California Bearing Ratio Test - Soil Properties: Testing, Measurement, and Evaluation - page 394

The CBR may be expressed in equation form as

penetration stress psi required to penetrate 0 . 10 in .

1,000 psi

1 2

CBR

100

(24-1)

Note —The 1,000 psi in the denominator is the standard penetration
stress for 0.10-in. penetration.
On occasion, the bearing ratio based on a penetration stress re-
quired to penetrate 0.20 in. with a corresponding standard penetration
stress of 1,500 psi may be greater than the one for a 0.10-in. penetra-
tion. When this occurs, the test should be run again, and if the retest re-
sult is similar, the ratio based on the 0.20-in. penetration should be
reported as the CBR.

CBR test apparatus: compaction mold (6-in. diameter and 7-in.
height), collar, spacer disk (5 15 ⁄ 16 -in. diameter and 2.416-in. height),
adjustable stem and perforated plate, weights, penetration piston
(3 in. 2 in area) (for more details, including additional dimensions,
weights, etc., see Figure 24-1)
Loading (compression) machine: with load capacity of at least
10,000 lb and penetration rate of 0.05 in./min (see Figure 24-2)
Expansion measuring apparatus
Two dial gages (with accuracy to 0.001 in.)
Standard compaction hammer
Miscellaneous equipment: mixing bowl, scales, soaking tank, oven

APPARATUS
AND SUPPLIES

(1) The specimen(s) for compaction shall be prepared in accordance
with the procedures given in ASTM Test Methods D 698 or D 1557
for compaction in a 6-in. (152.4-mm) mold except as follows:

SAMPLE [1]

(1.1) If all material passes a 3 ⁄ 4 -in. (19-mm) sieve, the entire gra-
dation shall be used for preparing specimens for compaction
without modification. If material is retained on the 3 ⁄ 4 -in. (19-mm)
sieve, the material retained on the 3 ⁄ 4 -in. (19-mm) sieve shall be
removed and replaced by an equal amount of material passing
the 3 ⁄ 4 -in. (19-mm) sieve and retained on the No. 4 sieve obtained
by separation from portions of the sample not otherwise used for
testing.

(1) Bearing Ratio at Optimum Water Content Only —Using mate-
rial prepared as described above, conduct a control compaction test
with a sufficient number of test specimens to definitely establish
the optimum water content for the soil using the compaction
method specified, either ASTM Test Methods D 698 or D 1557.
A previously performed compaction test on the same material may

TEST
SPECIMENS [1]