TM 5-852-4/AFM 88-19, Chap. 4
ment
with
non-frost-susceptible
material.
For
depth of burial in frozen material exceeds six times the
nonpermanent facilities it may sometimes be necessary
plate diameter; soil stresses on the anchor rod should be
to accept heavy maintenance costs, however.
included in the analysis. Anchors closer to the surface
d. If foundation soils are susceptible to
than six times the diameter of the plate may be analyzed
in terms of stresses on an assumed 30 truncated cone.
settlement on thaw, thermal analysis should be made of
In both cases, the presence of a thawed layer at the
the proposed culvert, adapting the methods of TM 5-852-
14
surface, of distinctly different characteristics from the
6/AFM 88-19, Chapter 6 . The final design should
underlying frozen material, will usually require that it be
provide a thermally stable condition.
e. Headwalls of culverts may be heaved,
handled as a separate element within the analysis.
d. Conventional plate, screw-in type or various
undercut, tilted and fractured by frost action unless
patented earth anchors can be installed in inclined or
properly designed and constructed. They should be
vertical augered holes in permafrost with slurry or backfill
designed using the same structural approaches as
of soil-water mixture, compacted moist soil, or crushed
outlined in paragraph 4-10, and with conventional
rock. The capacity of such anchors is greatly increased
provisions against piping through unfrozen material
by freezing and keeping the backfill frozen. If the anchor
under the culvert.
relies significantly upon the strength and resistance of
4-14. Anchorages.
thawed soil after installation, all efforts should be directed
a. Economical construction of anchors in
to selection of the most suitable backfill and the
frozen ground is a difficult and challenging problem
attainment of good compactive effort.
because of the marked tendency for anchors to yield and
e. In recent years, special helical anchors
creep when anchoring in a frozen soils' and also
have been developed for installation in permafrost.
because of frost heave forces within the annual frost
These anchors often have multiple helixes increasing in
zone. While anchors in frozen soil may be capable of
diameter from the bottom and are designed to resist the
sustaining relatively high, short-duration loads without
large torques required for installation in frozen soil by
difficulty, they can exhibit unacceptable yield and creep
truck-or crane-mounted power equipment. Conventional
under much lower long-term loadings. The latter is most
helical anchors used for unfrozen soils may fail during
pronounced when frozen ground temperatures are only
installation by shearing the rod from the helix.
slightly below the freezing point. Possible long term or
f.
The design capacities for the various sizes
transient changes in thermal regime must be carefully
and shapes of commercial earth anchors, as published
evaluated.
in various tables in handbooks or manufacturers'
b. Laboratory experiments have shown that
literature for a range of unfrozen soils, should be
plate type anchors entirely in frozen soil (no thawed layer
reduced by 75 percent for anchors in thawed soil above
at the surface) fail in two distinct modes depending on
permafrost. Unless protected in the annual frost zone by
the depth of burial. When the ratio of depth below
anti-heave devices or treated backfill, all anchors
surface divided by diameter of plate is greater than six,
embedded in permafrost should be designed so that the
failure of the anchor occurs by punching of the plate on a
anchor rod is capable of resisting 60 psi of frost thrust
cylindrical surface through the overlying material in a
within that part of the rod which will be in the annual frost
manner similar to that by which an overstressed footing
layer, and a total frost uplift force would be computed by
may punch downward into a foundation.
This is
assuming the average of 40 psi acting over the depth of
illustrated in b through f of figure 4-93. At stresses less
the annual frost zone; the latter should be added to the
than those which will produce rupture under relatively
design tensile load imposed on the anchor. For anti-
rapid loading, creep deformation will occur under long
heave protection, see paragraph 4-31. Provisions should
term loading in the same manner as described for
be made for adjusting the tension of guy lines in both
footings in paragraph 4-7. When the plate is buried less
summer and winter, since the pole, tower or structure
than six times the diameter of the plate below the
being guyed and anchored may experience heave or
surface, failure may be expected to take place by
settlement quite different from that of the anchor(s).
punching out of a truncated cone of material starting at
g. Conventional metal expanding anchors
the plate and widening out at a 30 angle from the
should not be used in frozen soil or in rock containing ice
vertical, as shown in figure 4-93g.
As this cone
as the extremely high local stresses developed with such
approaches the ground surface, the angle may change
anchors cause rapid plastic deformation and creep in the
abruptly to perhaps 20 with the ground surface.
ice component. Only anchors which develop very low
However, it is conservative to assume in the analysis that
level
the 30 angle continues to the surface. If the anchor acts
at an angle with the ground surface, the failure surface
may be expected to be altered as illustrated in figure 4-
94.
c. Therefore, plate type anchors may be
analyzed in the same manner as footings when the
4-156
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