TM 5-852-4/AFM 88-19, Chap. 4
drainage problem presently exist except discharge into
into the frost susceptible material and to slow rate of
the building sewer system in permafrost areas or, in
thaw into this material in spring. Its loading effect serves
seasonal frost areas, discharge into the ground at
to assist reconsolidation of frost-loosened material. It
sufficient depth to be below the zone of freezing.
serves as a relatively high strength reinforcing material
within the zone potentially involved in the sliding action.
4-19. Stability of slopes during thaw.
It also provides drainage. Because it is designed as a
a. Frost sloughing in areas of seasonal frost.
filter, seepage may emerge through it without loss of fine
(1) In seasonal frost areas slopes
particles from the frost-susceptible zone or plugging of
composed of finegrained soils tend to experience "frost
the voids in the blanket material.
sloughing," as illustrated in figure 4-98a, during spring
(4) Successful use of both cinders and
thaw. The sloughing occurs when the ice-filled soil
bank-run gravel as the blanketing material has been
thaws relatively rapidly. Impervious underlying frozen
reported in numerous cases. However, gravel will be the
material prevents drainage of excess water in that
normal blanketing material in arctic and subarctic areas.
direction. The resulting very wet, low shear strength soil
Good quality crusher run rock can also be used.
therefore slumps or flows downward as illustrated in
(5) Where frost and ordinary seepage
figure 4-98a. The effect can be intensified by earthquake
sloughing and erosion of slopes are definitely anticipated
accelerations. Emerging excess water may also move
in seasonal frost areas, protective blankets should be
some soil downward by erosion. The phenomenon
specified as part of the original design when they provide
occurs typically in frost-susceptible fine-grained soils
the most cost-effective approach. Although the blankets
under conditions where sufficient moisture is available to
require initial extra expense, as compared with untreated
build up substantial excess water in the annual frost zone
slopes, maintenance costs resulting from unstable
in the form of ice lenses.
slopes can be eliminated. Blankets may also be used to
(2) Frost-sloughing is more common in cut
correct unanticipated problems, but added expense to
slopes than fills because of the greater availability of
prepare the sloughed face of the slope will then be
moisture.
However, it is not uncommon in
involved. Blanket thickness should be between 6 inches
embankments, especially if the slopes are made very
and 30 inches, with the larger thicknesses used for the
steep. It is also more common and more severe on
most severe cases. In most cases 18 inches or 24
north-facing than south-facing slopes (in the Northern
inches will be needed. Opportunity to use as little as 6
Hemisphere), probably because north-facing slopes tend
inches is expected to be rare in arctic and subarctic
to be wetter and although onset of thaw is delayed, its
areas. Vertical to near-vertical slopes have been tried in
progress is fast once it starts. In wet cuts in which
highway cuts in Alaska in search of a more economical
seepage emerges from the face, sloughing and erosion
but still satisfactory solution. The wind-deposited silts in
may also occur during non-frost periods.
Alaska, when free of ground ice, have significant loess or
(3) Slope flattening can control frost
loess-like properties and in areas of low precipitation
sloughing but may be very costly or not feasible.
have appreciable capacity for standing vertically for
Drainage to reduce the amount of moisture available for
heights of 20 feet or more. However, a number of
ice segregation usually provides only partially effective
52
problems have been observed . These include a
control, especially when soils have strong horizontal
tendency for spalling to occur in slabs about 4 in. thick,
stratification, and may be very expensive. Turf helps to
more on south-facing slopes than on north, attributable
control sloughing but its effects is marginal. Both the
to such causes as moisture fluctuations and thermal
frost and non-frost types of slope problems can be
stresses, collapse from undercutting by erosion or by
controlled by blanketing the slope with granular pervious
loss of toe stability from moisture at the ditch level, and
material as illustrated in figure 4-98c. The blanket
erosion or gullying from the top of slope downward,
material should be graded to act as a filter but also be
caused by discharge of surface run-off over the top
sufficiently coarse-grained so that slow seepage can
during periods of heavy precipitation or snow melt. If the
emerge from it without movement of particles. At the toe
ditch must carry drainage flow, the slope is especially
of slope the blanket should be carried a short distance
vulnerable and blockage of drainage by collapse
below the adjacent surface as shown in figure 4-98c to
materials may cause secondary damage. Slumping onto
avoid supporting the toe on material which will
the roadway may be possible. Drifted snow and snow
experience sufficient frost weakening in spring and to
cast by snow removal equipment can cause a wet
avoid loss of support under the toe by seepage erosion
condition on thawing, not only in the ditch but to some
of fine-grained soil. The blanket functions in several
extent on the face itself. Low precipitation with long
different ways. It serves as a surcharge weight to reduce
periods of dry weather does not insure against
the amount of ice segregation per unit volume and hence
occurrence of wet conditions at some period of the year.
the volume of water to be released from the frozen frost-
In-
susceptible material in spring. It serves as a semi-
insulating layer, to reduce the amount of frost penetration
4-164
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