Lesson 5/Learning Event 3
Establish a moisture range for CBR investigation. OCE Military Standard (2) states that the
range is OMC + 4 percent. This is a time-saving guide as experience has shown that the maximum CBR
will normally occur at compaction moisture contents within this range and that testing soils behind these
limits is wasted effort. For this example, the moisture content range of investigation is 10 to 18 percent.
Compact samples within the moisture content range of investigation at different level of
compactive effort. You do this to allow for evaluation of soil strength when field placement is at
something other than 100 percent CE 55. The levels of compactive effort to be selected in the
laboratory are dependent upon the amount of compactive effort stipulated by the compaction
requirements (Table 2). If the soil is expected to be used as compacted subgrade or select material, it
will be placed at less than 100 percent CE 55. In this case you select laboratory compactive efforts of
12, 26, and 55 blows per layer.
If the soil is a very high quality subgrade (CBR > 20) or a subbase, the laboratory tests should include
samples compacted in excess of 55 blows per layer.* Normally 26, 55, and 72 blows per layer are
adequate. The only stipulation placed on the levels of compactive effort to be used is that a 55-blow-
per-layer compaction curve be obtained, and that data is developed at two other levels of compactive
effort encompassing the specified field placement densities. In this example, the soil is a CL (cohesive)
subgrade, so samples were compacted at 12, 16, and 55 blows per layer.
*NOTE: Table 2-4, TM 5-330, is used to estimate whether or not CBR is greater or less
than 20.
Soak samples and measure swell. The initial assumption from Table 9 was that this is a non-
swelling soil. It is emphasized that this categorization based upon USCS classification is only an
approximation. The swell must be monitored to verify the fact that the soil being tested is in fact non-
swelling.
Perform CBR penetration tests and determine corrected CBR for each sample. Accumulation of the
required data involves a considerable amount of work. At a minimum, 15 molds (five per level of
compactive effort) must be made. In this example 21 molds were compacted, the results of which are
given in the DATA SUMMARY on the next page.
Plot the data on graphs of dry density versus molding moisture content and corrected CBR versus
molding moisture content. The example is plotted on the next page.
Reformat data on CBR family of curves data sheet. This is done merely to estimate values of dry
density and CBR for whole integer moisture contents within the range of investigation at each level of
compactive effort.
Plot the CBR family of curves. The object of this step is to place the laboratory data in a form which
lends itself to analysis. You are establishing the trends of strength variation as the moisture content and
dry density change. The blank places between actual data points are filled in. Drawing a CBR family of
curves involves considerable practice, numerous attempts,
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