30 June 2001
depth of frost penetration and replaced with material similar to the material left in place. This
replacement should be completed before any required mixing and blending of the subgrade. This
will minimize the potential for large variations in frost heave and subgrade support. In fill sections,
the least frost susceptible soils shall be placed in the upper portion of the subgrade by temporarily
stockpiling, cross hauling, and selective grading. If the upper layers of fill contain frost susceptible
soils, the completed fill section shall be subjected to the subgrade preparation procedures, outlined
below for cut sections. In cut sections, no matter the type of frost susceptible subgrade soil, the
subgrade shall be scarified and excavated to a prescribed depth, and the excavated material
windrowed and bladed successively until thoroughly blended, and relaid and compacted.
Alternatively, a soil mixing and pulverizing machine may be used to blend the material in place.
Multiple passes of the machine will be required for proper blending.
a. Depth of Subgrade Preparation. The depth of subgrade preparation is applicable for limited
subgrade penetration and reduced subgrade strength design. The depth of subgrade preparation
measured downward from the top of the subgrade shall be lesser of:
(1) 0.6 meter (24 inches).
(2) Two-thirds of the frost penetration less the actual combined thickness of pavement,
base course, drainage layers, and subbase course under types A, B, or primary traffic areas.
(3) Under type C, D, and secondary traffic areas and under overruns and shoulder
pavements, it will be one-half the frost penetration less the actual combined thickness of
pavement, base course, drainage layers, and subbase course.
(4) 1.8 meters (72 inches) less the actual combined thickness of pavement, base course,
drainage layers, and subbase course.
The prepared subgrade must meet the designated compaction requirements for nonfrost areas
discussed in Chapter 6. The construction inspection personnel should be alert to verify that the
processing of the subgrade will yield uniform soil conditions throughout the section.
b. Exceptional Conditions. An exception to the basic requirements for subgrade preparation
are subgrades that are nonfrost susceptible or of very low frost susceptibility (NFS, S1, S2) to the
depth prescribed for subgrade preparation. These subgrades contain no frost susceptible layers or
lenses, as demonstrated and verified by extensive and thorough subsurface investigations and by
the performance of nearby existing pavements. Also, fine-grained subgrades containing moisture
well in excess of the optimum for compaction, with no feasible means of drainage nor of otherwise
reducing the moisture content, and which consequently are not feasible to scarify and recompact,
are also exceptions. If a wet fine-grained subgrade exists at the site, it will be necessary to
prevent frost penetration with fill material. This may be done by raising the grade by an amount
equal to the depth of subgrade preparation that otherwise would be prescribed, or by undercutting
and replacing the wet fine-grained subgrade to that same depth. In either case, the fill or backfill
material may be nonfrost susceptible or frost susceptible material. If the fill or backfill is frost
susceptible, it should be subjected to the same subgrade preparation procedures prescribed above.
c. Cobbles or Boulders. A critical condition requiring the attention of designers and inspection
personnel is the presence of cobbles or boulders in the subgrade. All stones larger than about
150 millimeters (6 inches) in diameter should be removed from fill materials for the full depth of
frost penetration, either at the source or as the material is spread in the embankment. Any such