UFC 3-260-03
15 Apr 01
given in table 7-4. These reduction factors are to be used as guides. If subgrade modulus values are
available for the thaw period, these values will be used.
Thaw Modulus ' RF ( Nonfrost Period Modulus
Table 7-4
Modulus Reduction Factors for use in Seasonal Frost Areas
Modulus Reduction Factors
Frost Group
(RF)
NFS
1.00
PFS
0.90
S1
0.75
S2
0.70
F1
0.60
F2
0.50
F3/F4
0.30
15. EVALUATION METHODOLOGY. The evaluation methodology requires the determination of allow-
able loads, allowable number of passes and PCN's to be reported for both thaw-weakened and normal
periods. Using this dual reporting system, PCN's are reported for both the thaw-weakened and normal
periods. The procedure utilizes the FASSI/FAIR or reduced modulus values for layer strengths during the
thaw-weakened condition and measured material strengths during the normal period. Material properties
for the normal period must be determined when the pavement has fully recovered from a thaw-weakened
condition. Strengths of the pavement materials may be based on direct sampling or nondestructive
testing. The evaluations are made for pass intensity levels I and II for Air Force pavements. The PCN is
determined for 50,000 passes of a C-17 for Air Force pavements and for either a C-130 or C-141 for Army
Airfields. Substantial pavement overloads may be allowed during the period that the pavement is solidly
frozen. The amount of overload and the period that the overload may be applied must be obtained from
Headquarters, U.S. Army Corps of Engineers (HQUSACE) (CEMP-ET), the appropriate Air Force Major
Command, or the appropriate NAVFAC Headquarters.
a. Evaluation Periods. The duration of the period of weakening and the normal period must be
determined and included in the evaluation report. The beginning and ending dates for each of the two
periods must also be included. Since a number of frost-melting periods may occur during a typical winter
period, it is essential that all periods of thaw weakening be included in the computation of the total period
of weakening. The time required for strength recovery following a thaw will vary depending on local condi-
tions. Principal factors affecting the recovery time are depth of frost penetration, type of frost-susceptible
material, and subsurface drainage. Normally, the time for recovery will be from several weeks to several
months. The thaw-weakened periods for different frost-susceptible soils are presented in table 7-4. This
table is to be used as a guide; the length of the thaw-weakened period can be changed based on local
experience. The total period of weakening must also include frost-melting periods during the winter; the
following will be used to establish those periods:
7-9
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