Lesson 5/Learning Event 3
The work done to this point is the job of the laboratory technician. However, it is the responsibility of
the engineer to insure that the CBR data is properly obtained and presented. More importantly, analysis
of the data with an understanding of how the result affects the design and impacts on economic
considerations is the key responsibility of the engineer.
For the purposes of this subcourse, it is essential that the engineer realize that soil placement at OMC
and the most compactive effort possible are not always the answers to good construction.
Establish a density range at which soil will be placed in the field. Minimum limits for field
compaction requirements are based on settlement criteria and are given in Table 6. To facilitate
construction, specify a reasonable range of densities which can be economically obtained and then
examine the strength values which would occur within that range. Establishment of the density range
depends to a large degree on economics. The greater the latitude given the builder, the better are the
changes of the soil being placed within established limits. However, if an extreme range is stipulated,
the CBR value on the Family of Curves Graph (Figure 37) allowed for design might be reduced and
thicker pavement structure could be required. Experience has shown that a 5 percent range (i.e., 90 to
95 percent of maximum CE 55 dry density) is reasonable.
For this example, the soil according to Table 6 must be placed at least 90 percent of maximum CE 55
dry density. It will be assumed that experience with the soil shows that 93 to 98 percent of maximum
CE 55 dry density can be achieved with reasonable effort. This means that the limits of densities will
0.93 x 130 = 120.9 pcf
0.98 x 130 = 127.4 pcf
The limits can be imposed on the CBR family of curves by drawing two vertical lines at 120.9 and 127.4
Determine the assured CBR values between the specific density limits, using the Family of Curves.
If the builder is going to be allowed to place the soil between 120.9 and 127.4 pcf, this step involves
determining the CBR values obtained for each moisture content. By observing the change in CBR for
any specific moisture content line between the two density limits, it can be seen that a range of strengths
is possible. Since the builder will be allowed to place the soil anywhere between the established density
limits, the CBR value selected as a potential density strength should represent the worst case. Using 10
percent moisture as an example, the CBR at 120.9 pcf is 8.5. At 127.4 pcf the CBR is a maximum of
11.3. Of the two, 8.5 is the