Lesson 3/Learning Event 4
Learning Event 4
The subgrade must be compacted for the same frost conditions as for temperate conditions.
There are compaction requirements for airfields and highways. The strength versus settlement criteria
should be compared for fill and then cut sections. The compaction design is then superimposed onto the
thickness design for fill and cut sections. Before compaction requirements are considered all of the
design methods are carried out.
Superimpose Compaction Requirements
After you have arrived at a design for frost action conditions and determined compaction requirements,
you must superimpose compaction onto the thickness for final design.
Compaction for frost action conditions must be incorporated into the regular temperate climate design
for final design. It is only after a method has been selected and used for design that compaction
requirements are superimposed to arrive at a final design (Figure 24).
Examples of Flexible Pavement Design
Example. Design a minimum operational flexible pavement for a C-121G cargo aircraft having a
design load of 69,400 pounds, twin tricycle gear configuration, and a tire contact area of 262 square
inches for the following conditions:
Subgrade. Uniform clay (CL), plasticity index 18, CBR
= 8 (normal period)
and an average water
content of 25 percent. It is assumed that the subgrade soil produces uniform heave.
Base CBR = 80. The highest ground water table is recorded to be 4 feet below the surface of the
natural subgrade. Soil type is GW, well-graded gravel.
Reduced subgrade strength design (Figure 23). The subgrade soil is a CL having uniform heave
characteristics and a plasticity index of 18, classifying the soil group F3. Using Figure 13-19 and a
design load of 69,400 pounds, it is found that 44 inches combined thickness of pavement and nonfrost-
susceptible base and subbase course is required for the design aircraft over the weakened subgrade soil.
A 4-inch filter blanket must be placed as an integral part of the base or subbase course, at the transition
between the frost-susceptible subgrade soil and the non-frost-susceptible base or subbase soil. Use a 10
percent reduction in traffic area "C."
Final pavement design. The frost design and the "temperate climate design" is then combined as
shown in Figure 24.