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)
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Date
2009-11
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Addis Ababa University
Abstract
The appropriate laboratory testing procedures while carrying out index property tests
of lateritic soils found in western Ethiopia (Nedjo, Jarso and Begi areas) have been
investigated. Classification and composition of these soils have also been studied. The
X-Ray Diffraction (XRD) analyses revealed that the soils in these areas are composed
of minerals of Kaolinite and Smectite groups; such as Kaolinite, Quartz, Hematite,
Vermiculite, Dickite, Borax, Illite, Nacrite. The X-Ray Fluorescent (XRF) analysis
showed that there is a high concentration of sesquioxide minerals and there is no
amorphous material present in the soils.
Comparison between the moisture contents determined using air drying (or
alternatively 50oC oven drying temperature) and that of 105oC oven drying
temperatures indicates that there is no significant amount of structural water;
however, comparing to the non-lateritic soils from Addis Ababa, the western Ethiopia
lateritic soils have some amount of structural water or water of hydration.
In the analysis of Atterberg Limits, in majority of the soils difference was observed
between the Atterberg Limits obtained from test on a specimen mixed for 5min and
that of 30min, this indicates that the cementation of the clay particles is to be
disaggregated with higher mixing time.
Analysis of specific gravities of the soils using various pretreatment and testing
procedures showed that, increase in drying temperature from its in situ condition to an
air-dried or equivalent of 50oC oven drying temperature and further to 105oC oven
drying temperature reduces specific gravity of the soils.
Analyses of the grain-size distributions (grading curves) with respect to the effects of
different testing procedures (pre-test treatment methods), the effects of specific
gravity variation within a sample, and the variation of sample location /depth/ and
other physical properties indicated that the soils were not homogeneous within the
laterite zone. The dry sample preparation and testing procedures tends to decrease the
percentage of coarser particles as the coarser particles became highly fractured during
the pulverization process. As a result these dry sieving techniques were found to be
inappropriate. The analyses of the grading curves using mass proportion and the
modified grading curve using volume proportion, which uses separate specific
gravity, showed that the two grading curves are nearly the same; hence, the need of
modifying the grading curves by volume proportion is not as such important
especially when wet sieving method is used. Moreover, the analysis of grading curves
with respect to sampling depth showed that, generally, the soils become coarser as the
sampling depth increases. This confirms that the degree of weathering decreases with
depth.
Soil classification using Unified Soil Classification System (USCS), AASHTO
Classification System, Wesley’s Classification Method (classification using
mineralogical composition) and soil grouping using genetic basis and soil forming
factors were analyzed. According to the USCS chart the soils under investigation falls
in the MH zone, which means that the soils have ‘poor’ engineering properties that
are considered unsuitable for various engineering purposes. However, in practice,
such soils frequently have good engineering properties especially for construction of
sub grades and embankments. Contrary to the USCS, the AASHTO Classification
System classify majority of the soils under sub group A-2-7 with a group index less
than 4, which mean the soils are good sub grades and embankments construction
materials. Using the Wesley’s Classification Method the soils fall under Group C,
sub-group (c). Soils under this group have good engineering properties. The soils
under investigation have fallen under ferrisols group where erosion has kept pace with
profile development.
The analysis of the test results obtained from different laboratory testing procedures
used for moisture content, plasticity tests, grain-size analysis and specific gravity tests
for the lateritic soils found in Western Ethiopia has shown that the soils are sensitive
to the type of testing procedures as a result an appropriate laboratory testing procedures attached in Appendix-A are proposed.
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Keywords
Soils ; Western ;Ethiopia