Correlation of California Bearing Ratio with Soil Index Properties for Subgrade Soil in Jimma Town

Abstract

CBR is a commonly used indirect method to assess the stiffness modulus and shear strength of subgrade soils in pavement design. However; it is always difficult and time taking to obtain representative CBR value for design of pavements. In this study a method is proposed for correlating CBR values with the liquid limit, plastic limit, plasticity index, optimum moisture content, and maximum dry density of cohesive soils of various zones of Jimma city. With the objective of predicting CBR value from index properties of soil, having all the associated test results with corresponding CBR values collected from laboratory tests and Ethiopia road authority’s archival secondary data analysis were carried out. As a result, these study has examined the feasibility of single linear regression analysis and multiple linear regression analysis in correlating CBR value with soil index properties. Specific to this research, statistical software (SPSS) is employed to investigate the significance of individual independent variables. The correlation is developed in the form of an equation of CBR as a function of grain size parameter, Atterberg limits and compaction parameters by considering the effect of an individual soil properties and effect of a combination of soil properties on the CBR value. The developed correlation entailed a moderate determination coefficient of 2 R = 0.462 using single regression analysis, while multiple regression analysis generated relatively an improved correlation of 2 R = 0.604, for a sample size of fifty. After validating the developed correlation with control test results, it was noted that the correlation of CBR value with soil index properties is more applicable for preliminary characterizing the strength of subgrade soil in Jimma town.