TM 5-852-4/AFM 88-19, Chap. 4
expensive than that in figure 4-89b (which in turn is more
accumulation on frost penetration under the interior of
costly than that in 'fig 4-89a), and unless the mat is of
the mat is difficult to predict. It is even more difficult to
substantial diameter there is a real possibility that the
predict effects in the perimeter transition zone. For both
frozen moist granular mat, forming a relatively
above-surface and below-surface mats there is at
continuous slab with the adjacent frozen soils, may be
present no rational technique for analytically determining
lifted by heave of these surrounding soils. If such lifting
the pattern of vertical deformation in this zone or of the
is possible, careful analysis of possible frost heaving
diameter of mat required to isolate the footings from the
forces on the bottom structural members of the tower or
upward thrust of the surrounding frost heaving materials.
k. Therefore, for designs which require frost
of possible lifting of the tower itself will be required.
g. The center and right hand diagrams in
heave to be predicted with a high degree of confidence,
figure 4-89d show the same mat and footing
prototype/scale test installations should be constructed
arrangements as in figure 4-89a except that the mat is
under representative field conditions; heave and frost
placed below the surface, to gain the advantages of
penetration should be measured on these in at least one
higher moisture retention potential and corresponding
winter, correlated with soil moisture and freezing
reduced frost penetration. The disadvantages are the
conditions, and projected to the worst anticipated winter
same as explained above for figure 4-89c except that
conditions during the life of the structure.
l.
Figure 4-90a shows, schematically, tower
possible heave forces on the non-perpendicular bottom
structural members are avoided. The self-supporting
foundation designs for non-frost-susceptible foundation
design also lacks the added safety against overturning
materials using surface footings. Since these are little
provided by load of the mat on the footings. The left
affected by freeze and thaw except for thermal
hand diagram of figure 4-89d illustrates how the design
contraction and expansion of the ground surface,
might be further combined with the mat in figure 4-89a to
foundation designs may be essentially the same as in
further increase the surcharge and better control frost
non-frost areas.
m. In figure 4-90b, footings in a frost-
penetration.
h. The development of frost penetration and
susceptible foundation are shown placed at a level below
frost heave in above-surface and below-surface granular
the zone of seasonal freeze and thaw. In seasonal frost
mats is illustrated diagrammatically in figure 4-91 for
areas, these footings may rest directly on natural soil. In
three relative depths of frost penetration, disregarding
permafrost areas a granular working surface of nominal
possible effects from non-uniform snow cover. In figure
thickness should be employed directly under the footing.
4-91a, freezing has only partially penetrated the mat and
By backfilling over the footings with the same material as
there is no frost heave in the interior of the mat. In figure
that removed, the depth of seasonal frost penetration
4-91b, frost penetration has reached the bottom of the
under the tower will experience minimum change from
mat but frost heave of the mat is confined to the
the natural conditions, the major remaining cause of
shoulders. In figure 4-91c frost has penetrated below the
difference then being the effects of destruction of the
mat; the entire mat has lifted and the surface is dish
surface vegetation during construction. If the soils have
shaped. At the same time heave of the natural ground
high moisture holding capacities, the depth of frost
tends to be restrained near the mat; heave at the mat is
penetration, and hence the needed depth of excavation,
less than it would have been without the mat. Because
will be a minimum. As shown in figure 4-90b, however,
the transition zone conditions at the boundary of the mat
the structural members passing through the frost zone
develop gradually over the winter, upward bending of the
must parallel the direction of frost heave; this
frozen granular material may occur by creep.
requirement may make this type design impractical in
i.
It will be apparent from these diagrams that
hilly country. Sometimes it may be possible to modify
tower footings should be located a prudent distance
the topography locally, in the area of the foundation,
away from the mat boundaries in order to minimize frost
sufficiently so that all frost penetration will be vertical.
n. Figures 4-90c and d show details of two
heave problems. No tower footing edge should be closer
than 5 feet to the top edge of the granular embankment
possible alternate footings. Figure 4-90c illustrates the
under average conditions.
use of two timber courses to provide a firm, semi-
j.
Although some guidance in selection of mat
insulating working surface and footing base. In some
thickness is given in paragraphs 2-4, 4-2b, and 4-2e, it is
areas, timber may be more readily available and more
difficult to estimate the actual maximum frost heave
easily handled than gravel. Figure 4-90d shows a steel
accurately, even at the center of the mat, in the present
grillage resting on a shallow gravel working course. If
state-of-the-art. For example, the projection of a mat
the backfill is frost-susceptible, frost heave forces acting
above the surface, as shown in figure 4-89a, will affect
on the vertical member of the foundation in figures 4-90c
the thickness and uniformity of snow cover developed
and d must be analyzed and the footings designed so
locally, but the exact effect of the variable snow
that they
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