TM 5-822-5/AFM 88-7, Chap. 1
and the completed fill section shall be subjected to
material. This may be done by raising the grade by
the subgrade preparation procedures required for
an amount equal to the depth of subgrade prep-
cut sections. In cut sections the subgrade shall be
aration that otherwise would be prescribed, or by
scarified and excavated to a prescribed depth, and
undercutting and replacing the wet fine-grained
the excavated material shall be windrowed and
subgrade to that same depth. In either case the fill
bladed successively until thoroughly blended, then
or backfill material may be nonfrost-susceptible
relaid and compacted. The depth of subgrade prep-
material or frost-susceptible material meeting
aration, measured downward from the top of the
specified requirements. If the fill or backfill material
subgrade, shall be the lesser of 24 inches; two-
is frost susceptible, it should be subjected to the
thirds of the frost penetration for class A, B, and C
same subgrade preparation procedures prescribed
roads, streets, and open storage areas or one-half of
above.
c. Cobbles or boulders. A critical condition re-
the frost penetration for roads, streets, and open
storage areas of class D, E, and F less the actual
quiring the attention of inspection personnel is the
combined thickness of pavement, base course, and
presence of cobbles or boulders in the subgrades.
subbase course. The prepared subgrade must meet
All stones larger than about 6 inches in diameter
the designated compaction requirements for non-
should be removed from fill materials for the full
frost areas. The construction inspection personnel
depth of frost penetration, either at the source or as
should be alert to verify that the processing of the
the material is spread in the embankments. Any such
subgrade will yield uniform soil conditions
large stones exposed during the sub-grade
throughout the section. To achieve uniformity in
preparation work also must be removed, down to
some cases, it will be necessary to remove highly
the full depth to which subgrade preparation is
frost susceptible soils or soils of low frost suscepti-
required. Failure to remove stones or large roots
bility. In that case the pockets of soil to be removed
can result in increasingly severe pavement
should be excavated to the full depth of frost
roughness as the stones or roots are heaved gradu-
penetration and replaced with material surrounding
ally upward toward the pavement surface. They
the frost-susceptible soil being removed.
eventually break through the surface in extreme
a. Exceptions conditions. Exceptions to the
cases, necessitating complete reconstruction.
d. Changes in soil conditions. Abrupt changes in
basic requirement for subgrade preparation are sub-
grades known to be nonfrost susceptible to the
soil conditions must not be permitted. Where the
depth prescribed for subgrade preparation and
subgrade changes from a cut to a fill section, a
known to contain no frost-susceptible layers or
wedge of subgrade soil in the cut section with the
lenses, as demonstrated and verified by extensive
dimensions shown in figure 18-6 should be removed
and thorough subsurface investigations and by the
and replaced with fill material. Tapered transitions
performance of nearby existing pavements. Also,
also are needed at culverts beneath paved areas, but
fine-grained subgrades containing moisture well in
in such cases the transition material should be clean,
excess of the optimum for compaction, with no fea-
nonfrost-susceptible granular fill. Other under-
sible means of drainage nor of otherwise reducing
pavement pipes should be similarly treated, and
the moisture content, and which consequently it is
perforated-pipe underdrains should be constructed.
not feasible to scarify and recompact, are also ex-
These and any other discontinuities in subgrade
ceptions.
conditions require the most careful attention of
b. Treatment of wet fine-grained subgrades. If
construction inspection personnel, as failure to
wet fine-grained subgrades exist at the site, it will
enforce strict compliance with the requirements for
be necessary to achieve frost protection with fill
transitions may result in serious pavement distress.
18-17
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