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What is California Bearing Ratio(CBR) Test?

CBR Testing Apparatus
CBR Testing Apparatus
The California Bearing Ratio (CBR) test is a penetration test developed by the California State Highway Department and is used to determine the mechanical strength of a soil. The test can be carried out on site or in the laboratory. The CBR test is carried out using a cylindrical plunger which penetrates the soil at a given rate. The relationship between the force and penetration can then be used to calculate the CBR value of the soil being tested.

The California Bearing Ratio test, described in ASTM D 1883, is a penetration test commonly used to evaluate the potential strength of subgrade, subbase and base course material. To perform the test, a technician uses a cylindrical piston with a 3-square-inch cross section to penetrate the soil at a rate of 0.05 inch per minute. At each 0.1 inch penetration up to 0.5 inch, the technician records the stress needed to push the piston into the soil. The CBR value is the ratio of this stress at different penetration levels to the bearing value of a standard crushed rock. In most cases, CBR decreases as the penetration increases, so the ratio at 0.1-inch penetration is used as the recorded CBR value. Sometimes designers use this value to choose an appropriate slab thickness for anticipated loadings.

Field CBR Test
The test is carried out off the back of a 4WD vehicle or and excavator either at the ground surface or in pits up to around 1.0 m deep. The test area is leveled and any unrepresentative material is removed. Sufficient surcharge discs are then placed around the test area and plunger to correspond with the over burden pressure for the test. The test is carried out by forcing the plunger to penetrate the soil over a given time. The results of the penetration are used to provide a CBR value for the soild horizon.

However, the test is only appropriate to use on material with a particle size that does not exceed 20 mm. An alternative technique to gain CBR values is to carry out a hand held Dynamic Cone Penetrometer (DCP) test.
CBR% = 100 (P/Ps)
P = Measured Pressure for Site Soil(N/mm²=Mpa
Ps = Pressure to achieve equal penetration on standard soil(N/mm²)
Ps = 6.9 Mpa(1000psi) for 0.1"(2.54mm) ; 10.3 Mpa(1500psi) for 0.2"(5.08mm)
Sample CBR Graph for various soil samples
Sample CBR Graph for various soil samples

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