30 June 2001
c. Soils With Expansive Characteristics. Expansive soils are generally those with a liquid limit
more than 40 and a plasticity index more than 15. Soils with expansive characteristics give the most
trouble when significant changes occur in moisture content of the subgrade during different seasons of
the year. TM 5-818-7 may be helpful in identifying expansive soils.
7. SOIL COMPACTION TESTS. Soil compaction tests will be used to determine the compaction
characteristics of soils. The degree of compaction required is expressed as a percentage of the
maximum density obtained by the test procedure used. Table 5-1 shows test methods to be used for
determining density. The laboratory compaction control tests should not be used on soil that contains
particles easily broken under the blow of the hammer. Also, the unit weight of certain types of sands and
gravels obtained by this method is sometimes lower than the unit weight that can be obtained by field
methods. Density tests in these cases should be made under some variations of the test methods, such
as vibration or tamping (alone or in combination) to obtain higher laboratory density. In some cases, it
may be necessary to construct field test sections to establish compaction characteristics.
8. SOIL STRENGTH. Soil strength is measured by the CBR for use in designing flexible pavements
and by the modulus of soil reaction (k) for the design of rigid pavements. Strength tests must be made
on material that represents the field condition that will be most critical from a design standpoint. Details
of the CBR test procedure are given in CRD-C 654 and details of the modulus of soil reaction test are
given in CRD-C 655. Figure 5-3 shows approximate relationships between soil classifications and soil
strength values. The relationships will not be used for design of pavements. They are given for
checking and estimating, not as a substitute for testing. Guidance in determining soil strength values are
presented in Chapters 6 through 8.
9. IN-PLACE SOIL STRENGTH TESTS. Test pits for in-place soil strength tests and associated
moisture-density tests should be located at approximately 305-meter (1,000-foot) intervals for runways
and taxiways. For parking aprons and pads, one test pit should be located for each 16,720 square
meters (20,000 square yards). The number and spacing of test pits may be modified whenever
variations in soil conditions or unusual features are encountered.