Lesson 5/Learning Event 3
minimum strength that can be counted on for the specified density range. We followed this procedure
for the remaining moisture contents, using the Family of Curves, and tabulated the results as follows:
w(moisture)
CBR
10
8.5
11
9.2
12
10.2
13
11.3
14
13.6
15
10.3
16
7.4
17
5.4
18
4.2
Determine the CBR value for potential moisture content specification ranges. As with the density
range specified in Step 9, it is desirable to have a moisture content range which can be economically
achieved in the field. Within the overall range of investigation (OMC +4 percent, a smaller specification
range which gives the greatest assured CBR will be determined. Experience has shown at a 4 percent
range (i.e., some value +2 percent) is a reasonable requirement, but not an absolute rule. The engineer
may specify a smaller range to achieve a larger design CBR and a reduction in pavement thickness
requirements, but this savings in pavement materials may be offset by increased costs associated with
difficulties in meeting the more stringent requirements. Conversely, for some soils an expanded
moisture content specification may have little effect on the design CBR. For the example problem a 4
percent specification range will be considered. One possible range is 10 to 14 percent. If the engineer
specified that the soil be placed within these limits, the worst possible strength that would be realized is
a CBR of 8.5. Such an analysis can be done for the other possible 4 percent ranges, and the result is as
follows:
w(range)
CBR
10 - 14
8.5
11 - 15
9.2
12- 16
7.4
13 - 17
5.4
14 - 18
4.2
Select the moisture content range which gives the greatest design CBR. In steps 10 and 11, CBR
values were selected based upon the premise that the engineer is going to allow the builder to place the
soil anywhere between potential moisture content and density specification limits. Now it is time for the
engineer to select the set of limits he desires. From the tabulation in Step 11, it can be seen that the soil,
if placed between 11 and 15 percent moisture, will give the largest of the possible CBR values. Thus 9.2
becomes the design CBR, and the field quality control measures to be used to insure that a CBR = 9.2 is
achieved are:
Compaction from 93 to 98 percent of maximum
CE 55 dry density of 120.9 to 127.4 pcf.
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