30 June 2001
subgrade strength method is used with an F3 or F4 subgrade soil, rigorous control of subgrade
preparation is required. In situations, based on previous experience, where use of the reduced
subgrade strength procedure has resulted in pavement thicknesses allowing objectionable frost
heave, but use of the greater base-course thickness obtained from the limited subgrade frost
penetration method is considered over conservative, intermediate design base-course thickness
may be used. However, these must be justified on the basis of frost heaving experience developed
from existing pavements where climatic and soil conditions are comparable.
a. Thickness of Flexible Pavements. The thickness design procedure is identical to the
thickness design for nonfrost conditions, with the exception that instead of using the subgrade
CBR, Frost Areas Soil Support Index (FASSI) values are used. The flexible pavement design curves
are used in connection with the reduced subgrade strength procedure. In place of the estimated or
determined subgrade CBR, use the applicable FASSI values outlined in Table 20-3 with the design
curves. The FASSI values for the F1 to F4 subgrade soils were backcalculated from performance
data of in-service pavements, and are the weighted average CBR valued for an annual cycle. These
values cannot be determined by CBR tests. The FASSI values for S1 and S2 materials meeting
current specifications for base and subbase will be determined by conventional CBR procedures.
The reduced subgrade strength design procedure is included in the design computer program PDSF
discussed in Chapter 1.
Frost Area Soil Support Indexes (FASSI) for Subgrade Soils
Frost Group of Subgrade Soil
F1 and S1
F2 and S2
F3 and F4
Once the overall thickness of the pavement structure has been determined, criteria for nonfrost
design should be used to determine the thickness of individual layers. It should also be ascertained
whether it will be advantageous to incorporate bound base layer(s) in the system. Although the
use of bound bases will reduce the thickness of the base and subbase layers, it is possible that
deeper frost penetration may occur leading to increased frost heave. The base-course requirements
set forth in this chapter must be followed rigorously.
(1) Design of overrun pavements. The runway overrun pavement thicknesses for providing
adequate strength during frost melting periods are determined from the appropriate flexible
pavement design curves and the applicable FASSI values outlined in Table 20-3. The thickness
established by this procedure shall have the following limitations:
(a) It shall not be less than required for nonfrost condition design.
(b) It shall not exceed the thickness required under the limited subgrade frost
penetration design method.