Lesson 5/Learning Event 3
TEST PROGRAM FOR FREE-DRAINING SOILS
Determination of a design CBR for this group of soils requires the least testing of the three molded
laboratory test programs. Table 5 gives the USCS classification and the uses of the soils that fall into
this group. The ease in testing is due to the free-draining characteristics or lack of fines in the soil. For
such soils, field placement is relatively easy. To insure maximum dry density for any level of
compactive effort, the only necessary control measure is to have more water available than that required
for the maximum dry density at the appropriate level of compactive effort. From the corrected CBR
versus molding water content curves, the same pattern can be seen in relation to moisture content as to
dry density. Soils placed wetter than the limiting moisture content achieve the maximum CBR possible
for that level of compactive effort. In other words, moisture contents of loose soils above the limiting
values have little bearing on the strength of a soil after compaction. This makes laboratory testing, field
placement, and field control a relatively easy matter. To illustrate what must be done to arrive at a
design CBR, the steps as outlined in the previous examples will again be followed.
Establish the OMC of the soil at 55 blows/layer. The OMC for this example is the limiting moisture
content or 8 percent. To insure that a free-draining soil is being tested, this curve should display a
maximum dry density at a limiting moisture content, and will display a concave upward shape.
Compact samples at different levels of compactive effort. Compaction curves must be made for three
levels of compactive effort up to the limiting moisture contents. As these soils frequently have CBR
values greater than 20, the placement compactive effort is in excess of 100 percent of maximum CE 55
dry density. Therefore, 25, 55, and 72 blows/layer are usually used.
Soak samples and measure swell. Swell measurements are not required. Eliminate soaking when it
is determined that saturation does not affect strength.
Perform CBR penetration tests. Only the samples at the limiting moisture contents for each level of
compactive effort need be tested. Normally more than one sample at the limiting conditions will be
made for each level of compactive effort and all should be tested.
Plot the data on graphs of dry density versus molding moisture content and corrected CBR versus
molding moisture content. Only the plot of dry density versus molding water is required. The corrected
CBR versus molding water content graph (Figure 40) is presented only for discussion.