Characterization of Black Cotton Soil from Index Properties Along Sebeta-Tefki Road Section
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Date
2005-07
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Addis Ababa Universty
Abstract
This MSc thesis work is entitled "Characterization of Black Cotton soil from Index Properties in
Sebetta area". The study area located at 32km, Southwest of Addis Ababa in the Oromia
regional state; covering an area of 126 km2.
The main objective of the study is to characterize the black cotton soil of the area from index
properties. The index properties define certain physical properties used basically for
classification but also for correlation with engineering properties.
The methodologies employed to achieve the objective are literature review pertaining to the
area, filed data collection on soil and rocks, laboratory testing of soil samples and data analysis
and interpretation.
The study revealed that the geology of the area is represented by rocks of Tertiary age (65my)
namely; basalt, andesite, and ignimbrite, and Quaternary soil units. Four engineering geological
rock units and three soil units were mapped.
Disturbed and undisturbed soil samples were analysed for index tests, namely, particle size
distribution, Atterberg limits, free swell, and shrinkage limits, in the Central Laboratory of
Geological survey according to BS 1377: part 2: 1990 standard.
Laboratory test results from the present and previous studies have shown that fines are over 90
%. Liquid limit (LL) and plastic limit (PL) values are ranging from 50-110 %, 24-50 %
respectively. The free swell varies from 118-140 %, while the shrinkage limit test values range
from 15-20 % respectively.
The main index properties used for identification and classification of black cotton soil are clay
fraction (minus 2 micron), Atterberg limits, free swell, and shrinkage limits. Accordingly, the
black cotton soil is potentially active and requires countermeasures
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Keywords
Black Cotton Soil