Abstract:
The classification properties and composition of lateritic soils, developed on
Tertiary basalt, granite and metamorphic rocks like gneiss and Shists in Arbaminch areas have
been investigated. X- ray diffraction analyses showed that the soil comprised minerals of Kaolinite
and Smectite group such as Kaolinite, Quartz, Dickite, Nacrite, Muscovite, Vermiculite, Geigerite,
Zektzerite, Tridymite and Gibbsite. Chemical analyses showed there was a Concentra tion of
sesquioxide minerals but no amorphous material present in the soil.
For the lateritic soil of Arbaminch , unlike the oxidation of the free secondary clay minerals, the
fact that there is no pronounced structural water found and/or amorphous clay minerals showed
that, dehydration does not significantly affect the index and/or engineering properties of lateritic
soils of this area.
Analysis of specific gravity of the soils shows that, increase in temperature from its in situ
condition to an air- dri ed equivalent of oven temperature reduces specif ic gravity.
Analyses of the grain size distribution and other physical properties indicated
That the soil was not homogeneous within the laterite zone. Atterberg Limit test results for the
soil falls below the Casagrande A- line. There were significant differences between the values of
plasticity index determined for as received condition compared with air dried samples. High
values for both the liquid and plastic limits may be due to the water retention capacity of porous
micro- aggregations in the soil. This can be correlated with the degree of laterisation, which
depends on the ambient climatetopographyandvegetation of the area.
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