Thursday, September 5, 2013

CBR(California Bearing Ratio) Test


CBR loading frame

  • Aim: To find out the CBR(California bearing ratio) value of the given soil sub-grade.
  • Apparatus: 
  1. CBR Apparatus
  2. soil sample with known OMC and MDD.
    CBR apparatus - accessories
  • Theory: California Bearing Ratio test was invented by California State Highway Department. This test is used to design the thickness of the flexible pavement. 
CBR value signifies the strength characteristics of the soil sub-grade which is compacted to the MDD using the OMC. The samples used for the testing are prepared in the laboratory. If a new pavement is to be constructed then, sample are prepared by compacting it with OMC and then sample is soaked in water for four days.

If the test is done for the overlay design then the sample is prepared by compacted it to the density of the soil at the site. Four days soaking is necessary in order to achieve the worst site conditions.
Higher the CBR value more is the strength of the soil sub-grade. Empirical charts are prepared by the IRC to find out the thickness of the flexible pavement corresponding to the given CBR value and for the given design wheel loading.

If the CBR value obtained with the 5 mm penetration exceeds the value obtained with the 2.5 mm penetration then this test is performed again and if the value are still same then CBR value at 5 mm is taken as the final value.

  • Procedure: 
    CBR apparatus
  1. Take the soil from the given soil sub-grade remove the gravel of size more than 20 mm and and find out its OMC.
  2. Compact the soil in three layers in the CBR mold, with 25 numbers of blows given to each layer.
  3. Put the surcharge weight and then immerse the sample in the water for four days.
  4. Put the sample with the surcharge weight on the loading base of the CBR apparatus.
  5. Apply the loading with a standard rate 1.25 mm/min with the 5 cm plunger and note down the load values corresponding to the penetrations of 1mm, 2 mm, 3 mm, 4 mm, 5 mm, 7.5 mm, 10 mm and 12 mm.
  6. Plot the graph and apply the correction if any.
  7. Repeat the test for two more samples.
  • Calculations: 
Check if there is any correction to be applied to the penetration and loading curve.
CBR value at 2.5 mm penetration = (Load at 2.5 mm penetration/ Standard load at 2.5 mm penetration i.e. 1370kg)*100
CBR value at 5 mm penetration = (Load at 5 mm penetration/ Standard load at 5 mm penetration i.e. 2055kg)*100
Use the CBR value at the 2.5 mm penetration if it is more than that at 5 mm penetration otherwise, repeat the test. If the value at the 5 mm penetration still comes out to be more, then use it.
  • Result: CBR value of the given soil sub-grade sample = .....%
Check the relevant books:

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