Effects of Subgrade and Sub-Base Materials Quality on Flexible Pavement: A Case Study along Ajip to Gebrel Church of Jimma Town

Abstract

Flexible pavements are composed of Asphaltic layer, base course; sub base materials which laid over a well compacted and strong sub grade foundation. Sub grade and Natural sub base materials basic engineering properties are required for Design and Coarse grained soils serves as good subgrade for supporting pavements whereas fine grained soils, particularly clayey soils pose problem to pavement. Jimma is a big trade center and coffee production areas but, the current existing condition most parts of the road are deteriorated. So, the objective of this research was to investigate effects of subgrade and sub-base materials quality on flexible pavement. The researcher was identified the distress and categorized by level of severity according to ERA standard specification manual to get 4-stations high deterioration of block crack, 6-stations medium deterioration of Alligator crack and 5-stations low deterioration of edge crack of the studied stretch along Ajip to Gebrel church (total 10kms) of Jimma town. Then, randomly samples taken from the deteriorated area of the subgrade and sub-base materials a total of 15 stations and the laboratory work was conducted and also the results were discussed compared with the standard specification of ERA, AASHTO and ASTM. Finally, Conclusions would be developed and recommended appropriate remedial measure would be drowning. Laboratory tests result NMC>OMC indicated that excess water in the subgrade and subbase materials according to AASHTO D-180 standard specification, compaction of subgrade and sub-base layers are below the minimum requirement (MDD>97% laboratory density) of AASHTO standard specification, poor strength (CBR<1.76gm/cc) of AASHTO T-193 and ERA specification, sieve analysis for gradation out of the upper and lower limit of AASHTO T-27 standard specification for the subgrade and sub-base materials. These can be causes of pavement deterioration on flexible pavement. But, the Atterberg’s limit test results satisfy the standard specification of ASTM D-4318. The researcher recommended that the designer and contractor should be follow the minimum requirement set by standard specification regarding the engineering properties of materials, the side ditch must be constructed so as to prevent infiltration of water to the under lining strata, seal coats shall be applied to prevent infiltration of water through cracked surfaces to subgrade layer of the pavement.