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Title: Stress - strain - strength characteristics of decomposed phyllite in the triaxial compression test
Authors: Acquaah, Vincent Kofi Abaka
Issue Date: 21-Mar-1999
Series/Report no.: 2665;
Abstract: Decomposed phyllite constitutes one group of soils termed as “problem soils” in Ghana. The design and construction of civil engineering structures require the knowledge of the strength and the deformation characteristics of the foundation material. To determine these characteristics, undisturbed samples are used for laboratory tests and analyses. The practice in Ghana is to use borehole and pit samples to obtain these undisturbed samples which are considered to be soils in their natural condition without disturbance of their structure, water content density, porosity and stress conditions. This study investigates some of the stress-strain characteristics and strength of a decomposed phyllite in Kumasi and how the method of sampling affects these characteristics. The decomposed phyllite used in this study was obtained from two different locations at depths up to 3.0m from the ground surface. The strength characteristics were studied by investigating parameters from the consolidated undrained triaxial compression test of both undisturbed and remoulded samples. The remoulded samples in this study were achieved by compaction to similar density and water content to simulate the undisturbed in- situ nature. Seismic refraction surveys and standard penetration tests were also conducted to evaluate the in-situ strength and stiffness characteristics of the decomposed phyllite. The stress - strain curve and the Secant Modulus against the axial strain relationships are presented and discussed. The stress path is also presented and discussed. The results show that the shear strength defined in terms of normalised maximum deviator stress was higher for the Pit specimens at consolidation pressure of 1 50kPa as compared to specimens from the borehole and that remoulded. This normalised shear strength averages 2.1 followed by that of the remoulded with a value of 1.9. The borehole specimens have the least value of about 1.6 at the same consolidation pressure of 150kPa. With consolidation pressures of 300 and 400kPa, the pit specimens gave the least normalised shear strength averaging as 1.1 for both consolidation pressures and that of the borehole and remoulded specimens giving normalised shear strength of about 1.5. In terms of effective stresses, the angle of friction for the site ranged between 33 and 38° with the Pit specimens having the least angle of friction, averaging as 34°, with the highest occurring in the case of the remoulded specimens, averaging as 37°. The cohesion intercept showed very small variation irrespective of method of sampling, ranging between 12 and 1 5kPa. In terms of total stresses, the angle of friction ranged between 15 and 26° with the pit specimens again giving the least angle of friction, averaging as 16° and the highest occurring in the case of the remoulded specimens averaging as 24.5°. The results show that the undisturbed samples obtained either by the percussion drilling or the use of tube samples from the pits have been subjected to various degree of disturbances. The average Shear Modulus obtained from the consolidated undrained triaxial compression is 39MN/m2, and that obtained using the seismic refraction survey for the horizon studied is approximately 300MN/m2. The N value obtained from the standard penetration tests for the studied horizon ranges between 7 and 8 blows per 300mm drive.
Description: A thesis submitted to the Board of Postgraduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirement for the award of the Degree of Master of Philosophy in Civil Engineering, 1999
URI: http://hdl.handle.net/123456789/3261
Appears in Collections:College of Engineering

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