Modified California Bearing Ratio Method (CBR) - IRC 37 code

 Modified California Bearing Ratio (CBR) method is further advancement of California Bearing Ratio test. Both CBR tests are used for flexible pavement design. CBR method based upon strength of sub grade soil and subsequent pavement material. In modified CBR method, The concept of cumulative standard axle is adopted instead of total number of all commercial vehicle as in simple CBR test.

To know the about the testing procedure of California Bearing Ratio Test, Click Here 

In Modified CBR method, Design traffic is defined in term of cumulative standard axle loads of 8170 Kgs (or 8.2 tons) during the design life of the roads, If the design traffic is ore than 1500 vehicles per day. 

The following expressions is used to calculate the number of cumulative standard axle N_S :

  

 Vehicle Damage Factor (VDF): It is a multiplier to convert the given number of commercial vehicles which have different axle configurations and different axle weights into an equivalent numbers of standard axle load repetitions

Lane Distribution Factor: It is required for estimating the design traffic to be considered for structural design of pavement. Design traffic means equivalent standard axle load application.

While Designing the Flexible pavement by Modified CBR test, You need only two parameter: CBR test value and Traffic in MSA. After that you need find Indian Road Congress IRC- 37 code: Design of Flexible Pavement. 

After finding the Catalogue for calculated CBR value (for reference see the below picture), Refer the Required value of Traffic, and then read the indicated layers and their respective thickness. .