Detail Investigation on Index Properties of Lateritic Soils: The Case of Nedjo-Mendi-Assosa
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Date
2008-03
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Addis Ababa University
Abstract
The classification properties and composition of lateritic soils, developed on
Tertiary basalt, granite and metamorphic rocks like gneiss and Shists in Nedjo,
Mendi and Assosa 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
concentration of sesquioxide minerals but no amorphous material present in the
soil.
Comparison between moisture content at air-dry temperature (50oC with 30%
relative humidity) and that of oven drying at 105oC indicates that there is no
structural water.
For the lateritic soil of Nedjo, Mendi and Assosa, 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.
In all the cases there is no significant difference observed between the Atterberg
limits obtained from test on a specimen mixed for 5min and that of 25min, this
indicates that the cementation of the clay particles is too high to be
disaggregated with in 25min further mixing and can also be related to the degree
of laterization. Analysis of specific gravity of the soils shows that, increase in
temperature from its in situ condition to an air-dried equivalent of 50oC oven
temperature reduces specific gravity by an average of 4%.
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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 microaggregations
in the soil. This can be correlated with the degree of laterisation,
which depends on the ambient climate, topography and vegetation of the area.
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Keywords
Nedjo;Mendi ;Assosa