Study Of Index Properties And Shear Strength Parameters Of Laterite Soils In Southern Part Of Ethiopia The Case Of Wolayita - Sodo

Most geotechnical problems such as bearing capacity, lateral earth pressure and slope stabilityrnare related to the shear strength of a soil. The shear strength of a soil can be related to thernstress state of the soil. The stress state variables generally used for an unsaturated soil are thernnet normal stress (_ –ua) and the matric suction. (Ua- Uw).The index properties of soils arernalso essential parameters for soil classification as well as for indirect estimation of itsrnpotential strength.rnBecause of climatic conditions, ground water tables in tropical and subtropical regions arernoften depressed. Evapotranspiration often exceeds infiltration. This leads to deep desiccationrnof the soil profile. Therefore, residual soils frequently exist in an unsaturated state withrncontinuous air in their voids. The pore air pressure will usually equivalent to atmosphericrnpressure, but the pore water pressure will be sub – atmospheric, i.e. negative due to capillaryrneffects in the small pores of soils. This negative pore water pressure or suction produces anrnadditional component of effective stress, or in other words: the effective stress becomesrngreater than the total stress.rnThe extended Mohr Circle failure envelope defines the shear strength of unsaturated soil.rnIdentifying the soil characteristic is essential to determine the type of test and test procedurernthat is applied during sampling, sample preparation and testing.rnIn this thesis, several tests have been conducted to understand the effect of unsaturated soilrnmechanics on the determination of shear strength parameters and comparison of the resultrnwith tests on saturated samples. To this effect the geotechnical and geo-chemicalrncharacteristics of soils sampled from Wolayita – Sodo have been investigated.rnFor shear strength parameters determination, undisturbed soil samples were collected andrnunconsolidated undrained tests and unconfined compression tests were carried out because ofrnthe triaxial machine’s limitation to suction measurements. Both saturated and unsaturatedrntests were conducted to make systematic comparison of the test results.rnxirnMoisture content determination, using oven temperatures of l05°C, 50°C and 35°C wererncarried out on the soil samples to investigate the variation of structural water.rnAtterberg Limits were investigated for different testing procedures on the soil samples tornevaluate the effect of test manipulation on cement bonds between clay clusters. The liquidrnlimit tests were carried out on soil specimens mixed for 5 minutes and 30 minutes durations.rnFrom the test results, the soil under investigation has been affected by manipulation.rnSpecific Gravity Test, Particle Size Distribution, Atterberg Limit Tests and Free Swell Testsrnwere conducted at three different test temperatures (as received moisture, air-dried and ovenrndried at a temperature of 105°c). These tests were conducted in order to understand thernbehavior of the soil in the area and classification.rnAccording to the result of this research, Moisture content ranges between 26 - 41%, PlasticityrnIndex ranges between 19 – 30%, Clay fraction ranges between 48 - 70, Shrinkage limitrnranges11-22, Free swell ranges between 28-38% and Specific Gravity ranges between 2.61-rn2.97. The shear strength parameters(C - _ value) as determined from UU Test ranges betweenrn150-173 Kpa and 12 º -20 º for unsaturated sample respectively. For saturated soil sample,rnthe C- value obtained is 118Kpa and unconfined compression strength (qu) ranges from 215-rn385 Kpa.rnLaterite soils are characterized by high concentration of Iron Oxide, Aluminium Oxidern(Sesqueoxide) and Kaolinite minerals. The soils samples subjected to tests fall below A-linernunder MH (inorganic clay with medium strength) and contain Kaolinite mineral.rnThe activity number which is the ratio of plasticity index to the percent of clay –size frictionrnby weight, is below 0.75, confirming that the soil is inactive.

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