Correlation of California Bearing Ratio (CBR) Value with Soil Properties of Road Subgrade Soil

The California Bearing Ratio (CBR) is the most utilised parameter for dimensioning flexible pavements in tropical countries. Often this test is expensive, laborious and time consuming, and to overcome this, some regression analysis (single and multiple) was considered between the soil’s index properties (liquid limit-LL, Plastic limit-PL and Plasticity index-PI), compaction characteristics (maximum dry density-MDD and optimum moisture content-OMC), percentage of particle sizes (gravel, sand and clay/silt) and CBR. The study was carried out along an ongoing roadway construction project where thirty-three soil samples were collected and transported to the laboratory for analysis. Routine geotechnical tests were carried out and thereafter correlation and regression analysis were run on the obtained results to assess the relationship between these index properties, compaction characteristics and the experimental CBR obtained. The results of this analysis showed relatively fair coefficients of determination of R² = 0.772 between CBR and MDD using single linear regression analysis and R² = 0.841 between CBR and all the parameters using multiple linear regression analysis (MLRA). Though the MLRA improved the R² from 0.772 to 0.841, the inclusion of additional properties results in a marginal increase of R² indicative of weak correlators of CBR hence practically not cost effective for pavement design .