Unsaturated Shear Strength Characterstics and Stress - Strain Behaviour of Silt Soils of Hawassa
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2011-11
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Addis Ababa University
Abstract
Most of the shear strength tests and studies done in this country particularly in Addis Ababa are using the conventional triaxial machine, which is unable to measure the matric suction of the soil. However, a few research works have recently been done on the shear strength characteristics for unsaturated soils in Addis Ababa. These previous studies on the unsaturated soil are only on red clay and expansive soils of certain areas of Addis Ababa and a single study on Arbaminch expansive soils. This thesis provides the results of triaxial tests which were carried out to study the unsaturated shear strength characteristics of Hawassa silt soil with suction measurement. The soil tested was reconstituted using dynamic compaction. Consolidated undrained (CU) triaxial tests were performed on both saturated and unsaturated samples.
In this work, disturbed silt soil samples were collected from test pits dug to a depth of 2.5m and the shear strength tests were conducted on compacted samples. The saturated shear tests were carried out for consolidation pressures of 100 kPa,200 kPa and 300 kPa.The unsaturated shear tests were carried out for same net confining pressure of 200 kPa but with varying matric suctions of 30 kPa,60 kPa,100 kPa,120 kPa and 150 kPa. All the specimens tested were compacted at the same initial moisture content and dry density corresponding to field conditions.
The results from the consolidated undrained triaxial tests disclosed that as the matric suction increases from 0 to 150 kPa, the maximum deviator stress increases from 202.48 kPa to more than double of 443.14 kPa under the net confining pressure of 200 kPa. This signifies that shear strength of the silt soil sample considerably increases due to matric suction and the rate of increase has been found to be significantly higher compared to what was observed in clays under unsaturated condition. Moreover, the shape of the stress-strain curve of the unsaturated cases resemble that of the saturated samples and for almost the whole range of the applied matric suction, the increase in matric suction did not affect the shape of the stress–strain curve.
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Civil Engineering