CALIFORNIA BEARING RATIO
TEST
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CALIFORNIA BEARING RATIO (CBR) :
The ratio of the force per unit area required to
penetrate a soil mass with standard penetration
plunger at a uniform rate of 1.25 mm/min., to the
corresponding penetration load of the standard
material or crushed stone is called CBR.
CBR (%) =
Test load x 100%
Penetration load (standard material)
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SIGNIFICANCE :
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It is generally used to classify & evaluate the soil
subgrade & base coarse materials for the flexible
pavement.
This test is empirical & the results cannot be
accurately related with any fundamental
properties of the material.
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PROCEDURE :
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Take 4.5-5 kg of soil & mix it thoroughly with
required quantity of water (OMC) or field
moisture content.
The spacer disc is placed at the bottom of the
mould over the base plate & a coarse filter paper
is placed over the spacer disc.
The moist soil is to be compacted over this in the
mould by adopting either IS light or heavy
compaction.
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After compacting the last layer, the collar is
removed & the excess of soil above the top of the
mould is evenly trimmed off.
The clamps are removed & the mould with the
compacted soil is lifted, leaving below the base
plate & spacer disc is removed.
A filter paper is placed on the base plate, the
mould with compacted soil is inverted & placed
over the base plate& clamps are tightened.
Weight of 2.5-5 kg are placed over the soil in the
mould. Then the whole mould is placed in the
water tank to allow soaking for 4 days.
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A swelling measuring device are placed on the
top edge of the mould & the spindle of the dial
gauge is placed touching the top of the sample.
Initial dial gauge reading are recorded & the test
sample is kept undisturbed in the water tank.
After 4 days of soaking, the mould is clamped
over the base plate & the same surcharge
weights are placed on the specimen as the test
could be conducted.
The complete assembly is placed under the
loading machine.
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Penetration plunger is brought in contact with
the top surface of the soil by applying 4 kg of
load.
The dial gauge of proving ring & the penetration
dial gauge is set on 0.
The load is applied through penetration rate of
1.25 mm/min.
The load readings are taken at 0.0, 0.5, 1.0, 1.5,
2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0, 12.5 mm of
penetration.
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The max. load value & the corresponding
penetration value are recorded.
A graph is plotted by penetration (mm) on x-axis
& load (kN) on y-axis & the value of load at
penetration 2.5 & 5.5 mm are found from the
graph.
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Finally, the CBR value is calculated from the
formulae.
As per IRC recommendation the min. value of
CBR required for a subgrade should be 8%.
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