30 June 2001
of the eight groups may be found as subgrade soils. Soils are listed in approximate order of decreasing
bearing capability during periods of thaw.
(a) Required Thickness. Where there are frost-susceptible subgrades, determine
section thickness according to the reduced subgrade strength method. The reduced 3-5 subgrade
strength method uses the frost area soil support indexes (FASSI) in Table 3-5. Use FASSI like CBR
values. The term CBR is not applied, because FASSI are weighted average values for an annual cycle
and their values cannot be determined by CBR tests. Enter Figure 3-4 with the soil support indexes
(vice CBR values) to determine the required section thickness.
Frost Area Soil Support Indices (FASSI) of Subgrade Soils
F1 and S1
F2 and S2
F3 and F4
(b) Pavement Section Layers. When frost is a consideration, recommend the pavement
section consist of layers that will ensure the stability of the system, particularly during thaw periods. The
layered system may consist of a 150-millimeter- (6-inch-) thick minimum wearing surface of fine crushed
stone, a coarse-graded base course, and/or a well-graded subbase of sand or gravely sand. To ensure
the stability of the wearing surface, the width of the base course and subbase should exceed the final
desired surface width by a minimum of 0.35 meter (1 foot) on each side.
(c) Wearing Surface. The wearing surface contains fines (material passing the #200
sieve) to provide stability in the aggregate surface. The presence of fines improves the layer's
compaction characteristics and helps to provide a relatively smooth surface.
(d) Base Course. The coarse-graded base course is important in providing drainage of
the granular fill. Base course should be non-frost-susceptible to retain strength during spring thaw
(e) Subbase. A well-graded subbase provides additional bearing capacity over the frost-
susceptible subgrade. It also provides a filter layer between the coarsegraded base course and the
subgrade to prevent migration of the subgrade into the voids in the coarser material during periods of
reduced subgrade strength. Therefore, the material must meet standard filter criteria. The subbase
must be either non-frost susceptible or of low frost susceptibility (Sl or S2), The filter layer may or may
not be necessary depending upon the type of subgrade material. If the subgrade consists principally of
gravel or sand, the filter layer may not be necessary, and may be replaced by additional base course if
the gradation of the base course meets filter criteria. For finer grained soils, the filter layer will be
necessary. If using a geotextile, the sand subbase/filter layer may be omitted, as the fabric will be
placed directly on the subgrade and acts as a filter.
(f) Compaction. The subgrade should be compacted to provide uniformity of conditions
and a working platform for placement and compaction of subbase. Compaction will not change a
subgrade's frost-area soil support index. However, because frost weakens the subgrade, compacted