Lesson 5/Learning Event 3
SUMMARY
This technique for determining a design CBR provides for a strength measure of at least 9.2 when the
associated density and moisture content ranges are adhered to. Greater strengths will be realized within
the specified limits, but the value obtained allows the engineer to size the structure for the worst
condition. It should be noted that for this soil and the size of the limits used, the greatest assured
strength did not occur for the 4 percent moisture content range centered on OMC. This is a phenomenon
which has been found to occur for many soils having appreciable clay contents. Also of importance is
the fact that the analysis was based upon an initial selection of density limits that meet the minimums
given in Table 6 to see if an adjustment to these limits will yield greater strengths.
TEST PROGRAM FOR SWELLING OR EXPANSIVE SOILS
There is a small group of soils which can display the characteristics of objectionable volumetric
expansion after being compacted and subjected to subsequent saturation. This presents a problem in
pavement design as this expansion or swell can damage the structure by means of a "reverse-settlement"
mechanism. The measure of swell used by the Corps of Engineers is termed the "free swell index"
which is expressed as a percentage of the initial sample height. Objectionable swell is defined as that in
excess of 3 percent. Experience has shown that the plasticity index (PI) is an excellent indicator of
expansive soils. The following guidelines have been suggested:
Swell Potential
PI 0-14
Not Expansive
PI 14-25
Marginal
PI 25-40
Critical
PI 40
Highly Critical
Although a high PI does not guarantee that the soil is expansive, critical soils should be checked more
closely for swell tendencies.
The potentially expansive soils by USCS classification are CHs, MHs, and OHs. The test procedure for
determining a design CBR for a expansive soil is similar to that previously discussed for the non-
swelling soil, but the objective is not exactly the same. For non-swelling soils, the object is to find the
greatest assured CBR value for some range of densities and moisture contents, whereas the objective of
the test program for the expansive soils is to find the moisture content ranges which will preclude
objectionable swell and provide the highest soaked CBR. Generally, the minimum swell and the highest
soaked CBR will occur at a molding moisture content of OMC. Follow along the example of such a test
program for a CH soil taken from the subgrade at the proposed "Airfield Delta." As the majority of the
steps are similar to those developed for the case of non-swelling soils, only differences will be
discussed.
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