UFC 3-260-03

15 Apr 01

Mean freezing length = 129 days

Mean start of freezing occurs on November 11.

Mean end of freezing occurs on March 19

b. Determine the Depth of Frost Penetration. For complete frost protection the depth of frost pene-

tration (d) estimated from MODBERG program is 1,422 mm (56 inches).

c. Determine if the Base/Subbase and/or Subgrade is Frost-Susceptible. From table 7-1, the sub-

grade is classified as a F3 frost-susceptible soil.

d. Evaluate for Complete Frost Penetration. With a 127-millimeter- (5-inch-) thick pavement, the

thickness of base course (c) for zero penetration of the subgrade is 1,422 - 127 = 1,295 millimeters

(56 - 5 = 51 inches). The thickness of the pavement, base, and subbase layer (x) is 660 millimeters

(26 inches). Since x # c, then the pavement structure was not designed for complete frost protection.

e. Evaluate for Limited Subgrade Frost Penetration. The ratio of subgrade to base-course water

content r = 24/3 = 8. From figure 7-5, using the maximum permissible ratio r of 2.0 applicable to traffic

area A, the required total base thickness b that would hold subgrade frost penetration within the allowable

limit is 813 millimeters (32 inches). In this case, the 660-millimeter- (26-inch-) thick section of pavement

and base does not provide adequate protection against frost action, and evaluation for frost and nonfrost

conditions are required.

f.

Evaluate for Reduced Subgrade Strength.

(1) Determine allowable load and PCN during thaw-weakened period. The soil is classified as

a F3 frost-susceptible soil. From table 7-3, the FASSI value is 3.5. Use either figure 5-27 and the

procedure outlined in chapter 5 or the computer program Airfield Pavement Evaluation (APE) to determine

the allowable load for the traffic area during the thaw-weakened period using the FASSI value of 3.5. The

allowable loads for pass intensity levels I to IV and the respective PCN are tabulated below.

I

15,000

97,296

19/F/D/W/T

(214,500)

II

3,000

114,623

27/F/D/W/T

(252,700)

III

500

146,692

40/F/D/W/T

(323,400)

IV

100

201,667

64/F/D/W/T

(444,600)

(2) Determine the Period of Thaw Weakening. From WORLDINDEX, the design beginning of

freezing is Nov 11th and ends in March 19th. The average length of the freezing season is 129 days. The

thaw weakened period after the end of winter from table 7-4 is estimated to be 2 months. Since the

airfield is located in an area with a DFI of 2903 EF days, an additional 1 month (to cover for intermediate

thaw periods during the freezing period) is added to the 2 months to obtain the total weakening period of 3

months. The mean thaw weakened period is from Feb 17th to May 18th.

7-11