Developing Correlation Between DCP And CBR For Locally Used Subgrade Materials

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Addis Ababa University


California Bearing Ratio (CBR) is a commonly used indirect method to assess the sub grade soil strength in pavement design works. However, since CBR cannot be easily determined in the field, civil engineers always encounter difficulties in obtaining representative CBR values for design of pavement. Consequently, prediction of CBR va1ues from other direct soil field tests such as Dynamic Cone Penetrometer (DCP) is found to be a valuable alternative. The dynamic cone Penetrometer (DCP) is the most versatile rapid, in situ evaluation device currently available. Its correlations to CBR and its use in performance evaluation of pavement layers make it an attractive alternative to more expensive and time consuming procedures. Many useful correlations had been proposed by various researchers for soils of their locality in which the DCP penetration index was used to develop these correlations. A study was carried out to find the correlation between Cone Penetrometer (DCP) with CBR values that best suit the type of soils in Ethiopia. Accordingly, several laboratory tests and field tests on Jimma – Mizan, Contract 1: Jimma – Bonga road project from km 100+000 to km 105+000 has been conducted. From the tests, the Atterberg limits (PI, LL, PL), In situ density, classification (sieve analysis, hydrometer analysis), California bearing ratio, insitu Moisture Content, and Dynamic cone penetration results are acquired. Based on this laboratory and field test results analyses were carried out using SPSS software and the applicability of the existing correlations for Ethiopia soil on the aforesaid data was checked. From the analysis results, it is observed that the published correlations are not suitable to be used in Ethiopia. Consequently, a correlation had been proposed in the study to predict the CBR values of the sub grade soil from dynamic cone penetration test results. The relation obtained from statistical analysis relationship has an R 2 of 0.943 and is shown below. log (CBR) = 2.954 – 1.496log (DCPI) with R 2 = 0.943 The results of the statistical analysis show that good correlation does exist between the dynamic cone penetration indexes (DCPI) and unsoaked CBR values.



DCP, CBR, Sub grade Materials