Lesson 1/Learning Event 2
Treatment of soils with expansive characteristics. Soils with expansive characteristics occur and give
the most trouble in certain areas of the world where climatic conditions are conducive to significant
changes in moisture content of the subgrade during different seasons of the year. Such soils can also
give trouble in any region where construction is accomplished in a dry season and the soils absorb
moisture during a subsequent wet season. If highly compacted, these soils will swell and produce uplift
pressures of considerable intensity if the moisture content of the soil increases after compaction. This
action may result in differential heaving of flexible pavements that is unacceptable. Where the amount
of swell is less than about 3 percent, special considerations will not normally be needed.
A common method of treating a subgrade with expansive characteristics is to compact it as a moisture
content and to a unit weight that will minimize expansion. The proper moisture content and unit weight
for compaction control of a soil with marked expansion characteristics are not necessarily the optimum
moisture content and unit weight determined by the modified AASHO (CE 55) test, but may be
determined from a study of the relationships between moisture content, unit weight, percentage of swell,
and CBR for a given soil, as determined by the CBR test. A combination of moisture, density, CBR,
and swell which will give the greatest CBR and density consistent with an acceptable amount of swell
must be selected. The CBR and density values so selected are those which must be considered in the
design of overlying layer thickness.
Field control of the moisture content must be carefully exercised since if the soil is too dry when
compacted, the expansion will increase; and if too wet, low unit weight will be obtained and the soil will
shrink during a dry period and then expand during a wet period. Special solutions to the problem of
swelling soils are sometimes possible and should not be overlooked where pertinent. For instance,
where climate is suitable, it may be possible to place a permeable layer (aquifer) over a swelling soil and
limit or prevent drainage therefrom. Moisture buildup in this layer maintains the soil in a stable, swelled
condition. Designs must, of course, be based on the swelled CBR and density values of such a material
when so treated.
Selection of Subgrade and Subsoil Design CBR Values
The CBR test described in TM 5-530 and Engineer Subcourse 5453, Soils Engineering, includes
procedures for making tests on samples compacted in test molds to the design density and soaked 4
days, for making in-place CBR tests, and for making tests on undisturbed samples. These tests are used
to estimate the CBR that will develop in the prototype structure. Where the design CBR is above 20, the
subgrade must meet the requirements for subbases.