E. M. PART XV
CHAPTER 3
October 1954
may be little or no detrimental frost action. There is some evidence that pavements constructed on varvcd clay
subgrades, subject to freezing, in which tile deposit and depth to ground water are relatively uniform, l have performed
satisfactorily. When subgrade conditions are uniform and there is local evidence that the degree of heave is not
exceptional the varved clay subgrade soil should be assigned a group F3 frost susceptibility classification.
Road and runway design over frost-susceptible soils must take into account frost and permafrost conditions. Soils
information should be obtained from borings and test pits to at least the depths indicated for the final stage of site
selection as described in chapter 2. The number of holes or pits required can only be determined from physical
conditions and the experience of the engineer.
b. Effect of frost and permafrost conditions. Frost and permafrost conditions may be altered by changed surface
conditions produced by construction. Such challenges have a considerable effect on the stability of roads and runways
and must be considered in their design. Depths of freeze and thaw before and after construction may differ considerably.
Figure 1 illustrates the effect of clearing and stripping on the depth to the permafrost table after a period of 5 years.
Figure 2 shows an idealized sketch of the maximum thaw beneath a road and adjoining areas where thaw is caused by
surface heat and the effect of running water in road ditches. The surface temperature for a given air temperature is
strongly affected by a considerable variety of factors, and the mean annual surface temperature may differ considerably
from that of the air. The effect of surface type on depth of thaw is illustrated by table 1.
The change of the level of the permafrost table after construction should be considered in design. Construction may
result in a degradation of the permafrost (a condition resulting if annual thaw exceeds annual freeze) or in a condition
where thaw and annual freeze penetrate into soil which has previously existed as permafrost. Degradation of permafrost
in cuts may lead to slides or sloughing, often difficult to stabilize. Where degradation is anticipated, soils should be
investigated to a depth sufficient to determine the eventual effects of such progressive action. The depth of thaw and of
freeze which will occur after construction may be calculated by methods given in chapter 6
Thawing of previously unthawed subgrades which have a high ice content, such as with thick closely spaced ice
lenses, buried ice masses, or ice-filled polygon fissures will result in serious and
Table 1 - Measured Depth of Thaw Below Various Surfaces in the Subarctic After 5 Years.. (Fairbanks, Alaska, Mean
Annual Temperature 26 F.)
Type of surface
Color of surface Thickness
Nature of
Thick-
Approximate
Observed
Depth of
of
base
ness
elevation of
total
thaw in
pavement
course
of
water table
depth of
feet into
in feet
base
thaw in
silt
course
feet
subgrade
in feet
Gravel...........................
Natural .......................................S.......and.................4..........o.t.t........f.bas.e.......8.0 - 10.5
..and .....
. 0 B . om o. ..... .
4.0 - 6.5
gravel.
course.
Concrete .......................
Natural .................................5....S.......and
0 .. ..and .....
4.0 Bottom of base
8.5 - 9.5
4.0 - 5.0
gravel
course.
Concrete .......................
Natural .................................5....S.............................4..........o.t.t........f.bas.e.......8.5..-.9.5........4.0 .-.5.0
0 .. ..and
. 0 B . om o. ..... .
.. . . .. .
.. .. . .. ..
course.
Asphalt .........................
Black ....................................4....S.......and
0 .. ..and .....
4.0 Bottom of base
8.5 - 10.0
4.0 - 5.5
gravel.
course.
Trees, brush, grass,
Natural vegeta
................................................................u.r.f.a.c....................3.0..-.4.0........3.0 .-.4.0 .......................
S . . .e
.. . . .. .
.. .. . .. ..
and moss
tion.-
Grass and moss ...........
Natural minus
...............................................................u.r.f.a.c....................5.0..-.6.0........5.0 .-.6.0 .......................
S . . .e
.. . . .. .
.. .. . .. ..
trees and
brush.
Grass without moss
Natural grass
................................................................u.r.f.a.c....................8.0..-.9.0........8.0 .-.9.0 .......................
S . . .e
.. . . .. .
.. .. . .. ..
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