UFC 3-260-02
30 June 2001
CBR, and swell that will give the greatest CBR and density consistent with a tolerable amount of swell
must be selected. The CBR and density values so selected are those that must be considered in the
design of overlying layer thickness. Field control of the moisture content must be carefully exercised
because if the soil is too dry when compacted, the expansion will increase; and if it is too wet, low unit
weight will be obtained and the soil will shrink during a dry period and then expand during a wet period.
This method requires detailed testing and extensive field control of compaction.
(2) Overburden load. In order to limit swell of expansive soils, it may be desirable to provide
overburden if expansion cannot be limited by other procedures to acceptable amounts. Special swell
tests normally will be needed to determine the amount of weight (overburden) necessary to restrict the
swell to tolerable magnitudes. These tests can be variations of the standard soaked CBR test described
in CRD-C 656, or they can be specially designed tests using a consolidometer apparatus.
(3) Special solutions. 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 to maintain the swelling soil in a
saturated condition. 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. Other possible solutions are treatment with lime (TM 5-822-14/AFJMAN 32-1019), replacement
of the swelling soil, or working the soil to make it more uniform.
d. Design Considerations for Special Cases. Whenever subgrades are given special treatments
that cause their resulting strength or their resulting density to be less than when normally treated, these
lesser values must be considered in design of the overlying layers. When a low CBR results, sufficient
thickness of overlying structure must be provided to protect a subgrade of such low strength. When a
low density results, the thickness of overlying material must be such that the density versus depth
requirements of the specifications are met.
8. STABILIZED SUBGRADES. Subgrades can be stabilized by the addition of lime, cement, or a
combination of these materials with flyash. Design of pavements using stabilized soils is discussed in
Chapter 9 of this document and in TM 5-822-14/AFJMAN 32-1019. Lime should not be used with soils
containing sulfates.
9. SUBGRADES IN FROST AREAS. In areas where frost susceptible subgrade soils will be
subjected to cycles of freeze-thaw, pavements must be designed in accordance with the requirements of
Chapter 20.
6-13
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