California Bearing Ratio Test - Soil Properties: Testing, Measurement, and Evaluation - page 397

(1.2) Where the CBR is desired at optimum water content and some
percentage of maximum dry unit weight, compact three specimens
from soil prepared to within ±0.5 percentage point of optimum water
content and using the specified compaction but using a different
number of blows per layer for each specimen. The number of blows
per layer shall be varied as necessary to prepare specimens having
unit weights above and below the desired value. Typically, if the CBR
for soil at 95% of maximum dry unit weight is desired, specimens
compacted using 56, 25, and 10 blows per layer are satisfactory.
Penetration shall be performed on each of these specimens.
(2) Bearing Ratio for a Range of Water Content —Prepare speci-
mens in a manner similar to that described in (1) except that each
specimen used to develop the compaction curve shall be penetrated.
In addition, the complete water content-unit weight relation for
the 25-blow and 10-blow per layer compactions shall be developed,
and each test specimen compacted shall be penetrated. Perform all
compaction in the CBR mold. In cases where the specified unit
weight is at or near 100% maximum dry unit weight, it will be
necessary to include a compactive effort greater than 56 blows
per layer (Note 3).
Note 3 —A semilog plot of dry unit weight versus compactive
effort usually gives a straight-line relation when compactive
effort in ft-lb/ft 3 is plotted on the log scale. This type of plot is
useful in establishing the compactive effort and number of
blows per layer needed to bracket the specified dry unit weight
and water content range.
(2.1) If the sample is to be soaked, take a representative sample of
the material, for the determination of water content, at the begin-
ning of compaction and another sample of the remaining material
after compaction. Use ASTM Method D 2216 to determine the mois-
ture content. If the sample is not to be soaked, take a moisture con-
tent sample in accordance with ASTM Test Methods D 698 or
D 1557 if the average moisture content is desired.
(2.2) Clamp the mold (with extension collar attached) to the base
plate with the hole for the extraction handle facing down. Insert the
spacer disk over the base plate and place a disk of filter paper on
top of the spacer disk. Compact the soil-water mixture into the
mold in accordance with (1), (1.1), or (1.2).
(2.3) Remove the extension collar and carefully trim the compacted
soil even with the top of the mold by means of a straightedge. Patch
with smaller size material any holes that may have developed in
the surface by the removal of coarse material. Remove the perfo-
rated base plate and spacer disk, weigh and record the mass of the
mold plus compacted soil. Place a disk of coarse filter paper on the
perforated base plate, invert the mold and compacted soil, and
clamp the perforated base plate to the mold with compacted soil in
contact with the filter paper.