Study of Index Properties and Shear Strength Parameters of Laterite Soils in Southern Part of Ethiopia the Case of Wolayita - Sodo
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Date
2008-05
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Addis Ababa University
Abstract
Most geotechnical problems such as bearing capacity, lateral earth pressure and slope stability
are related to the shear strength of a soil. The shear strength of a soil can be related to the
stress state of the soil. The stress state variables generally used for an unsaturated soil are the
net normal stress (_ –ua) and the matric suction. (Ua- Uw).The index properties of soils are
also essential parameters for soil classification as well as for indirect estimation of its
potential strength.
Because of climatic conditions, ground water tables in tropical and subtropical regions are
often depressed. Evapotranspiration often exceeds infiltration. This leads to deep desiccation
of the soil profile. Therefore, residual soils frequently exist in an unsaturated state with
continuous air in their voids. The pore air pressure will usually equivalent to atmospheric
pressure, but the pore water pressure will be sub – atmospheric, i.e. negative due to capillary
effects in the small pores of soils. This negative pore water pressure or suction produces an
additional component of effective stress, or in other words: the effective stress becomes
greater than the total stress.
The extended Mohr Circle failure envelope defines the shear strength of unsaturated soil.
Identifying the soil characteristic is essential to determine the type of test and test procedure
that is applied during sampling, sample preparation and testing.
In this thesis, several tests have been conducted to understand the effect of unsaturated soil
mechanics on the determination of shear strength parameters and comparison of the result
with tests on saturated samples. To this effect the geotechnical and geo-chemical
characteristics of soils sampled from Wolayita – Sodo have been investigated.
For shear strength parameters determination, undisturbed soil samples were collected and
unconsolidated undrained tests and unconfined compression tests were carried out because of
the triaxial machine’s limitation to suction measurements. Both saturated and unsaturated
tests were conducted to make systematic comparison of the test results.
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Moisture content determination, using oven temperatures of l05°C, 50°C and 35°C were
carried out on the soil samples to investigate the variation of structural water.
Atterberg Limits were investigated for different testing procedures on the soil samples to
evaluate the effect of test manipulation on cement bonds between clay clusters. The liquid
limit tests were carried out on soil specimens mixed for 5 minutes and 30 minutes durations.
From the test results, the soil under investigation has been affected by manipulation.
Specific Gravity Test, Particle Size Distribution, Atterberg Limit Tests and Free Swell Tests
were conducted at three different test temperatures (as received moisture, air-dried and oven
dried at a temperature of 105°c). These tests were conducted in order to understand the
behavior of the soil in the area and classification.
According to the result of this research, Moisture content ranges between 26 - 41%, Plasticity
Index ranges between 19 – 30%, Clay fraction ranges between 48 - 70, Shrinkage limit
ranges11-22, Free swell ranges between 28-38% and Specific Gravity ranges between 2.61-
2.97. The shear strength parameters(C - _ value) as determined from UU Test ranges between
150-173 Kpa and 12 º -20 º for unsaturated sample respectively. For saturated soil sample,
the C- value obtained is 118Kpa and unconfined compression strength (qu) ranges from 215-
385 Kpa.
Laterite soils are characterized by high concentration of Iron Oxide, Aluminium Oxide
(Sesqueoxide) and Kaolinite minerals. The soils samples subjected to tests fall below A-line
under MH (inorganic clay with medium strength) and contain Kaolinite mineral.
The activity number which is the ratio of plasticity index to the percent of clay –size friction
by weight, is below 0.75, confirming that the soil is inactive.
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Keywords
Wolayita - Sodo