TM 5-852-4/AFM 88-19, Chap. 4
columns resulted and the slabs returned to their original
potential; however, potential for increasing tangential
grades in the spring. Figures 4-44 and 4-45 show the
shear strength by increasing roughness is limited by the
magnitude of heave forces actually developed on plain
shear strength of the adjacent frozen soil. Creosoted
steel pipe and creosoted wood piles at Fairbanks,
wood, steel with a mill varnish or red lead or other
51
Alaska .
coating substances have the least adfreeze bond
(2) By a nominal estimate of the effective
potential.
area of the slab of seasonally frozen soil which
(5) In tests in a permafrost area at
contributes to heave or thrust forces on a structure and
Fairbanks, Alaska, average maximum adfreeze bond
by use of the maximum heave pressure data presented
stresses as high as 60 psi have been measured on
in figure 2-9, and discussed in paragraph 2-4, a rough
uncoated steel piles embedded in a silt, under natural
approximation of the total heave or thrust forces on a
freezing conditions. Since this is an average value over
given structure can be made. A comparison of these
the area of adfreeze, higher unit values undoubtedly
forces with structural foundation loadings or passive
were developed in the coldest upper levels of the
resistive forces will give an indication of their relative
seasonal frost zone. Values decreased rapidly when the
40 66
By laboratory tests , , using either
balance.
rate of advance of the freezing plane slowed, as stress
undisturbed or remolded materials as applicable for the
relaxation occurred in creep. Thus, any design measure
particular frostsusceptible soil involved and by more
which results in slower frost penetration, and/or higher
rigorous analysis of the interaction of the structure and
temperatures at the adfreeze bond surface and smaller
frost forces, a more accurate estimate is possible in
bond area will lower peak adfreeze bond forces and
theory. However, because of the many variables of soil
increase probability of stability against frost heave. The
conditions, moisture availability and frost penetration,
same is not necessarily true of direct uplift as illustrated
precise quantitative predictions are not usually practical
in figure 4-42a or of frost thrust as illustrated in figure 4-
in the present state-of-the-art.
42b, as direct forces may remain high so long as ice
(3) When frost penetrates downward
segregation is occurring, even with a stationary position
along vertical faces of walls, footings and piles in contact
of the freezing plane.
with earth, as illustrated in figure 4-42c, an adfreeze
(6) When a structure is permitted to "float"
bond develops between the soil and the concrete, wood
on the annual frost zone as in figure 4-42a, detrimental
or other material of the foundation. If the soil is frost
frost effects can be best avoided, first by minimizing the
susceptible and heaves, the wall or footing tends to be
magnitudes of seasonal displacement through the heave
lifted with the layer of frozen soil because of this
reducing effect of a surcharge provided by a non-
adhesion. The weight of the structure at the same time
frostsusceptible granular mat and, second, by insuring
tends to restrict frost heaving, the reduction diminishing
uniformity of the remaining frost effects through (a)
with distance from the face. The maximum upward force
selection of sites with as nearly uniform soil conditions as
which can be exerted on the structure is usually not
possible, (b) careful control of thickness, soil
limited by the uplift force which can be developed at the
characteristics, and drainage of the mat, (c) extending
plane of freezing, but by the unit tangential adfreeze
the mat a sufficient distance beyond the structure
bond strength and the area of adfreeze contact on the
perimeter so that possible edge effects on footings are
wall, footing or pile itself. The total uplift force which is
minimized, and (d) providing supplementary shading of
thus imparted to the structure is a function of the
the foundation if appropriate.
thickness of the frozen layer; the total force increases as
(7) A situation similar to that shown in
the depth of frost penetration and the total bond contact
figure 4-42a may develop inadvertently in temporarily
area increase.
exposed footing or foundations on frost-susceptible
(4) The tangential shear stress which can
materials during construction if adequate protection
be developed under a given rate of loading, or allowed in
against winter frost heaving is not provided. In two
design, on the surface of adfreeze is limited by creep,
documented cases, at Anchorage, Alaska; and at a
which occurs down to stresses as low as 5 to 10 percent
group of mid-western construction sites, erected but
of the rupture strength measured under relatively rapid
incomplete structures were bodily lifted as much as 4 to
loading. Tangential shear stress is a function of such
6 inches by frost action before the heaving was
148
noticed . In the case of the mid-western installations
factors as the temperature, surface material (as
tilting due to differential heaving also occurred. At both
concrete, steel, wood or paint), presence of salt or other
locations the foundation soils were allowed to thaw
chemicals in the soil moisture, direction and sequence of
gradually and evenly; the foundations returned
freezing, and rate and duration of loading. Tangential
essentially to their design grades and the structures were
shear stress values are discussed in paragraph 4-8. In
completed successfully. In an inverse case, frost heave
the present state of knowledge, dean metal, untreated
of various parts of an Alaskan schoolhouse was caused
smooth wood or smooth concrete surfaces may all be
when artificial refrigeration
assumed to have similar adfreeze bond potentials.
Rough concrete and rough wood (timber) have greater
4-68
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