COMPARISON OF CORRELATIONS DEVELOPED IN ETHIOPIA AND TRL, OVERSEAS ROAD NOTE 8 BETWEEN DCP AND CBR FOR LOCALLY USED SUB GRADE MATERIALS

California bearing ratio is the parameter used in determining strength of subgrade materials in pavement design. But it is time taking and expensive method; to solve this situation many researchers find better way of finding strength of the subgrade material representing CBR. From this prediction of CBR value from DCP has been found good alternative. Dynamic cone penetration test method is very quick, economical and easy way of subgrade and sub-base field test instrument. DCP is field testing method is used to evaluate the thickness and bearing capacity of sub-base and subgrade of pavement. Many correlations have been found to correlate CBR and DCP test methods to get better and accurate estimated value. Now a day Ethiopian researchers have found correlation for local soils in Ethiopia (Ytagesu Desalegn 2012, Gebremariam G. feleke). The correlation developed by ytagesu shows better than TRL. Ytagesu has recommended that correlations developed in his study the test conducted at in-situ density and moisture content rather than soaked CBR. Hence, it may not be employed directly for design purpose unless the DCP be made when the soil is at its worst condition. Therefore, in order to use this correlation for design and/or analysis directly it is recommended that future research on the strength reduction factor should be conducted. In addition he recommended that his research has been done on specific one project; so it is better to check at several locations of the country. This project was carried out to compare correlation between DCP with CBR values that best suit the type of soils in Ethiopia rather than TRL. Several laboratory tests from Adama-Awash Expressway road project have been selected from lab and field test data’s from Beza consulting Engineers P.L.C (designer of the project). From the tests Atterberg limits, gradation, lab CBR and DCP data were selected. Based on this laboratory and field test results analyses were carried out using correlation developed by Ytagesu Desalegn (log (CBR) = 2.954 -1.496log (DCPI) with R2=0.943), G/mariam G. feleke (log10SCBR=2.015-0.906log10DCPI (R2=0.93)) for soaked fine grained soil and TRL (log (CBR) = 2.48 – 1.057log (DCPI)) and compare and check the applicability locally developed correlation. Percentage difference prediction CBR from lab CBR and scatter plot methods were used for comparision. From the analysis results and comparision, it is observed the prediction found by Ytagesu Desalegn’s correlation shows good and near to lab test CBR values