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A Semi-Emperical Model for Small Starin Shear Modulus of Unsaturated Low Plasticity Soils

Gheibi, Amin | 2015

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 48190 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Khosravi, Ali
  7. Abstract:
  8. The shear modulus of soils is a key material quantity which is frequently used in the evaluation of wave propagation through soil layers in dynamic response analyses for many geotechnical systems. The shear modulus, G, is defined as the slope of the shear stress-strain hysteresis loop and is significantly influenced by the shear strain amplitude induced in a soil specimen. Of particular interest in geotechnical engineering, the value of shear modulus for strains less than 10-6 defined as the small strain shear modulus, Gmax. The early experimental studies on the shear modulus of water-saturated or dry soil showed that shear modulus is highly dependent on mean effective sress, p´, and void ratio, e. However, despite the significant body of work has been developed to evaluate the magnitude of Gmax for saturated and dry soils, the varbiables that impact the magnitude of Gmax for unsaturated soils were less well understood. In later studies on unsaturated soils, the degree of saturation and matric suction were observed to play an important role in the magnitude of Gmax. These studies led to the development of different empirical relationships between Gmax, unsaturated stress state variables (mean net stress and matric suction), the degree of saturation, and the void ratio, e. A major deficiency in these relationships and interpretation of Gmax results is the empirical manner in which the coupling between mechanical and hydraulic loadings was considered. In this study, a semi-empirical model was modified to predict the impact of effective stress state, hydraulic hysteresis, voild ratio on the small strain shear modulus of unsaturated low plactisity soils. Unlike previous empirical relationships, this model consideres coupling between effective stress and void ratio, as well as possible effects of net stress on degree of saturation. The model was then experimentally validated using small starin shear modulus data for two types of soils in reported in litereature as well as data from an independent testing program with an unsaturated triaxial shear test device equipped with bender elements. 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. 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. The model was observed to follow the experimental data along different paths of hydro-mechanical loading
  9. Keywords:
  10. Unsaturated Soil ; Bender Element Test ; Semi Emprical Model ; Maximum Shear Modulus ; Coupled Behavior ; Bony Silt ; Void Ratio ; Hydraulic Hystersis

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