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Characterizing the Impact of Void Ratio and Stress Path on the Small Strain Shear Modulus of Unsaturated Silt

Rahimi, Mehrzad | 2015

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 48179 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Khosravi, Ali
  7. Abstract:
  8. Early efforts in soil dynamics have shown that the analysis of soil behavior under cyclic or random loading conditions is highly dependent on some representative material properties, the dynamic shear modulus, the Poisson’s ratio, and the damping ratio. Of particular interest is the value of dynamic shear modulus for strains less than 10-6 defined as the maximum or small strain shear modulus Gmax. Improvements in experimental unsaturated soil mechanics have led to the definition of trends between Gmax with different parameters most notably unsaturated stress state, degree of saturation and void ratio. However, the tests in these studies were mostly conducted on specimens experiencing hydraulic hysteresis under constant net confining stress conditions with small/no change in volume and the effect of volume change and stress path on the small stress shear modulus were not fully understood. As a type of complicated engineering material, the stress-deformation behavior of fine grained soils is known to be highly dependent on the initial stress state, the current stress path and the stress path direction. Therefore, in order to allow for a satisfactory analytical description of the behavior, it is important to accurately investigate the behavior of soils under different stress state conditions and stress path directions. In this study, the results of the bender element tests were used to investigate the role of void ratio and stress path on the small strain shear modulus of an unsaturated compacted silty soil. The test setup used in this study incorporated the axis translation technique for suction control, a bender element system to measure shear wave velocity, a high pressure hydraulic pump to apply high confining pressures to the specimens, and a Digital Image Processing (DIP) method to measure volume changes during loading. A signal processing technique was also modified for the analysis of bender element test results. The small strain shear modulus was measured on specimens subjected to two different stress paths: Isotropic compression under constant matric suction conditions; and drying under constant net confining stress conditions. Results indicate that the small strain shear modulus is highly dependent on the extent of loading and drying/wetting. For the same values of mean net stress, Gmax of unsaturated soils subjected to isotropic compression were often larger than those subjected to drying. The results presented in this study and values of Gmax reported in literature were also used to evaluate the reliability of an existing approach in predicting the small strains shear modulus of unsaturated soils under different loading conditions
  9. Keywords:
  10. Bender Element Test ; Small Strains ; Maximum Shear Modulus ; Bony Silt ; Hydraulic Hystersis ; Void Ratio

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