A Study on Correlation of California Bearing Ratio (CBR) With Index Properties of Soils on Welkite –Arekit-Hossana Road.

Abstract

Soil properties vary from region to region and season to season as it appears naturally. studying this variation in different soil type and origin are a very important task for geotechnical engineers. To overcome the effects from this variation geotechnical engineers as well as other professionals attempt to develop empirical equations specific to a certain region and soil type in order to use the soil for different purpose. However, these empirical equations are more reliable for the type of soil where the correlation is developed .Hence, it is good practice developing empirical equations that best fit for the soil available in the area that we can access. In the flexible pavements sub-grade is considered to be an ideal layer to resist wheel load and its CBR value is considered as the strength measuring parameter. Conducting CBR test is an expensive and time consuming procedure , moreover it is very difficult to mould the sample at a desired in-situ density in the laboratory. Furthermore, if the available soil is of a poor quality, suitable additives are mixed with soil and resulting strength of soil is assessed by CBR value which is cumbersome. To overcome such problems, the other method such as regression based models (simple & multiple) has to be used from quick and easily determined parameters. Therefore ,this study is conducted to develop the correlation between CBR values with soil index properties specifically located along the way Welkite- Arekit –Hossana Road which is 121 km long. The study was carried out using thirty samples retrieved from this road and tested in a laboratory. By using the test result regression based statistical analysis is carried out to develop the intended correlation. The correlation development is performed in the form of an equation of CBR as a function of grain size parameters, 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. Based on both simple and multiple linear regression analysis relatively fair correlation is obtained by combining plasticity index, percentage of fine content and maximum dry density which are basically strength determinant of fine grained soils. From the correlation analysis the equations developed are , with coefficient of determination of R 2=0.731 for multiple linear regression and with coefficient of determination of R 2=0.682 single linear regression respectively. Statistical data analysis commercially available soft wares namely MINITAB, SPSS and Microsoft Excel used. After developing the correlation, comparison of predicting capacity of the developed model with control samples and previous researchers correlation have been applied for conformity. The result shows that the correlation is sufficiently accurate in determining the CBR and hence can be used for preliminary characterization purpose within the soil property ranges used in the study