Basic Procedure of Performing the California Bearing Ratio (CBR) Test


CBR stands for California bearing ratio; It is the penetration test which is used to evaluate the mechanical adaptability of the soil mass in the natural ground level. The rate is measured to assess the bearing capacity of the soil surface.

The basic procedure of conducting the CBR test on the soil is as follows:

Ø  First, weight of the empty mould is taken.

Ø  And then, keep the spacer disc on the base plate and then place the filter paper on the disc and then fix the mould to the base plate with the plate inside a shape and append the collar over the mould.

Ø  Add water to the specimen collected and then compact the same by the standard or modified proctor test.

Ø  After the compaction, you need to remove the collar and then the surface needs to be levelled-up by using the cutting edge.

Ø  Then the base plate needs to detach and removed from the spacer disc.

Ø  Now take the total weight of the mould with the compacted specimen and then determine the density of the sample collected.

Ø  Now take the sample to determine the moisture content of the soil and hence to find the dry density of the same.

Ø  Now the filter paper needs to be placed on the base plate (perforated)

Ø  Now fix the mould into the upside down to the base plate (perforated).

Ø  Now, fix the mould upside down to the base plate.  So, the surface of the specimen which is already in the downwards position contacting with the spacer disc during the compaction process is now turned upwards and on that the penetration test needs to be performed where it must be done on the unsoaked condition.

Ø  Whereas for the soaked condition, now fix the adjustable stem and the base plate (perforated) in to the compacted soil sample into the mould with the surcharge load of 2.5 kg.

Ø  Now this is the step which needs to be performed in the case of soaked CBR only, place the above set up which is mentioned in the XI step in the soaking tank for the four days.

Ø  After fourth day of soaking, measure the reading of the swell and find the percentage of the swell with the use of the dial-gauge reading tool.

Ø  Now the moulds need to be removed from the tank and water is drained.

Ø  Now the specimen needs to be placed under the examination piston with the total load of the surcharge as 4 kg which is 2.5 kg during the soaking process with the 1.5 kilograms during the testing process.

Ø  Now the gauges of the load and deformation needs to be set to zero.

Ø  Now the Load applied to the plunger in the soil @ the rate of 1.25 mm/min.

Ø  Take the reading of load at penetration of 0, 0.5, 0.635, 1.0,1.27,1.5 ,2.0, 2.54, 3.82,4.0 ,5.08,7.62, 10.0, 10.16 ,12.5 and 12.7 mm

Ø  Now remove the plunger and calculate the water consisting content of the soil.

Ø  At the end plot the load in comparison of deformation/penetration curve and the CBR value is measured.

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