UFC 3-260-02
30 June 2001
Table 20-2
Frost Susceptibility Classification
Heave Rate
Frost Susceptibility
(mm/day)
Thawed CBR
Classification
Frost Group
<1
>20
Negligible
NFS, PFS
<2
>15
Very Low
S1, PFS
<4
>12
Very Low
S2, PFS
<6
>10
Low
F1
<8
>6
Medium
F2
<16
>3
High
F3
No Limit
<3
Very High
F4
It will not be used to design pavements to serve conventional traffic, except when approved by
appropriate written waiver.
b. Reduced Subgrade Strength Method. The reduced subgrade strength method does not seek
to limit the penetration of frost into the subgrade. It determines the thickness of pavement, base,
and subbase that will adequately carry traffic. This approach relies on uniform subgrade
conditions, adequate subgrade preparation, and transitions for adequate control of pavement
roughness resulting from differential frost heave.
c. Limited Subgrade Frost Penetration Method. The limited subgrade frost penetration method
requires a sufficient thickness of pavement, base, and subbase to limit the penetration of frost into
the frost susceptible subgrade.
4. SELECTION OF DESIGN METHOD. In most cases the choice of the pavement design method
will be made in favor of the one that gives the lower cost. The limited subgrade frost penetration
method will be used, even at higher costs, in areas where the subgrade soils are extremely variable
(e.g., in some glaciated areas) and the required subgrade preparation could not be expected to
sufficiently restrict differential frost heave, and when special operational demands on the pavement
might dictate unusually severe restrictions on pavement roughness, requiring that subgrade frost
penetration be severely restricted or even prevented. If the use of limited subgrade frost
penetration method is not required, tentative designs must be prepared using both methods for
comparison of costs.
5. REDUCED SUBGRADE STRENGTH METHOD. The thickness design is based on the seasonally
varying subgrade bearing capacity that includes sharply reduced values during frost melting periods.
This design procedure usually requires less thickness of pavement and base than that needed for
limited subgrade frost penetration. The method may be used for pavements where the subgrade is
reasonably uniform or can be made reasonably uniform horizontally by the required subgrade
preparation techniques discussed later in this chapter. This will prevent or minimize significant or
objectionable differential heaving and resultant cracking of pavements. When the reduced
20-3
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