California Bearing Ratio Test

California Bearing Ratio (CBR) :- The ratio of the force per unit area required to penetrate a soil mass with standard penetration plunger at a uniform rate of 1.25 mm/min., to the corresponding penetration load of the standard material or crushed stone is called CBR. CBR (%) = Test load x 100% Penetration load (standard material)

Significance :- It is generally used to classify & evaluate the soil subgrade & base coarse materials for the flexible pavement. This test is empirical & the results cannot be accurately related with any fundamental properties of the material.

Procedure :- Take 4.5-5 kg of soil & mix it thoroughly with required quantity of water (OMC) or field moisture content. The spacer disc is placed at the bottom of the mould over the base plate & a coarse filter paper is placed over the spacer disc. The moist soil is to be compacted over this in the mould by adopting either IS light or heavy compaction.

After compacting the last layer, the collar is removed & the excess of soil above the top of the mould is evenly trimmed off. The clamps are removed & the mould with the compacted soil is lifted, leaving below the base plate & spacer disc is removed. A filter paper is placed on the base plate, the mould with compacted soil is inverted & placed over the base plate& clamps are tightened. Weight of 2.5-5 kg are placed over the soil in the mould. Then the whole mould is placed in the water tank to allow soaking for 4 days.

A swelling measuring device are placed on the top edge of the mould & the spindle of the dial gauge is placed touching the top of the sample. Initial dial gauge reading are recorded & the test sample is kept undisturbed in the water tank. After 4 days of soaking, the mould is clamped over the base plate & the same surcharge weights are placed on the specimen as the test could be conducted. The complete assembly is placed under the loading machine.

Penetration plunger is brought in contact with the top surface of the soil by applying 4 kg of load. The dial gauge of proving ring & the penetration dial gauge is set on 0. The load is applied through penetration rate of 1.25 mm/min. The load readings are taken at 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0, 12.5 mm of penetration.

The max. load value & the corresponding penetration value are recorded. A graph is plotted by penetration (mm) on x-axis & load (kN) on y-axis & the value of load at penetration 2.5 & 5.5 mm are found from the graph.

Finally, the CBR value is calculated from the formulae. As per IRC recommendation the min. value of CBR required for a subgrade should be 8%.