Investigating the Index Properties of Residual Tropical Soils of Western Ethiopia (The Case of Asossa) BY Fekede Wakuma January
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Date
2007-01
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Addis Ababa University
Abstract
The engineering behavior of soils, wheather formed under temperate or tropical
conditions, is determined by certain physical characteristics designated as engineering
properties. In practice, since the determination of all the engineering properties of soils is
expensive, often index properties, simpler and cheaper engineering characteristics which
are indicative are investigated.
Residual soil is a soil material derived from the in-situ weathering and decomposition of
rock, which has not been transported from its original position.
Residual tropical soils can have characteristics that are quit distinctively different from
those of transported soils, That is: the conventional concept of soil mechanics is not
applicable to many residual soils as it consists of aggregates or crystals of weathered
mineral matter that break down and become progressively finer under continuous
manipulation.
Western part of Ethiopia is characterized by such soils. As mentioned in ERA (Ethiopian
Roads Authority) design manual (ERA, 2001) Residual laterites are widely available soil
material in these areas. The index properties of such soils have not been studied in detail
as per the test recommendations for residual tropical soils. This Thesis is conducted to
indicate the index properties of soils of Asossa Town (Capital of Beneshangul Gumuze),
which is located in western Ethiopia and covered by residual soils.
In this thesis work, the index properties of residual tropical soil have been investigated on
the soil specimens taken from Asossa Town by following the test procedures
recommended for residual soils.
The specimens were tested at different sample preparation conditions prior to conducting
the laboratory test and resulted in different end values. The index property investigation
in this research includes; Specific Gravity Test, Particle Size Distribution, Atterberg
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Limit Tests and Free Swell Tests, all tests were conducted at three different test
temperatures (at received moisture, air-dried and oven dried at a temperature of 105o
c ).
Generally it was found that, sample pre-treatment and commonly used soil test
procedures for temperate soils when applied to Asossa soils resulted in completely altered
test result as compared to the actual test result values obtained by following testing
procedures recommended for tropical soils. It was found that there is considerable
structural water that could be destroyed by oven drying at a temperature of 105 degrees
and must be deducted from every moisture content determinations in Atterberg Limit
tests. Sample manipulation is also found to alter the results of the Atterberg Limit values
by affecting the bond between soil structures. Therefore, the appropriate test procedures
for tropical soils are strongly recommended for research and practical engineering
application where such soils are likely to exist.
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Western Ethiopia