TM 5-852-4/AFM 88-19, Chap. 4
U. S. Army Corps of Engineers
Figure 4-91. Effects of granular mats on frost penetration and heave.
be placed within the frost heaving material, the possibility
with the footings, to resist the heave forces generated in
would exist that the footing would be moved
them.
f.
The same concepts as involved in figure 4-
progressively upward with successive annual cycles of
freeze and thaw, in the same way that boulders work
89b are also represented in the approach shown in figure
upward in the seasonal frost zone. Therefore, where
4-89c. On the plus side, the latter design offers the
foundation soils are frost-susceptible, footings must be
additional advantages that moisture content of the
placed either on top of the frost-susceptible material or
granular fill will tend to be higher and the total seasonal
granular mat or below the zone of seasonal frost, never
frost penetration less than in the figure 4-89b case,
in between. If in the case shown in figure 4-89b, the
because of the poorer NFS drainage situation, the
footings were to be placed below the seasonal frost
footings rest on material not subject to freeze and thaw
zone, care would have to be taken that the direction of
which thus will retain its bearing capacity and snow cover
frost heave and the axes of the structural members
will develop relatively normally because through the year
within the annual frost zone had the same orientation so
the backfill is flush with the surrounding ground.
that the heaving soil could "slide" on the structural
However, the scheme has the inherent disadvantages
members. In hilly country this requirement might be
that the additional cost of excavation and removal of
impossible to achieve.
Of course, the structural
material in the annual frost zone is bound to make this
members would also have to be free of projections or
scheme more
obstructions and would have to be designed, together
4-150
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