California Bearing Ratio (CBR) test is a type of test on roads that are carried out to check the strength of the subgrade of pavement. By comparing the results of this test with a set of standard curves, the thickness of the subsequent layers can also be determined. It is a type of penetration test and extensively used in the design of flexible pavements. This test was developed by the California Division of Highways of the United States.
CBR 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
Apparatus Required :
Steel Cutting Collar
Disc and loading machine
Cylindrical Mould and Rammers
Procedure in CBR Test:
The procedure for conducting the CBR test is as follows:
1. Firstly, an undisturbed sample specimen is prepared by attaching a cutting edge to the mould and pushing it gently in the mould to remove soil on the outside.
2. The density of the sample is determined and is remoulded at Proctor’s maximum density or Optimum Moisture Content. The sample must be such that it must pass through 20mm IS sieve but must be retained on 4.75 IS sieve.
3. About 4.5kg to 5.5 kg of the sample is mixed with water. The extension collar and base plate are then fixed to the mould and the disc is placed over the base plate.
4. The soil mix is then compacted in several layers by a rammer. Generally, for light compaction, the soil is compacted in three layers by 55 blows from a 2.6kg rammer.
5. The collar is then removed and the soil is trimmed. The mould is then turned over and the base plate and 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.
Standard Load Values for CBR Test
|Penetration(mm)||Standard Load(kg)||Unit Standard Load(kg/cm2)|
Observations during CBR Test
Weight of soil taken =
Weight of surcharge =
Area of the plunger, A =
|Sl No.||Penetration(mm)||Proving dial reading||Load on plunger(kg)||Corrected load||Unit Load|
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 =