Suitability assessment of selected material as subbase for road construction along Mazoriya to Limmu Genet in Limmu genet wereda quarry sites . (A case study in Jimma zone)

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.