TM 5-852-4/AFM 88-19, Chap. 4
property values which are typical for frozen soils under all
protected the ice from thawing, or by other especially
conditions.
favorable local conditions. The possibility of such ice will
b. Ground ice.
When ice is found in
be greater, the colder the climate of the location.
(4) Open-work gravel containing clear ice
permafrost in sufficient amounts that significant
tends to give a visual impression that substantial
settlement would occur upon thawing, it may have
settlement of this material may occur on thaw, but close
originated by the conventional ice segregation process
examinations of random samples of such material near
with ice lensing primarily parallel to the ground surface
Fairbanks, Alaska, have shown particle-to-particle
as the permafrost was gradually developed in the
contacts, and no actual thaw-settlement difficulties with
geologic past; it may have been formed as vertical ice
such soils have thus far been reported. On the other
wedges in the horizontal contraction-expansion process
hand, personnel of the Alaska District, Corps of
which results in the typical patterned ground features so
Engineers, report that many ice-saturated gravel
evident in far northern areas; it may be "fossil ice" buried
deposits do not shown particle-to-particle contact, and
by landslides or other events and preserved as
that tests of sands have shown consolidation on thaw. It
permafrost; or all three of these types may occur
may be visualized that if permafrost is formed in clean,
together. Often several soil formations of different ages
granular, non-frostsusceptible material by continuous
may be superimposed one on another, each containing
downward advance of the freezing front under conditions
ground ice. Figure 2-18 shows a typical cross section in
permitting escape of any excess water then there may
silt near Fairbanks, Alaska; note that ice concentrations
be no separation of the soil particles which would lead to
do not necessarily decrease with depth, contrary to a
consolidation on thaw. However, if opportunity for
frequent assumption.
escape of excess water is lacking, either in the original
(1) In the annual frost zone, ice is of the
freezing or in refreezing from a partially degraded
common ice segregation type; small amounts of ice may
condition, as from a climatic warming interval, then a
also be found in shrinkage cracks. Ice formations in this
bulked condition will exist in the zone of water
zone disappear every summer and are formed anew in
entrapment; this zone may be horizontally discontinuous.
the winter. Substantial ice concentrations are frequently
Finally, even essentially clean deposits may develop
found on permafrost at the bottom of the annual thaw
some ice segregation during downward freezing if even
zone which thaw only in occasional very warm summers.
thin inclusions of fine material are present, or possibly if
(2) Occasionally bodies of permafrost may
the freezing front advances very slowly or is stationary.
be encountered (such as near the top of a high, well-
Thus, no simple generalizations are safe.
drained bluff) which are less than 100 percent saturated;
(5) In
borderline
permafrost
areas
such permafrost may lack some of the detrimental
remnants of permafrost containing ground ice may
characteristics associated with ground ice. However, if
sometimes be found at depths such as 25 to 40 ft, with
ground ice exists in strata at lower levels in the
non-frozen soils both above and below. Such was the
foundation and thaw may reach the ice during the life of
experience in deep granular deposits at Clear, Alaska.
the structure, this must be taken into account in the
Extensive exploration is required to locate these
design.
remnants when they occur as small, scattered bodies.
(3) Ice masses in clean, granular deposits
(6) If fine-grained soils containing ground
are not uncommon in the Arctic because of the severe
ice are excavated, very wet, liquid-soft material unusable
environment, but their occurrence is less common in the
for earth fill and incapable of supporting equipment may
Subarctic.
U.S.
Army Engineer District, Alaska,
result on thaw. From coarse, free-draining granular soils
personnel have reported the occurrence of major ice
and broken rock, however, thaw-water may escape
inclusions in gravels at Cape Lisburne, Alaska, and at
almost as rapidly as thaw occurs. The settlement of
Gambel on St. Lawrence Island off the coast of Alaska,
foundations from degradation of permafrost and the
and others have reported ice wedges and masses in
development of frost heave in the annual frost zone both
gravels at Barrow and Umiat, Alaska, and Inuvik, N.W.T.,
46
Canada. Church, Pewe and Andresen , in their study of
typically occur differentially between points across the
patterned ground in the Donnelly Dome area near Ft.
foundation. The thawing of polygon ice wedges such as
Greely, Alaska, found evidence that ice wedges had
shown by figures 2-2 and 2-18 may tend to develop
formed in the outwash gravels of the area during a
cavern-like voids in the foundation which may lead not
period colder than at present and subsequently thawed
only to sudden and dangerous collapses but also to
during a period warmer than now exists.
Similar
development of underground drainageways which may
evidence has been observed at Clear, Alaska. Thus, the
serve to extend thaw. Loess-type frozen soils may be
possibility should be considered that, even in the
particularly susceptible to subsidence and erosion on
Subarctic, ice wedges formed in clean granular materials
thaw.
Where foundations include anchorages in
in a previous colder climatic period may be found
permafrost, thaw may lead to anchorage failures.
preserved by overlying accumulations of soil which have
2-24
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