California Bearing Ratio Test on Subgrade Soil -Procedure and Values

The California Bearing Ratio(CBR) test is a measure of the resistance of a material to penetration of standard plunger under controlled density and moisture conditions. It was developed by the California Division of Highways as a method of classifying and evaluating soil- subgrade and base course materials for flexible pavements.


CBR test may be conducted in a remolded or undisturbed sample. The test consists of causing a cylindrical plunger of 50mm diameter to penetrate a pavement component material at 1.25mm/minute.

The procedure of California Bearing Ratio Test
Sieve the sample through 20mm IS sieve. Take 5 kg of the sample of soil specimens. Add water to the soil in the quantity such that optimum moisture content or field moisture content is reached.

Then soil and water are mixed thoroughly. Spacer disc is placed over the base plate at the bottom of mold and a coarse filter paper is placed over the spacer disc.

The prepared soil water mix is divided into five. The mold is cleaned and oil is applied. Then fill one-fifth of the mold with the prepared soil. That layer is compacted by giving 56 evenly distributed blows using a hammer of weight 4.89kg.

The top layer of the compacted soil is scratched. Again second layer is filled and process is repeated. After 3rd layer, collar is also attached to the mould and process is continued.

After fifth layer collar is removed and excess soil is struck off. Remove base plate and invert the mould. Then it is clamped to baseplate.

Surcharge weights of 2.5kg is placed on top surface of soil. Mould containing specimen is placed in position on the testing machine.

The penetration plunger is brought in contact with the soil and a load of 4kg(seating load) is applied so that contact between soil and plunger is established. Then dial readings are adjusted to zero.

Load is applied such that penetration rate is 1.25mm per minute. Load at penetration of 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 7.5, 10 and 12.5mm are noted find out more information here.