Recalibration of Structural Layer Coefficient and Drainage Coefficient for Sub –base and Base

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Addis Ababa University


The performance of flexible pavement structures is governed by the supporting capacity of the component layer determined by the structural layer coefficient and drainage coefficient. Also, the performance of the pavement will depend on the pavement material, the traffic level, the environmental condition, and the failure criterion. The value of structural coefficients and drainage coefficients used for pavement design are taken directly from AASHTO 1993. Using AASHTO value has a great influence on the structural thickness of the pavement conversely on the capacity of the pavement. The focus of this study were concentrated on recalibration of the structural layer coefficient and drainage coefficient values for some base and sub-base pavement material in Addis Ababa by the AASHTO material characterization method. Sieve analysis, Atterberg limit, proctor test, specific gravity, and CBR were conducted for subgrade, sub-base, and base. However, a UU Triaxial test was conducted for subgrade and sub-base to determine the resilient modulus (Mr). The constitutive equation for resilient Modulus was also developed after material constants K1, K2, and K3 were determined from multiple linear regressions using STATA software. Structural layer coefficient (a) was determined from respective SN by rearranging and averaged for each segment. Drainage coefficient (m) was calculated from the ratio of site layer coefficient to AASHTO layer coefficient. New correlation equations were developed from average structural layer coefficients and resilient modulus (Mr). Then, the results were compared with each other as well as with that of given on AASHTO 1993, AACRA, and ERA value. From the result obtained for Shiro Meda – Kidane Mihiret Hamere Nooh, the average thickness obtained for base course is 99.17cm which is greater than the value of 50cm obtained from engineering report which shows addition of 49.17cm. For sub - base course, the average thickness is 25.75cm which is less than the value of 50cm obtained from engineering report which shows a save of 24.25cm. However, this method should be calibrated by cyclic triaxial test to be used routinely.



Recalibration, structural layer coefficient, drainage coefficient