UFC 3-260-02
30 June 2001
(3) Base- and subbase-course materials of borderline quality should be tested frequently
after compaction to ensure that the compacted material meets requirement (1). For material
expected to exhibit serious degradation during placement and compaction (>3 percent finer than
0.02 millimeters by weight), a test embankment may be needed to study the formation of fines by
the proposed compaction method. If the test embankment shows serious degradation, the material
gradation should be changed to account for the fines obtained during compaction. If experience
indicates that the base- or subbase-course materials degrade rapidly under traffic loads or due to
environmental effects, consideration should be given to stabilizing the material with asphalt or
Portland cement.
(4) Mixing of base or subbase course material with frost susceptible subgrade soils should
be avoided. The subgrade should be properly graded and compacted prior to the placement of the
base or subbase course. Separation layer requirements must be met.
8. DRAINAGE LAYER REQUIREMENTS. A minimum 100-millimeter (4-inch) thick nonfrost
susceptible drainage layer must be placed at the bottom of the asphalt concrete layer, the PCC
layer or below the bound base for all pavements constructed in frost areas. The rapid draining
nonfrost susceptible material must meet the gradation requirements shown in EI 02C202/
AFJMAN 32-1016. The drainage layer will be designed in accordance with the requirements in
EI 02C202/AFJMAN 32-1016. The layer is considered as a structural component of the pavement
and will serve as part of the base course. In seasonal frost areas, as frost penetrates into the frost
susceptible subgrades, water is drawn to the cold front and ice lenses form. During the frost
melting period, the ice lenses will melt and the water will have to be removed. In extremely wet
conditions or with F3 and F4 subgrade soils, a drainage layer should be considered between the
subbase and the subgrade in lieu of a drainage layer under the surfacing as illustrated in
Figure 20-7.
9. SEPARATION LAYER. If subgrade freezing will occur, a minimum of a 100-millimeter (4-inch)
granular separation layer as specified in EI 02C202/AFJMAN 32-1016 will be placed between the
subgrade and the overlying base course. Over weak subgrades, a 152-millimeter (6-inch) or greater
thickness may be necessary to support construction equipment and to provide a working platform
for placement and compaction of the base course. This layer is not intended to be a drainage layer.
The gradation of this separation layer should meet the requirements in EI 02C202/
AFJMAN 32-1016. An additional requirement is that the separation layers must be nonfrost
susceptible or of frost group S1 or S2. Alternatively, where stable foundation already exists,
geotextile fabrics meeting the requirements of EI 02C202/AFJMAN 32-1016 can be used in lieu of
a granular material as a separation layer. No structural advantage will be attained in the design
when a geotextile fabric is used. The fabrics must meet the requirements of EI 02C202/
AFJMAN 32-1016.
10.
SUBGRADE REQUIREMENTS. In addition to the requirement outlined for subgrades in
nonfrost areas in Chapter 6, the following additional requirements shall be required for subgrades in
frost areas. It is a basic requirement for all pavements constructed in frost areas that subgrades in
which freezing will occur be as uniform as possible. This will be done by mixing nonhomogeneous
soils, eliminating isolated pockets of soil of higher or lower frost susceptibility, and blending the
various types of soils into a single, relatively homogeneous mass. This attempts to produce a
subgrade of uniform frost susceptibility and thus create conditions tending to make both surface
heave and subgrade thaw weakening as uniform as possible over the paved area. To achieve
uniformity in some cases, it will be necessary to remove high frost-susceptible soils or soils of low
frost susceptibility. In that case, the pockets of soil to be removed should be excavated to the full
20-11
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