A series of consolidated drained triaxial tests took place in clay-fine sand specimens of different proportions (number of specimens 12) and in clay-fine sand specimens of a constant proportion containing 2 to 12% cement (number of specimens 12). Microfine cement was used with Blaine over 4500 cm~2/g, produced by Titan Co., Greece. Siliceous sand used was uniform with Hazen coefficient 2.45 and its range of particle size was 74 to 840 μm. The basic characteristics of the clay used are: Liquid limit 43.54 %, Plastic limit 25.32%, Water content 61.2%, Activity 0.43, Bulk unit weight 16.68 KN/m~3, Dry unit weight 10.17 KN/m~3. The angle Θ, defined by the exact failure plane and the horizontal line was measured for each specimen. Continuously, the friction angle Φ_f was calculated by using the equation Θ_f = 45 + Φ_f /2 and compared with the one which was drawn from Mohr circles. Concluding, the difference between the values of theoretical and reliable friction angle, for clay-fine sand mixtures, is ranging from 1.5 to 6 degrees with the reliable angle always higher. In the case of clay-fine sand specimens containing cement, the difference between the values is fluctuating from 1 to 4.5 degrees.
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