Development of empirical correlations between California bearing ratio (CBR) and soil index properties.

The California Bearing Ratio is a penetration test for evaluation of the mechanical strength of road sub-grades and base-courses. This can be used as a mean of designing the road pavement required for a particular strength of sub-grade by comparing the strength of different sub-grade materials. However; civil engineers always encounter difficulties in obtaining representative CBR value for design of pavement. Over the years, many correlations had been proposed by various researchers in which the soil index properties were used to develop these correlations. A study was carried out to find correlations between CBR value with soil index properties those best suit the type of soils in Sri Lanka. Analyses were carried out based on the published correlations and soil data obtained from several Sri Lankan project sites. Based on the results, it is observed that the current published correlations are not in good agreement with Sri Lanka soils. In addition, no typical range could be found based on the soil index properties. Mechanical Strength of soil depends not only on the soil type but also on the observable physical characteristics which significantly influence on a soil’s behavior. Therefore, a method is proposed for correlating soaked CBR value and compaction parameters with such index properties, for Sri Lankan soils. This research covers the entire soil types according to Unified Soil Classification System which are generally used as sub-grades and base-courses. Among the several soil index properties, Atterberg Limits and grain size distribution data are used in this regard as these tests are much more economical and rapid than Compaction and CBR tests. The correlations are established in the form of an equation as a function of different soil properties by the method of regression analysis. Finally, results of the laboratory test are used to compare with the results of regression equation for the compiled data for the validation of the correlation. Key Words : California Bearing ratio, Compaction Parameters, Index Properties, Regression Analysi