30 June 2001
a. Subgrades with CBR values above 20.
(1) Army and Air Force. One hundred percent density from ASTM D 1557 except where it is
known that a higher density can be obtained practically. Then, the higher density will be required.
(2) Navy and Marine Corps. Compact to 95 percent of ASTM D 1557 maximum density.
b. Subgrades with CBR values of 20 or less.
(1) Fills. Subgrades in fills shall have densities equal to or greater than the values determined
from Tables 6-2 through 6-7. Cohesionless fill will not be placed at less than 95 percent nor cohesive fill
at less than 90 percent of maximum density from ASTM D 1557. The top 6 inches of subgrade will be
compacted to 95 percent of maximum density from ASTM D 1557.
(2) Cuts. Subgrades in cuts shall have natural densities equal to or greater than the values
determined from Tables 6-1 through 6-6. When they do not, the subgrade shall be (a) compacted from
the surface to meet the densities required, (b) removed and replaced (then the requirements given
above for fills apply), or (c) covered with sufficient select material, subbase, and base so that the
uncompacted subgrade will be at a depth where the in-place densities are satisfactory. The top
152 millimeters (6 inches) of subgrade will be compacted to 95 percent of maximum density from
ASTM D 1557.
c. Natural Densities. The natural densities occurring in the subgrade should be compared with the
compaction requirements to determine if densification at the deeper depths under design traffic is a
problem. If such densification is likely to occur, means must be provided for compacting these layers, or
the flexible pavement structure must be established so that these layers are deep enough that they will
not be affected by aircraft traffic.
d. Compaction Levels and Moisture Content. Compaction of soils and aggregates accomplishes
two specific purposes: (1) it achieves sufficient density in each layer of material such that future traffic
will not cause additional densification and consequent rutting and (2) it achieves the designer's desired
engineering properties, normally strength used for the pavement design. The requirements for density in
Tables 6-2 through 6.6 coupled with proof rolling (paragraph 9 of Chapter 8) accomplish the first
objective. The interaction between specified compaction levels and moisture contents and design
strength is described in paragraph 3 of this chapter and Figure 6-1. Controlling field compaction of soils
and aggregates using a specified percent of a laboratory compaction value and a specific range of
allowable compaction moisture contents based on the laboratory optimum has proven simple and
effective in practice for over a half century. Compaction curves of actual rollers in the field conform to
the general shape and characteristics of the laboratory compaction curves but will deviate slightly from
the actual laboratory curve. This deviation is not generally significant. Failure to control compaction
moisture is probably one of the most common causes of failure to achieve specified density in the field.
The contractor must thoroughly mix and disperse the moisture in the soils and aggregates and must
allow for evaporation which can be significant on clear or windy days in many soils. Some soils such as
silts have very steep compaction curves requiring fairly close control of the moisture to achieve
compaction. Truly cohesionless soils compact best saturated but a relatively small increase in fines in
such materials can make them spongy and uncompactable at saturation. Experience and field
evaluation of each soil's behavior under compaction is usually needed to meet the stringent compaction
standards used in military airfield construction. It is important to meet both the minimum specified
density and to accomplish the compaction within the specified ranges of moisture content.