TM 5-852-4/AFM 88-19, Chap. 4
frost-susceptible soils the formation of ice lenses at (and
2-3. Groundwater.
in the finer-grained soils behind) the freezing plane
a. If a freezing soil has no access to free water beyond
during the freezing process tends to produce an upward
that contained in the voids of the soil immediately below
movement or heaving of the soil mass. Ice segregation
the plane of freezing, frost hicave will necessarily be
is most commonly in the form of lenses and layers
limited. However, if free water can be easily drawn to the
oriented principally at right angles to the direction of heat
plane of freezing from an appreciable distance below the
flow; the surface heave is approximately equal to the
plane of freezing or from an underlying aquifer, heave
total thickness of the ice layers. The raising or heaving
can be large. A water table within 5 feet of the plane of
6
of the ground surface in a freezing season may vary from
freezing is favorable for significant frost heave .
nothing in confined, well-drained non-frost-susceptible
However, lowering of a water table to even great depth
sands and gravels to a foot or more in saturated, frost-
cannot be depended upon to eliminate frost heave; the
susceptible silts and some clays if there is an unlimited
percentage of water that can be drained by gravity from
supply of moisture. The magnitude of seasonal heaving
most frost-susceptible soils is limited and may be
is dependent upon such factors as rate and duration of
negligible"'. The remaining water in the voids will still be
frost penetration, soil type and effective pore size,
available to migrate to the plane of freezing.
In
40,59,60
. Figure 2-3
surcharge, and availability of moisture
permafrost areas the supply of water available to feed
illustrates, in a laboratory freezing test, the typical gain in
growing ice lenses tends to be limited because of the
moisture content in upper layers of soil, with withdrawal
presence of the underlying impermeable permafrost
of moisture from lower strata even though water was
layer, usually at relatively shallow depths, and maximum
provided at the base. In frost-susceptible soils, ice
heave may thus be (but is not necessarily) less than
segregation can occur by such withdrawal of moisture
under otherwise similar conditions in seasonal frost
from lower layers even without an outside source of
areas. Uplift forces on structures may nevertheless be
moisture.
more difficult to counteract in these more northerly cold
c. Clean GW, GP, SW, and SP gravels and sands with a
regions because of lower soil temperatures and
negligible percentage of material smaller than 0.02 mm
consequently higher effective tangential adfreeze
are so relatively nonheaving that foundation design on
strength values.
such materials is usually not governed by seasonal frost
b. As illustrated in figure 2-4, the water table may
effects. (Frost susceptibility criteria are discussed in
disappear rapidly in the first part of the freezing period as
more detail in TM 5-818-26). If saturated, it is possible
water is withdrawn from the unfrozen layers of soil to
for such soil to heave a small amount on freezing
form ice lenses t the plane of freezing. However, even
because of the expansion of water on changing to ice.
when the free water table disappears a substantial
However, if the expansion of the water which freezes can
volume of water remains still available, and ice
be balanced by movement of an equal volume of
segregation and frost heave may continue for many
unfrozen water away from the freezing plane, there will
weeks thereafter, while availability of moisture, surcharge
be negligible expansion of nominally confined non-frost-
weight at the freezing plane and rate of frost penetration
susceptible materials. The same expansion relief can be
are progressively changing.
Full saturation is not
obtained in non-frost-susceptible soils by a condition of
necessary for ice segregation in fine-grained soils,
partial saturation. Superficial fluffing of the surface of
though below about 70 percent saturation some soils do
unconfined, relatively clean sands and gravels is
not heave significantly. Perched water tables can be as
frequently observed, but this effect is small to negligible
important as base groundwater tables.
when these materials are confined. In permafrost areas,
c. Susceptibility of the soil to particle break-down under
however, the fact that the foundation materials are non-
freeze-thaw cycles is a function not only of the durability
frost-susceptible does not justify assuming, without
characteristics of the particles themselves (which can be
investigation, that ground ice masses are not present or
evaluated by standard laboratory tests), but also of the
that settlement on thawing will not occur.
degree of saturation. For building stone and mineral
d. When fine-grained soils thaw, water tends to be
aggregate it has been found that 87 percent saturation of
released by melting of segregated ice more rapidly than
the material itself is a good maximum if the stone or
it can be drained away or redistributed in the thawed soil.
aggregate (or concrete made therefrom) is to resist
This results in a very wet, soft condition of the soil, with
freeze-thaw damage (communication from K.B. Woods).
substantial loss in shear strength. The shear strength of
Thus, the degree of natural drainage existing in
the thawed soil is dependent upon the same factors as
foundation soils during freeze-thaw cycles may be an
would apply under non-frost-related conditions but is very
important design consideration if particle degradation
difficult to measure meaningfully by conventional
may significantly affect the long range performance of
approaches.
the construction.
2-5
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