The California Bearing Ratio (CBR) test measures the bearing capacity of a soil by determining the ratio of the force required to penetrate a soil mass with a standard plunger to that of a standard material. It is used to classify and evaluate soils for flexible pavement subgrades and bases. The procedure involves compacting a soil sample, soaking it for 4 days, and then applying a load through a plunger at a rate of 1.25 mm/min while measuring penetration. Load readings are recorded and used to calculate the CBR value based on standard pressures at 2.5 and 5.0 mm penetrations.

1. CALIFORNIA BEARING RATIO TEST
By :- ABHIK BORTHAKUR
NITESH KHOKER
ANJALI MEENA
NAGENDRA MALAPATI
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2. 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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3. SIGNIFICANCE :-
 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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5. PROCEDURE :-
 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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6.  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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7.  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. 7

8.  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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9.  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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11.  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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12.  OBSERVATION TABLE:
 Following are the observed values of loading in kg
against penetration values in mm on a sample
Sr.no. Penetration Load(kg) Sr.no
.
Penetrati
on
Load(kg)
1. 0.0 7. 3.0
2. 0.5 8. 4.0
3. 1.0 9. 5.0
4. 1.5 10. 7.5
5. 2.0 11. 10.0
6. 2.5 12. 12.5

13. OBSERVATIONS
 Area of Plunger, 𝐴 = 𝜋𝑟2
= 3.14x2.5²(d=5cm)
=19.625 cm²
 Pressure at 2.5 mm Penetration =
 Pressure at 5.0 mm penetration =
 Standard pressure for 2.5 mm penetration=
 Standard pressure for 5.0 mm penetration=
 C.B.R. At 2.5 mm =
 C.B.R. At 5.0 mm =

14.  Volume of mould : 2244 cm³
 Rate of loading : 1.25 mm/ min.
 CBR = Load carried by specimen x100
Load carried by std. crushed stone specimen

15.  Volume of mould : 2244 cm³
 Rate of loading : 1.25 mm/ min.
 CBR = Load carried by specimen x100
Load carried by std. crushed stone specimen

16. PENETRATION RESULT
◮ A load penetration curve of load against
penetration is then plotted with load in ordinate
axis and penetration in abscissa axis
◮ the loads corresponding to 2.5 and 5.0mm
penetration values are noted.
◮ Sometimes a curve with initial upward concave
may also be obtained due to surface CBR%
irregularities and in this case a correction is to be
done.
◮ A tangent is drawn at a point of greatest slope.
◮ The point where this tangent meets penetration
axis is the corrected zero reading of penetration.

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