Investigation Into The Appropriate Laboratory Testing Procedures For The Determination Of The Index Properties Of The Lateritic Soils Of Western Ethiopia (nedjo-jarso-begi Road Area)

The appropriate laboratory testing procedures while carrying out index property testsrnof lateritic soils found in western Ethiopia (Nedjo, Jarso and Begi areas) have beenrninvestigated. Classification and composition of these soils have also been studied. ThernX-Ray Diffraction (XRD) analyses revealed that the soils in these areas are composedrnof minerals of Kaolinite and Smectite groups; such as Kaolinite, Quartz, Hematite,rnVermiculite, Dickite, Borax, Illite, Nacrite. The X-Ray Fluorescent (XRF) analysisrnshowed that there is a high concentration of sesquioxide minerals and there is nornamorphous material present in the soils.rnComparison between the moisture contents determined using air drying (orrnalternatively 50oC oven drying temperature) and that of 105oC oven dryingrntemperatures indicates that there is no significant amount of structural water;rnhowever, comparing to the non-lateritic soils from Addis Ababa, the western Ethiopiarnlateritic soils have some amount of structural water or water of hydration.rnIn the analysis of Atterberg Limits, in majority of the soils difference was observedrnbetween the Atterberg Limits obtained from test on a specimen mixed for 5min andrnthat of 30min, this indicates that the cementation of the clay particles is to berndisaggregated with higher mixing time.rnAnalysis of specific gravities of the soils using various pretreatment and testingrnprocedures showed that, increase in drying temperature from its in situ condition to anrnair-dried or equivalent of 50oC oven drying temperature and further to 105oC ovenrndrying temperature reduces specific gravity of the soils.rnAnalyses of the grain-size distributions (grading curves) with respect to the effects ofrndifferent testing procedures (pre-test treatment methods), the effects of specificrngravity variation within a sample, and the variation of sample location /depth/ andrnother physical properties indicated that the soils were not homogeneous within thernlaterite zone. The dry sample preparation and testing procedures tends to decrease thernpercentage of coarser particles as the coarser particles became highly fractured duringrnthe pulverization process. As a result these dry sieving techniques were found to berninappropriate. The analyses of the grading curves using mass proportion and thernmodified grading curve using volume proportion, which uses separate specificrngravity, showed that the two grading curves are nearly the same; hence, the need ofrnmodifying the grading curves by volume proportion is not as such importantrnespecially when wet sieving method is used. Moreover, the analysis of grading curvesrnwith respect to sampling depth showed that, generally, the soils become coarser as thernsampling depth increases. This confirms that the degree of weathering decreases withrndepth.rnSoil classification using Unified Soil Classification System (USCS), AASHTOrnClassification System, Wesley’s Classification Method (classification usingrnmineralogical composition) and soil grouping using genetic basis and soil formingrnfactors were analyzed. According to the USCS chart the soils under investigation fallsrnin the MH zone, which means that the soils have ‘poor’ engineering properties thatrnare considered unsuitable for various engineering purposes. However, in practice,rnsuch soils frequently have good engineering properties especially for construction ofrnsub grades and embankments. Contrary to the USCS, the AASHTO ClassificationrnSystem classify majority of the soils under sub group A-2-7 with a group index lessrnthan 4, which mean the soils are good sub grades and embankments constructionrnmaterials. Using the Wesley’s Classification Method the soils fall under Group C,rnsub-group (c). Soils under this group have good engineering properties. The soilsrnunder investigation have fallen under ferrisols group where erosion has kept pace withrnprofile development.rnThe analysis of the test results obtained from different laboratory testing proceduresrnused for moisture content, plasticity tests, grain-size analysis and specific gravity testsrnfor the lateritic soils found in Western Ethiopia has shown that the soils are sensitivernto the type of testing procedures as a result an appropriate laboratory testing procedures attached in Appendix-A are proposed.

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