California Bearing Ratio Test Procedure

California Bearing Ratio Test ( CBR Test ) of Soil : Lab Procedure, Standard Values, Observation Table

 

California Bearing Ratio Test is carried out to check the strength of the subgrade of pavement.

By comparing the results of this test with a set of standard curves or values, the thickness of the subsequent layers can also be determined.

It is a type of penetration test and is extensively used in the design of flexible pavements. This test was developed by the California Division of Highways of the United States.

California Bearing Ratio is defined as the ratio of force per unit area that is required to penetrate a soil mass with a piston of 1.25mm/min corresponding to that required for the penetration of a standard material.


CBR = ( Test load / Standard load ) * 100

 


 

  1. Apparatus Required  

Soil Tests for Road Construction- California Bearing Ratio Test

a. Steel: Cutting Collar

b. Dial Gauges

c. Weighs

d. IS Sieves

e. Penetration Plunger ( Diameter: 50 mm & Height: 100 mm )

f. Spacer Disc ( Diameter: 148 mm &  Height: 47.7 mm )

g. Loading Machine

h. Cylindrical Mould ( Diameter: 150 mm & Height: 175 mm )

i. Rammers

 


 

  2. California Bearing Ratio Test Procedure

The procedure for conducting the CBR test is as follows:

1. Prepare the soil sample passing through a 20 mm IS sieve but retained on a 4.75 mm IS sieve.

3. About 4.5kg to 5.5 kg of the sample is mixed with water ( commonly distilled water ). A spacer disc is placed over the base plate and a coarse filter paper is placed over the spacer disc. Then inner oiling of the mould is done and the mould is fixed in its position. At the top of the mould, a collar is placed.

4. The soil mix is then compacted in several layers by static compaction or dynamic compaction.

For Static Compaction: Compacting soil in loading machine ( gradually increasing load ).

For Dynamic Compaction: Compacting soil by rammer ( impact load ).

For light compaction, the soil is compacted into 3 layers by 56 blows with a 2.6 kg rammer from a height of 31 cm.

For heavy compaction, the soil is compacted into 5 layers by 56 blows with a 4.89 kg rammer from a height of 45 cm.

Light CompactionHeavy Compaction
No. of layers35
Rammer Weight2.6 kg4.89 kg
Fall31 cm45 cm
Blows5656

 

5. The collar is then removed and the soil is trimmed. The mould is then turned over and the base plate and spacer disc are removed.

6. The mould is then weighed and bulk density, as well as dry density, is determined.

7. Then the mould along with the surcharge weight is placed on the penetration test machine.

8. The load is applied to the piston such that the rate of penetration is 1.25mm/min.

9. The load readings at penetrations of 0.5,1.0,2.0,2.5,3.0,4.0,5.0,7.5,10 and 12.5 mm are recorded.

10. The mould is then detached from the loading equipment and moisture content is determined.

 


 

  3. Standard Load Values for California Bearing Ratio Test  

Penetration(mm)Standard Load(kg)Unit Standard Load(kg/cm2)
2.5137070
52055105
7.52630134
10.03180162
12.53600183

 


 

  4. Observations During California Bearing Ratio Test  

Weight of soil taken =

Weight of surcharge =

Area of the plunger, A =

Proving Ring Calibration Factor =

 

Note: Tilt your mobile to view the full table. 

 

Sl No.Penetration

(mm)

Proving dial readingLoad on plunger (kg)Corrected loadUnit Load

 


 

 5. Result of California Bearing Ratio Test  

California Bearing Ratio at 2.5mm penetration =

California Bearing Ratio at 5.0mm penetration =

California Bearing Ratio of subgrade soil =

 

Read Also: Core Cutter Method