TM 5-852-4/AFM 88-19, Chap. 4
upward frost thrust acting directly against horizontal
h.
Factors of safety.
foundation surfaces and on vertical surfaces by
(1) On the basis of failure load
adfreeze bond of the seasonal frost layer to the
determination from pile loading tests (g(5) above) the
structure. They are also subject to lateral thrust from
factor of safety of friction type piles against ultimate
laterally acting frost heave forces; as indicated in figure
failure should be at least 2.5 for dead load plus normal
4-42, frost heave develops in a direction directly opposite
live load and 2.0 for dead load plust maximum live load.
to the direction of frost penetration.
If freezing
Since ultimate adfreeze bond strength is about 1.4 times
temperatures penetrate through a vertical face, such as
the sustainable, the factor of safety of 2.5 provides a
a wall, water migration, ice lensing and frost thrust will be
factor of safety of about 2.5 divided by 1.4 equal to 1.79
oriented in relation to the vertical surface in the same
with respect to the sustainable strength, on a gross
manner as they are to a horizontal surface when frost
basis. When the allowable design load in equation 14 is
penetration is downward. The force developed can be
computed analytically (f(1) above) the gross factor of
sufficient to move or break the wall. Progressive small
safety against ultimate failure contained in the resultant
movements, year after year, can produce substantial
value Qa should not be less than 3.0. These criteria
permanent tilt, because forces during the thawing period
apply for piles of average length of embedment in
do not act to return the structure toward its original
permafrost, i.e., 15 to 35 feet.
position, as is the case, for example, for pavements. For
(2) Factors of safety for end bearing piles
these reasons, the design of walls and retaining
should be the same as in TM 5-818-1/AFM 88-3,
structures requires even more care than the design of
Chapter 75.
conventional footings. The most satisfactory method is
(3) Because, as shown in figures 4-44 and
to place a backfill of non-frost susceptible material
4-45, peak frost heave forces act for only a fraction of
directly behind and adjacent to the wall structure, as
the year, avoidance of rupture of the adfreeze bond in
shown in figure 4-87a, b, to a thickness equal to the
permafrost under peak stresses is a more critical
depth of frost penetration, using, if necessary, a 12-inch
problem in considering pile safety against heave than is
filter layer next to the finegrained backfill. If differential
progressive upward movement under stresses of creep
frost heave would cause a problem on the ground
levels. The same is true for piles subject to intermittent
surface at the edge of the nonfrost-susceptible backfill,
external tension loads. If rupture of the bond occurs,
the latter should be tapered out over sufficient distance
major upward displacement may be expected, which, as
to eliminate the problem. Positive drainage of the backfill
noted in f(3) above, is not likely to stabilize. These types
should be provided; however, the possibility that the
of loading are also less predictable in magnitude than
drainage system may be blocked by freezing during a
downward compression type loadings.
Under
significant part of the year must be taken into account in
intermittent tension or frost heave loading of piles,
the hydrostatic pressure design assumptions for wall
factors of safety of 2.5 and 3.9 with respect to failure
stability analysis.
loads determined by tests and by computations,
b. The chart shown in figure 4-88 may be used
respectively, should be applied in equation 15 to forestall
failure. These values should also be used for other
to estimate the depth of backfill required behind concrete
types of foundations when critical stressing is in
walls in order to confine seasonal freezing to the backfill.
tangential shear of adfreeze bond.
For average wall conditions (assuming essentially
4-9. Grade Beams. Grade beams or similar horizontal
vertical wall faces) with average exposure to the sun, a
structural members placed at or just below ground level,
surface freezing index equal to 0.9 of the air freezing
which may be subject to uplift, should be avoided when
index should be used. The n-factor of 0.9 is greater than
frost-susceptible soils are involved.
Instead, full
the 0.7 used for pavements kept cleared of snow
foundation type walls should be substituted. In theory,
because of the more positive freedom from the insulating
an alternate procedure is to replace the soil at the grade-
effects of snow and ice, because of 3-dimensional
beam location for the full frost depth with non-frost-
cooling effects associated with a wall and embankment,
susceptible material for sufficient width so that the non-
and because of increased cooling effects of wind. If the
heaving soil under the beam will not be carried up by
wall receives no sunshine during the freezing period, is
heave of the adjacent soil.
However, no rational
exposed to substantial wind and remains free of snow or
procedure for determining the required width of non-
ice, an n-factor of 1.0 should be used. If the wall is
frost-susceptible material is yet available.
located in a southerly latitude, has a southerly exposure
4-10. Walls and retaining structures.
and therefore receives much sunshine, the n-factor may
a. Bridge
retaining
walls,
be as low as 0.5 to 0.7; however, in very high latitudes,
bulkheads and similar structures with unheated
the net radiational heat input may be very small or
foundations are susceptible to frost heave, settlement
negative and the n-factor may be 0.7 to 0.9.
and overturning forces, when the requisite soil, moisture
The chart may also be used for estimating the depth of
and freezing conditions are present. They are subject to
frost penetration vertically into granular soil below a
4-144
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