Lesson 5/Learning Event 3
Learning Event 3
IDENTIFY CBR STEPS FOR NON-SWELLING, SWELLING, AND FREE-DRAINING SOILS
It is important to remember that CBR values are most useful when the specimens tested duplicate the
density and moisture content expected in the field. In general, the most critical condition for most
materials exists when the maximum amount of water has been absorbed. For this reason, the CBR test is
made on specimens which have soaked for four days and were confined in molds under a surcharge
equal to the weight of the planned base and pavement.
SAMPLE TEST PROGRAM FOR NON-SWELLING SOILS
This test program is applicable to the majority of soils used in construction. As Table 9 indicates, soils
which fall into this grouping might be used as compacted subgrade, select, or subbase materials
depending upon their strengths and location in regard to the construction site.
To illustrate the methodology of evaluating the design CBR, the data given on the CBR analysis sheet
for "Hope Road" will be used. We took this data from the subgrade along a proposed road alignment.
The object of the following analysis is to determine a soil placement moisture content range for a
specified level of compactive effort which gives the greatest assured design CBR. To develop this data,
the soils analyst must accomplish the following:
Establish the OMC of the soil at 55 blows/layer. This is done using the standard CE 55
compaction test. For example, OMC = 14 percent. (Remember, the OMC is the peak of curve moisture
content for that soil. Thus, a number of tests are done and a compaction curve is developed. From the
curve one can find the OMC value for that soil.) See "Molding Water Content," Figure 36.
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