Evaluation of Airport Subsurface Materials
VINCENT C. JANOO, ROBERT EATON, AND LYNETTE BARNA
INTRODUCTION
Granular unbound bases containing frost-suscep-
tible materials may also weaken significantly
In areas of the world having seasonal frost, air-
during thaw-weakening periods, owing to
port pavements freeze and thaw annually. The
increased saturation and a decrease in moisture
effects that frost action have on a pavement struc-
tension, combined with reduced density that
ture are uniform or differential frost heave during
comes from ice expansion when the base course
the winter and subsidence in the spring and dur-
was frozen. Granular material with excess fines
ing intermittent thaws in the winter because of
can also weaken during the thaw-weakening
thaw-weakening. Pavements constructed in frost
period because its permeability is reduced. The
areas must accommodate the design aircraft load
presence of fines can also change the hydraulic
during thaw-weakening periods and minimize
conductivity of the material, i.e., as the fine con-
roughness caused by differential frost heave.
tent increases, the hydraulic conductivity of the
For frost heave to occur, three conditions must
layer decreases.
be present: 1) below-freezing temperatures, 2)
The objective of this study is to evaluate the
frost-susceptible soil, and 3) water close to the
current Federal Aviation Administration (FAA)
freezing front. As the frost penetrates into the
specifications for subsurface materials used in
pavement structure, moisture is drawn to the
airport pavement construction in cold regions.
freezing front by capillary action. Ice lenses are
We assume that below-freezing temperatures are
formed and the material is displaced, usually in
present in the winter and that the water table is
the vertical direction. The displacement is trans-
close to the surface. The evaluation will focus on
lated to the surface and is called frost heave.
estimating the frost-heave and thaw-weakening
Thaw-weakening takes place when the segre-
susceptibility of the subsurface materials on the
gated ice lenses melt. Depending on permeability,
basis of the properties of the subsurface materi-
the excess moisture from the melting ice can
als. The results and conclusions presented in this
become trapped between the surface course and
report are based on the limited materials data
the frozen layer below. When the surface is load-
available in the FAA (1989) Advisory Circular
ed, the water is unable to dissipate, an undrained
AC150/5370-10A. The material specifications
loading condition exists, and positive pore pres-
evaluated are presented in Table 1.
sures develop in the saturated layer. This leads to
a reduction of the effective stress that the layer
Table 1. FAA material specifications
can tolerate, thus reducing the bearing capacity of
evaluated in this study.
the pavement structure. So, an important soil
FAA
property that affects thaw-weakening is the soil's
designation
Material and layer type
hydraulic conductivity.
P-154
Subbase course
It is often assumed that frost heave and thaw-
P-155
Lime-treated subgrade
weakening occur primarily in the fine-grained
P-208
Aggregate base course
subgrade soils. Base courses are usually thought
P-209
Crushed aggregate base course
of as layers that are resistant to frost heave and
P-210
Caliche base course
P-211
Lime rock base course
thaw-weakening. However, this is not necessarily
P-212
Shell base course
the case. The presence of fines in a base or sub-
P-213
Sandclay base course
base layer can change the frost-heave-susceptibil-
P-301
Soilcement base course
ity of the material. In general, as the fine content
P-304
Cement-treated base course
P-306
Econocrete subbase course
increases, the frost-heave-susceptibility increases.
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