CBR (California Bearing Ratio) It is the ratio of force per unit area required to penetrate a soil mass with a standard circular piston at the rate of 1.25 mm/min. to that required for the corresponding penetration of a standard material. The California Bearing Ratio Test CBR Testing is a penetration test developed by the California State Highway Department (U.S.A.) for evaluating the bearing capacity of subgrade soil for the design of flexible pavement.

Tests are carried out on natural or compacted soils in water soaked or un-soaked conditions and the results so obtained are compared with the curves of a standard test to have an idea of the soil strength of the subgrade soil.

APPARATUS

  • Mould
  • Steel Cutting collar
  • Spacer Disc
  • Surcharge weight
  • Dial gauges
  • IS Sieves
  • Penetration Plunger
  • Loading Machine
  • Miscellaneous Apparatus

CBR Test PROCEDURE

Normally 3 specimens each of about 7 kg must be compacted so that their compacted densities range from 95% to 100% generally with 10, 30 and 65 blows.

  • Weigh of empty mould
  • Add water to the first specimen (compact it in five layers by giving 10 blows per layer)
  • After compaction, remove the collar and level the surface.
  • Take a sample for determination of moisture content.
  • Weight of mould + compacted specimen.
  • Place the mould in the soaking tank for four days (ignore this step in case of unsoaked CBR.
  • Take other samples and apply different blows and repeat the whole process.
  • After four days, measure the swell reading and find %age swell.
  • Remove the mould from the tank and allow water to drain.
  • Then place the specimen under the penetration piston and place surcharge load of 10lb.
  • Apply the load and note the penetration load values.
  • Draw the graphs between the penetration (in) and penetration load (in) and find the value of CBR.
  • Draw the graph between the %age CBR and Dry Density, and find CBR at the required degree of compaction.

California Bearing Ratio test

Data – Observations & Calculations

Graph of Penetration vs Loading in California Bearing Ratio Test

Graph of CBR vs % Percent Compaction Graph