
designs of the types shown in Figure 4-89 may be
to
considered. Figure 4-90 shows a number of possible
types of foundations requiring little or no granular non-
frost-susceptible material.
b. A tower supported on top of the annual frost
zone will experience frost heave if the freezing soil is
frostsusceptible and moisture is available. Depending on
the design and purpose of the tower, the seasonal
4-147
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