Abstract:
Subbase is one of the unbound materials which enable traffic stresses to be reduced to acceptable
levels in the subgrade. It acts as a working platform for the construction of the upper pavement
layers, and it acts as a separation layer between the subgrade and base course. Selection of good
quality materials for subbase is one of the problems facing during road construction, and such
material should adhere the minimum requirements of the standard specifications. This study had
been focused in assessing or identifying the suitability of the selected material as the subbase for
road construction along mazoriya to limmu genet of Jimma zone. In the study area, it is
predominantly covered with wide kilometer square of coffee beans which is the most exported
product in Ethiopia (e.g., Horizon coffee plant) and some agricultural products. Three quarry sites
were identified. Representative disturbed samples from the quarry sites of Limmu genet, Babu, and
Ambuye (i.e. designated by site one, site two and site three, respectively) were collected from
different parts. Laboratory tests of specific gravity, natural moisture content, Atterberg limits, Grain
size analysis, compaction test, CBR strength test and in-situ test of unit weight by Sand cone
replacement method, were conducted. Based on the laboratory tests results of the grain size
distribution from the three quarry sites were gravelly soils. The gravel content and sand content
have taken the significant percentage of the grain size analysis of the samples. Also, laboratory test
results showed natural moisture content ranges from 10.95% to 40.45%. The average specific
gravity of site one, site two and site three is 2.66, 2.58 and 2.68 respectively. This means the specific
gravity of site three is the highest, and that of site two is the lowest specific gravity. It implies the
construction of subbase layer by soil sample of site two is more prone to erosion and scour.
Likewise, the CBR value of laboratory test of site one, site two and site three is 40%, 24% and 34%
respectively. The sites in their previous order have a swelling of 0.00%-0.20%, 0.43%-0.50% and
0.26%-0.37% for 65 blows, 30 blows and 10 blows of compaction per layer of CBR test. This implies
that site one has less swelling than others. Therefore, it can be concluded that site one sample is
more suitable for subbase material than the others.
