Masters Thesis

Application of Shansep and Hvorslev's Theories to Evaluate the Shear Strength of Over-consolidated Clays in Mineralogical Framework.

This paper presents the results of a study, whose aim was to determine the undrained shear strength at different over-consolidation ratios, which is a very important parameter to evaluate the stability of natural and man-made slopes in soft clay. The undrained shear strength of clays was determined using a laboratory test method utilizing the Direct Simple Shear (DSS) apparatus in the geotechnical engineering laboratory at California State University, Fullerton. In this study, the change in undrained shear strength of soil with over-consolidation ratio in a mineralogical framework was studied. Four different soil samples were prepared by mixing commercially available clay minerals such as kaolinite and montmorillonite with quartz at different proportions by their dry weight. These samples included 100% kaolinite, a mixture of 70% kaolinite with 30% quartz, a mixture of 50% kaolinite with 50% quartz and a mixture of 50% montmorillonite with 50% quartz. The plasticity characteristics of these samples were evaluated. Each of the first three samples had five different specimens representing five different over-consolidation ratios (2, 4, 8, 16 and 32). The fourth sample had only two specimens for two different over-consolidation ratios, i.e. 2 and 4. The applied consolidation stresses were 600 kPa, 300 kPa, 150 kPa, 75 kPa and 37.5 kPa for five different over-consolidation ratios of 2, 4, 8, 16 and 32, respectively. Using the direct simple shear device, the undrained shear strength of these samples were measured using a strain rate of 5%/hour. The pore pressures generated at different applied stresses was also back calculated from the change in total stresses. The pore water pressure continuously increased up to certain displacement and then after tended to remain constant. The results showed that it was inversely proportional to the over-consolidation ratio. Using the results, the SHANSEP model and Hvorslev’s theory were utilized to check normalized shear strength, and true friction angle and true cohesion of each soil sample, respectively. The result showed that the shear strength depends up on the composition of clay minerals and stress history of the soil. The relationship of the normalized undrained shear strength ratio was directly proportional to the overconsolidation ratio of the soil. Similarly, the true friction angle of the soil depended up on the composition of the clay minerals, but not on the stress history. True friction angles of 19.28°, 20.63°, 21.06° and 35.24° were obtained for Sample Nos.1, 2, 3 and 4, respectively; whereas, the true cohesion of these sample were measured as 8.46°, 7.21°, 4.55° and 0.39° respectively.

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