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.
