Lesson 5/Learning Event 3
be set at 95 percent of maximum CE 55 dry density. The actual densities are:
110 x .90 = 99 pcf
110 x .95 = 104.5 pcf
As in the previous example, the above limits are shown by two vertical lines on the CBR family of
curves (see Figure 39).
Determine the assured CBR values between the specified density limits. As more than 3 percent
swell is not acceptable, evaluation of the CBR values at moisture contents less than 14 percent is
needless. The CBR values for the applicable moisture contents are as follows:
w
CBR
14
3.4
15
4.2
16
3.2
17
2.2
18
1.3
19
.9
20
.4
Determine the CBR values for potential moisture content specification ranges.
w(range)
CBR
14 - 18
1.3
15 - 19
.9
16 -20
.4
Select the moisture content range which gives the greatest design CBR. From the step above, it can
be seen that the 14 to 18 percent range gives largest CBR. Thus the design CBR is 1.3, an this value is
obtained when the soil is placed at:
w = 14 - 18 percent
d = 99.0 - 104.5 pcf
As an engineer the first option you must consider when you encounter an expansive soil is going some
place else to build the structure. When moving the site is not feasible, this technique allows for
determination of a design CBR at which swell is not excessive. It is emphasized that obtaining this
design value was done at the expense of strength. This technique does not provide for the advent of the
soil being dried to a moisture content less than the amount at placement. Should such extreme drying
take place, expect shrinkage and pavement failure. However, as an engineer, you have to your
advantage the fact that such soils will normally be protected from drying by the overlying pavement.
Another alternative to building on expansive soils is chemical stabilization. For example, addition of
small amounts of lime considerably reduces the potential for shrinkage and swell.
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