Table 11. Effect of lime on frost heave (after Lambe et al.
1971).
Heave ratio
New Hampshire
Sandy
Silty gravel
Additive
silt
clay
sand
Type
Percentage
(ML-CL)
(CL)
(SW-SM)
Lime
1
1.06
0.70
3
0.27
0.16
0.56
Quicklime
1
0.93
1.74
3
0.43
0.13
1:1 Limefly ash
25
0.18
0.21
1:4 Limefly ash
25
0.09
0.14
1:9 Limefly ash
25
0.08
0.66
ing the no. 200 sieve) in the current FAA specifica-
the Illinois pavement test track. Thompson also re-
tions for base courses is presented in Table 12.
ported that the failure (rutting) of the test section
Examining COE field tests in the 1940s on base
could not totally be attributed to the subgrade.
courses in the northern tier of states, Johnson
From measurements he found that, with the ex-
(1974) reported that bases with 10% fines passing
ception of the top 25 mm (1 in.), the subgrade
the no. 200 sieve showed serious thaw-weaken-
moisture content did not change when the test
ing and significantly reduced (up to 70%) bearing
sections were soaked. However, the base course
capacity (in terms of CBR). This reduction in
was in the range of 86 to 90% saturation. He con-
bearing capacity has to be related to the drainabil-
cluded that the failure of the test sections was at-
ity of the base course, which in turn is affected by
tributable to the reduction in the bearing capacity
the amount of fines it contains. Therefore, the crit-
of the base course.
ical property of the basesubbase material during
We analyzed base drainage using the method
thaw-weakening is its permeability (vertical and
developed by Casagrande and Shannon (1951).
horizontal). The horizontal permeability does not
This model was based on field observations of six
have to be the same as the vertical permeability. If
airfields in Maine, Michigan, Wisconsin, and
a criterion of 10% fines is set, then all of the FAA
North and South Dakota by the COE. Most of
unstabilized base courses, with the exception of
their observations of saturation of the base courses
P-209 crushed aggregate base, are prone to thaw-
were made during spring thaw. The model
weakening.
assumes symmetry along the centerline of the
One way to assess the thaw-weakening poten-
pavement; the equations represent drainage for
tial of the base course is to estimate how long it
one-half of the base course, ABCD (Fig. 11). The
will take the base course to drain to at least a
base drains in two parts. First, it is saturated and
saturation level of 80%. Haynes and Yoder (1963)
the free water surface changes from AD to AC
found that granular materials subjected to repeated
(Fig. 11). This assumption is based on the premise
loading became unstable when the degree of sat-
that water is allowed to drain freely through the
uration was greater than 80%. This was substanti-
open face CD. Second, the water surface changes
ated by Thompson (1969a), who used results from
from AC to BC. Other assumptions are that the
centerline and the bottom of the base course are
Table 12. Maximum allowable fines
impervious and that the phreatic surface is a
in FAA basesubbase specifications.
straight line. This assumption of an impermeable
subgrade is valid during spring thaw when the
Max.
allowable
subgrade is still frozen.
FAA
fines
Liu et al. (1983) reported on base course drain-
designation
(%)
age characteristics and showed that there was no
significant improvement in the results when a
P-154 Subbase
15
P-208 Aggregate base
15
parabolic instead of a straight line free surface
P-209 Crushed aggregate base
8
was used. An additional assumption made for
P-210 Caliche base course
15
this study is that the base is sloping. The calcula-
P-212 Shell base course
15
tions are done in two stages.
P-213 Sandclay base (A)
15
P-213 Sandclay base (B)
25
17
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