In-depth Investigation Into Engineering Characteristics Of Jimma Soils

Jimma is located in south-west Ethiopia and is classified as “Weyna Dega” with high degree ofrnhumidity. The topography is predominantly flat and rolling terrain. It is mainly covered with black,rngray and red colored plastic clay soils.rnIdentification of soils type, classification of soils, and determination of compression parameters andrnestimation of the soils shear strength is done. Samples of 16 disturbed and 12 undisturbed soils arerncollected from 8 different test pits. The locations of the test pits are selected so that they can wellrnrepresent soils found in Jimma town. Disturbed samples are used for grain size analysis, indexrnproperty, free swell, geochemical and compaction tests whereas undisturbed samples are used forrnconsolidation and undrained shear strength tests.rnThe study shows that the natural moisture content varies from 37% to 68%. The specific gravityrnvaries from 2.58 to 2.82. The soils are categorized as fine-grained soils from which more than 90%rnof the particle sizes are smaller than 0.075mm. The liquid limits range from 53% to 108% andrnplastic limits vary from 27% to 41%. According to the Unified Soil Classification System, the soilsrnare categorized as CH-clay with high plasticity (fat clay) with a potential of expansion. And as perrnAASHTO classification system these soils classified as plastic clay which are unfavorable for subgradernconstruction.rnThe soil in the study area consists of expansive and lateritic residual soils. From geochemical tests,rnthe silica sesquioxide ratio varies from 1.2 to 1.79 which proves that the presence of lateritic soils.rnTheir Atterberg limits are considerably affected by sample preparation methods for air dried andrnoven dried and testing procedures such as for 5 minute and 30 minutes mixing time.rnFor black and gray soils their Atterberg limits, free swell and consolidation tests indicate that thesernsoils are expansive. The free swell ranges from 80% to 130% and Swelling pressures vary fromrn135kPa to 260kPa. The activities of the soils fall within range of 0.58 to 1.43. From differentrnclassification schemes, the expansive soils of the area are rated medium to high to very high in theirrndegree of expansiveness.rnThe maximum dry unit weight and optimum moisture content of the studied soils, fromrncompaction test, range from 12.7 to 14.00kN/m3 and from 30% to 37% respectively. Forrnlateritic soils, pre-test drying temperature and test procedures also affect the result of thernmaximum dry densities and optimum moisture contents.rnCompression parameters such as coefficient of compressibility extends from 106x10-4 torn241x10-4 m2/kN, compression index ranges from 0.238 to 0.399, swelling index varies fromrnXIIrn0.0393 to 0.0947, preconsolidation pressure varies from 230 to 300kPa and the OCR is muchrngreater than 2. The coefficient of consolidation extends from 7.24x10-2 to 1.15x10-1cm2/minrnand coefficient of permeability ranges from 3.64x10-7to 8.24x10-7cm/min.rnThe undrained shear strength of the studied soils ranges from 54 to 157kPa for undisturbedrnsamples, 26 to 90kPa for disturbed samples. The sensitivity of the soils is less than 2, indicatingrnthat the soils are less sensitive to disturbance. Tangent modulus and secant modulus ofrndeformation varies from 3750 to19000kPa and 2160 to 6300kPa respectively.rnComparison is made for expansive and lateritic soils investigated in this study with previouslyrndone researches in other parts of Ethiopia. The comparison shows that the engineeringrnproperties of these soils from the study area lie within the range given by different researchersrnin the other part of the country.

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