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California Bearing Ratio(CBR Test) of Subgrade Soil -Procedure, apparatus, and use for pavement Design

The California Bearing Ratio Test(CBR test) is a penetration test that can be performed in the laboratory or in-situ. They can be considered as small scale bearing tests in which the ratio of penetration to the size of the loaded area is much greater than in bearing tests.

For penetration tests, California bearing ratio test and cone penetration tests are commonly known tests.

California Bearing Ratio(CBR) test is used to calculate the stability of soil sub-grade and other flexible pavement materials.

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Originally California bearing ratio test was developed in 1929  by California division of highway for evaluating the stability of soil sub-grade and other flexible Highway  Materials which was later modified by the U.S.  Corps of Engineers.  

 The abbreviation of the California bearing ratio is (C.B.R.).  C.B.R.  may  be defined  as the ratio  of the  test  load  required   to  force  a  cylindrical  plunger  of 19.355 cm2 cross-sectional area into  a soil mass at the rate of 0.25 cm/min to the  load  required for corresponding penetration of the plunger into a standard  sample  of crushed  stone,  the  later load  is known as the standard  load.


California Bearing Ratio Test of Subgrade Soil

Equipment for CBR test

The laboratory C.B .R. apparatus consists of a mold of 15  cm internal diameter with a base plate and collar. The internal effective height of the mould is 17.7 cm.

During the preparation  of the specimen, a displacer disc of 5 cm depth is placed in the mould which enables to obtain a specimen of 12.7 cm depth.

 As shown in Fig.  a  loading frame with the cylindrical plunger of 5cm diameter and dial gauges for measuring the expansion of specimen on soaking in water and penetration measurements is also needed.

Procedure of CBT test

The specimen is placed in mould in four layers. After soaking and swelling for four days water absorption values are noted as per which a surcharge load is placed at the top of the specimen.                                                                                           

The setup is as per the above fig. and load is applied at the rate of 1.25mm/min. The loads are recorded as per the penetration values of 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 7.5 12.0 mm.

During this operation, a 5 cm diameter plunger is penetrated into the pavement component material at the rate of  1.25  mm/min The load for 2.5 mm or 5 mm penetration is recorded.

To obtain the C.B.R. value,  this load is expressed in the percentage of a standard load value at a respective deformation level. The standard load values are obtained from the average of a number of tests on different crushed stones as given in the following table below.

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Table. Standard  load  values used in  C.B.R.  test

S.No. Penetration in mm Standard Load  in kg          Unit standard load in kg/cm2                            
1 2.5 1370 70
2 5 2055 105
3 7.5 2630 134
4 10 3180 162
5 12.5 3600 183

From the observations, the load penetration graph is drawn as penetration in X-axis and load in Y-axis. If correction on the curve is required then correction is done on the load penetration curve . Then  the load  corresponding  to 2.5 mm and 5 mm penetration is calculated.  The values corresponding to 2.5mm and 5 mm penetration are noted in each case of correct and corrected observation.

The C.B.R. value usually is selected at 2.5 mm penetration.  If the C.B.R. at 5 mm is greater than at 2.5 mm, the test should be repeated. If the check test results are similar, then the value for 5 mm penetration is used as.

Generally, the CBR test is conducted for three test specimens and the average value is taken of the three.

The C.B.R. value is calculated using the relation :

BCR formula

The C.B.R.  test essentially is an arbitrary strength test and thus cannot be used to evaluate the soil properties as the angle of internal friction or cohesion or shearing resistance etc. Dependable results can not be obtained unless the procedure is followed very strictly. The C.B.R. test results are used in the empirical method of flexible pavement design.

Recording of Observations

S.no. Penetration mm Proving ring reading Corresponding load, Kg Corrected load, Kg
1.  …. ….   ….   ….  

CBR (California bearing test) result

  1. CBR value at 2.5 mm penetration =
  2. CBR value at 5.0 mm penetration =
  3. California bearing ratio of sub grade =

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Calculation of pavement thickness using CBR value

Studies have shown that there’s a relation between CBR value (within a range of 10-12%) and the wheel load, tyre pressure and the thickness of pavement. The expression for pavement thickness is given by: –

t (cm) =  √{P[(1.75÷CBR)-(1÷pπ]}

≫ t (cm) =  √[(1.75p÷CBR)-(A÷π]


 p = P/A

P= wheel load (kg),

t = pavement thickness (cm),

CBR = CBR in %, 

p  =  tyre pressure (kg/cm2),

A = Contact area (cm2)

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