INVESTIGATION INTO SOME OF THE ENGINEERING PROPERTIES OF SOILS FOUND IN WEREDA 10 OF BOLE SUB CITY , ADDIS ABABA, ETHIOPIA

Soil is the ultimate foundation material which supports the structure. The proper functioning of the structure will, therefore, depend on the engineering properties of the underlying soil. The objectives of this research is to investigate the engineering properties of soils found in Wereda 10 of Bole Sub-City by conducting different types of laboratory tests, reconnaissance survey of the study area, and collection of secondary data, cross-section profiling and by evaluating relations between UCS and Index properties(P200, LL and PI). This research is useful in providing necessary data or information that can be used in designing civil engineering structures in Wereda 10 of Bole Sub City. Laboratory tests carried out and collected on disturbed and undisturbed samples revealed that the natural moisture content ranges from 5.47-67.37%, specific gravity of the soils ranges from 2.03-2.87%. Results of Atterberg limits tests show that soil of Wereda 10 of Bole Sub City have liquid limits ranging from 0-116%, plastic limit ranges from 0-84% and plastic index from non-plastic – 73%.This indicate that soils of the study area has heterogenetic nature and wide range of plasticity. This variation also reflected in their grain size distribution of particles passing 200 sieve that ranges from 10.7% - 99.6%. The free swell test result indicates that soils of the study area range from non-swelling to 200%, which means the soils in the study area ranges from non-expansive to highly expansive. According to USCS, the soil in the study area classified in to four main types as MH, ML, SM and CH, in their dominance order. Also soil types CL, GM, SC, CL – ML, & SC – SM rarely exist in the study area. The Compaction test result showed that maximum dry density (MDD) ranges from 1.67 to 1.7 g/cm3 and the optimum moisture content (OMC) ranges 18.5 to 23.4%. The unconfined compressive strength (UCS) test result of the area ranges from 29.94 to 285kN/m2 indicates the soil consistency ranges from soft to very stiff. Moreover, the one dimensional consolidation test result carried in the area showed ranges of initial void ratio (eo) from 0.7 to 1.52 and compression index (Cc) from 0.09 to 0.336. The permeability test result ranges from 2.8 to 5 (x 10-3) cm/sec, indicating the soil is impervious. In the effort to correlate the index properties to that of their engineering properties, the UCS of MH soil can be predicted using exponential equation by making use of LL as independent variable which is UCS = 14.692e0.0217(LL) with R2 of 0.5449. However, no clear correlations are observed between UCS and P200/PI. In the other hand, the UCS of CH soil can be predicted using either of exponential or linear equation by making use of P200/LL/PI as independent variable which is UCS = 9.1527e0.0315P200 , UCS = 106 e-0.156LL, UCS = 821768e-0.303PI, UCS = 3.3608(P200) – 137.19, UCS = -17.929(LL) + 1183.6, UCS = 34.072(PI )+ 1132.2. Their R2 are 0.8844, 0.8544, 0.7696, 0.7681, 0.856, and 0.7397 respectively. Generally, the cross-section profile of the area reveals that soil layers can be explained with the soil profile successions of black cotton (silty clay/clayey silt) as a top layer, silty clay/clayey silt soil layer, sandy silt/Silty sand layer, sandy gravel layer and weathered ignimbrite layers with variable thickness