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Characterizing the Effect of Fines Content on Small Strain Shear Modulus of Unsaturated Sandy Soils by Bender Element Test

Jebeli, Mohammad Reza | 2019

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 52456 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Haeri, Mohsen; Khosravi, Ali
  7. Abstract:
  8. Small strain shear modulus, Gmax, is one of the most important parameters for characterization of the behavior of earth structures subjected to static or dynamic loading conditions. This research presents an experimental laboratory study on the effect of non-plastic fines content and hydraulic hysteresis on the maximum shear modulus of unsaturated sand-silt mixtures. In this regard, clean Firoozkuh No. 161 silica sand which is classified as poorly graded uniform sand was mixed with different percentages of silica non-plastic silt. A set of bender element tests were planned to carry out implementing two modified triaxial devices. The modifications on these two apparatus were to add HAV ceramic discs for air-water control of unsaturated specimens, in addition to include piezoelectric bender elements to send and receive shear and compressional waves and measure their velocities within the unsaturated silty sand specimens. Axis translation technique was implemented for hydromechanical purposes and bender element for shear wave velocity measurements and calculating Gmax. The results of previous studies have shown that the Gmax of sandy soils changes in an up and down manner in drying path while Gmax of silty specimens increase in a nonlinear manner with matric suction. To keep the specimens in the same situation for comparison, all specimens were prepared with an initial void ratio of 0.7, and were desaturated under 100 kPa constant mean net stress to a suction of either 48 or 128 kPa. Adding fines to the sandy soil gradually leads to the reduction of small strain shear modulus when saturated. Ironically, adding a few percentages of fines content as low as 10 percent to an unsaturated sandy specimen causes increase of Gmax with matric suction. Besides, samples with at least 10% fines content have higher small strain shear modulus in the wetting path than those in the drying path despite observation on clean sands. Based on the results of this study, a new semi-empirical model is developed to predict small strain shear modulus of unsaturated sand-silt mixtures considering different Gmax behaviors of sandy and silty soils with matric suction
  9. Keywords:
  10. Small Strains ; Shear Modulus ; Bender Element Test ; Unsaturated Sand ; Non-Plastic Fine

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