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Evaluation of the Effects of Principal Stress Direction on Dynamic Properties of Unsaturated Sand by Hollow Cylinder Apparatus

Ahmadi Givi, Farid | 2018

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 51229 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Jafarzadeh, Fardin
  7. Abstract:
  8. Soils have an anisotropic response, and mechanical and dynamic properties of this material are greatly dependent on the direction and magnitude of principal stresses. A reliable assessment of the soil behavior and a good estimation of the soil parameters need to perform tests in similar condition with in-situ. From all the devices used for studying anisotropy of soils, hollow cylinder apparatus is the only apparatus that enables an independent control of both the direction and magnitude of the principal stresses. Moreover, one of the most controversial topics in the unsaturated soil dynamics is determining the shear modulus and damping ratio which are used for assessing many geotechnical problems. Therefore, it is important to study the variation of these two parameters versus matric suction and degree of saturation. The cyclic hollow cylinder apparatus of Sharif University of Technology has recently been developed for performing unsaturated tests. In this research, using this apparatus, the effects of principal stress direction and intermediate stress parameter on the shear modulus and damping ratio of unsaturated Babolsar sand were studied. The type of anisotropy studied in this research is induced anisotropy investigated through application of cyclic stress under controlled conditions of α (15, 30, 45, 60, 75 degrees) and b (0, 0.25, 0.5, 0.75, 1). In order to study the effects of matric suction and degree of saturation, the tests were carried out on samples in the initial state conditions and also under matric suctions equal to 3, 5, 9 and 30 kPa. Moreover, the effects of relative density on soil dynamic properties were investigated through three tests series performed on the initial state of the three different samples. The results show that the effect of anisotropy on the shear modulus and, consequently, the shear stiffness of sand is very significant. In all of the tests, with increasing α, the shear modulus decreases to a certain value, and then slightly increases at the end (at α=75°). Moreover, the maximum and minimum value of the shear modulus was obtained at α=15° and α=60° (in very few tests at α=45°) respectively. Moreover, the shear modulus increases and the damping ratio decreases by increase in matric suction. According to the results, it can be stated that the anisotropy does not affect the damping ratio, and there is no specific trend for its variation with α and b
  9. Keywords:
  10. Shear Modulus ; Damping Ratio ; Unsaturated Sand ; Matric Suction ; Hollow Cylinder Apparatus (HCA) ; Intermediate Principal Stress ; Principal Stress Axis Rotation

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