TM 5-852-4/AFM 88-19, Chap. 4
tercepting ditches above the top of cut can control
(4) If the excess ice content is very high it
discharge of moisture over the top, and may be needed
may be necessary to ultimately place a very substantial
regardless of degree of slope, but the consequences of
blanket designed to make up for the low amount of soil
seepage of water from the ditches into the soil directly
naturally present in the slope. Since relatively fine-
behind the face may be more severe with the steep
grained moisture-holding soil is more effective than
slopes. In fine-grained soil, especially silts, great care is
gravel for this purpose, the blanket may in this case
required to avoid gullying and progressive soil erosion
consist of two layers an underlying zone of random fill
where the surface vegetative mat is cut, as by an
and an overlying granular blanket not exceeding 3 to 5
intercepting ditch or at the cut slope, and surface flow of
feet in thickness. When possible, it is advantageous
water occurs. Such erosion can involve large areas and
under these conditions to make the initial cut slope quite
must be corrected in its earliest stages. The savings in
steep and to allow natural degradation and build-up of
initial construction costs obtained by employing nearly
protective cover to occur for up to several years before
vertical slopes as opposed to conventional flatter slopes
placing the final protective blanket. Required combined
must be balanced against such extra costs as providing
thicknesses of blanket materials and natural cover for
wider ditch area to allow for spalling, erosion and
complete thermal stability may be computed as
sloughing, placing gravel or crushed rock at the toe when
described in paragraph 4-2.
needed to insure stability at the ditch level, and
(5) Under conditions where a lengthy
maintenance efforts for periodic clean up and removal of
period of slope adjustment is acceptable, initial
slope-wasting materials, restoration of drainage, etc.,
construction cost may be significantly reduced by going
which are not required when positive slope stability is
even further and making cuts with vertical or near-
provided at the start.
vertical slopes with wider than normal ditches, leaving
b. Sloughing and thaw settlement in areas of
the natural cover undisturbed and allowing the slope to
permafrost.
seek a relatively stable condition by natural processes, at
expense of greater maintenance costs. This method
(1) When a cut is made in permafrost
was tried in 1970 along the Trans-Alaska Pipeline
containing substantial amounts of ground ice, frost-
Access Road from Livengood to the Yukon River,
sloughing and seepage erosion effects are intensified
195
Alaska . Cut faces contained up to 70 percent ice, as
because of the potentially much larger volumes and
illustrated by figure 4-99. During summers following
deeper extent of deposits of ice and because of the
construction, melting of the ice caused the cut faces to
irregular general settlement of the slope which may
assume quite irregular but gradually flatter slopes,
occur when this ice melts, as illustrated in figure 4-98b.
accompanied by dropping of the organic mat and thawed
If it is necessary to produce a reasonably stable slope
soil down over the initially exposed ground ice to provide
during the initial construction, the amount of protective
a progressively increasing thickness of protective cover
earth covering which will develop as permafrost
as shown in figure 4-100. Considerable sloughing and
degrades should be evaluated and a blanket thickness
down-slope erosion of fine-grained soils developed.
adopted which will not only control sloughing and erosion
Study of silt faces cut by gold mining operations in the
but ultimately limit further permafrost degradation. This
Fairbanks area indicates that in areas of relatively warm
will require the initial cut to be made with more or less
permafrost, slope instability and adjustment under this
conventional side slopes. The blanket should cover the
approach may continue for many years or indefinitely,
full height of the cut slope.
even after moderate-size trees have grown on the slope.
(2) If the excess ice content of the natural
Where significant ice is present, the slope may become
soil is relatively low, the same protective blanket criteria
very rough and unsightly. Periodic removal of silt from
as presented above for areas of seasonal frost should be
ditches and drainageways will be needed. If road-way
used, except that blanket thicknesses should be in the
ditches are allowed to fill, not only will drainage fail to
range of 18 to 36 inches. As ice masses melt and drain
function properly but snow and ice control in winter may
away, the blanket may develop an irregular surface, but
become more difficult. Silt in run-off may also cause
so long as the blanket remains intact, it will retain its
undesirable or unacceptable environmental effects
function. Except in most northerly areas, it will seldom
unless it is prevented from entering streams which
be economical to place sufficient thickness of blanket to
receive the drainage discharge. Where slopes are high
contain thaw within the blanket. Some redressing of the
the possibility that sloughing or slides may encroach on
slope may be done if needed, in future years.
the pavement must be considered. Progressive gullying
(3) If the ice content of the permafrost is
and erosion where the cut slope intercepts surface
higher and the sloughing penetrates deep it may be
drainageways must be corrected expeditiously.
necessary to use up to 3 to 5 feet of blanket material.
Ultimately, the natural stabilization processes may have
This was done successfully to stabilize a sloughing slope
to be supplemented.
in silt at the CRREL tunnel in permafrost at Fox, Alaska,
for construction of the tunnel portal.
4-166
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