TM 5-852-4/AFM 88-19, Chap. 4
subsurface drainage and water table conditions of the
Hanover, New Hampshire. Because of possible serious
general area is needed for accurate design. Under
structural and operational effects of differential frost
bodies of water, the permafrost table may be depressed
heave, the site investigations should ascertain the
or permafrost may be absent, particularly in marginal
horizontal variability of frost heave potential under the
permafrost areas, and subsurface movement of moisture
structure. Variation may occur from point to point as a
through these unfrozen zones may be an important
result of differences in soil type, properties or profile, or
factor influencing the thermal stability of the foundation.
in moisture availability. Figure 3-8 illustrates a special
It may also be possible to exploit such zones as water
case of such variability which resulted in serious
191,209
When residual thaw zones carrying
supply sources.
cracking, in the first winter, of a new rigid pavement at
subsurface drainage develop, the thaw zones tend to
the west end of the East-West taxiway at Elmendorf
deepen and channelize and when these develop near a
AFB, Anchorage, Alaska, with 72 inches combined
foundation, they may threaten its stability. Even where
thickness of pavement and non-frost-susceptible base
moisture-bearing residual or permanent perched thaw
over the natural soil subgrade. Maximum heave was
zones do not exist, substantial quantities of water and
about 0.4 feet. The pattern of cracking shown in figure
heat may be transported by subsurface flow in the
3-8, which also continued out through the unpaved
annual frost zone in the summer. Thus sufficient
shoulders, directly corresponded with a pattern of
information is needed so that both surface and
extreme subgrade soil variation. The soil conditions are
subsurface drainage conditions within the vicinity of the
illustrated by a profile recorded in a trench dug parallel to
structure and foundation after construction can be
the south edge of the taxiway, (fig. 3-9). The Alaska
anticipated.
District, Corps of Engineers, concluded that the
(1) Ground water levels encountered in
alternating strata of sands and silts had been contorted
subsurface explorations should be recorded routinely,
by glacial shoving into their near-vertical positions. It will
whether in seasonal frost or permafrost areas. In the
be apparent that design of structure foundations on such
saturated silty soils common in permafrost areas, as
soils would require special attention to the subsurface
illustrated in figure 2-4, the ground water table in the
details.
j.
Frost heave field observations. It is often
annual frost zone may drop rapidly during the fall and
early winter and disappear well before annual freezing
necessary or useful to have quantitative information on
reaches permafrost level. However, as further illustrated
the amount of frost heave which occurs at a project site.
in figure 2-4, frost heave may continue almost up to the
Since heave and settlement are cyclic, the amount of
time freeze-up is complete because, in frost-susceptible
frost heave can be determined by measuring either total
finegrained soils, 95 percent or higher degree of
heave, which occurs during fall and winter, or total thaw
saturation may still prevail at the moment when removal
settlement, which occurs in spring and summer. Aitken2'
of moisture causes a free water table to disappear. In
describes one type of apparatus used in measuring frost
permafrost areas the absence of a water table in the
heave of the ground surface. Where roads exist, the
annual frost zone in the freezing season should not be
amount of heave may be determinable at fixed structures
taken to indicate that high ground water will not exist in
such as bridge abutments or culverts. Effects of frost
summer.
heave or frost thrust can often be discerned by the
(2) Ground water considerations are
evidence of frost jacking out of the ground or tilting of
further discussed in Chapter 4 (paras 4-12 and 4-18) and
insufficiently anchored and inadequately constructed
Chapter 7 (para 7-6).
facilities. In summer, mud lines may often be discerned
i.
Frost
susceptibility.
Criteria
for
on surfaces of piles or structures as the heaved surface
susceptibility of soils to ice segregation based upon the
recedes with thaw. Thaw consolidation tests can also be
percentage of grains finer than 0.02 mm by weight are
performed on cores of frozen materials obtained from the
6
outlined in TM 5-818-2 . While 3 percent finer than 0.02
annual frost zone after maximum winter freeze had
mm is the most common dividing line between soils
occurred. Where some frost heaving of the facility under
susceptible and not susceptible to ice segregation, frost
design is to be permitted, as for a transmission tower,
heave and subsequent thaw weakening, and is used
but the amount must be limited, the design predictions
widely in pavement engineering, this is a very rough
may be verified by constructing a prototype foundation
measure, an engineering rule of thumb, signifying not
without superstructure but with equivalent ground cover,
zero frost susceptibility but a level which is acceptably
loading it, and observing its performance through a
small for most engineering requirements under average
freezing season.
k. Creep and solifluction. Slope creep is
conditions. For a specific soil, the actual limiting criterion
for frost susceptibility may be either below or above this
extremely slow downslope movement of surficial soil or
value. For borderline materials or where a measurement
rock debris, usually imperceptible except by long-term
of frost susceptibility is essential, freezing tests will be
obser-
carried out under the supervision of the U.S. Army Cold
Regions Research and Engineering Laboratory,
3-12
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