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A Microstructural Investigation on the Influence of Pore Fluid Osmotic Potential on Volume Changes and Soil Water Retention Curve, Low Plasticity Clay

Heydari, Ali | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 54662 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Sadeghi, Hamed
  7. Abstract:
  8. Understanding many issues related to environmental geotechnics and soil sciences such as: instability and collapse of soil slopes, sinkholes, leakage from landfills and isolation of nuclear landfills, and optimization Agricultural processing depends on our knowledge of the behavior of unsaturated soils and their volume changes. To better understand the behavior of unsaturated soils, it is important to study the soil-water retention curve (SWRC) and the factors affecting it. These include factors such as porosity, grain size distribution, soil minerals, pore size distribution, and pore water chemistry, which affect soil-water retention curves and volume changes. Since the simultaneous effect of microstructure and pore water chemistry on the soil-water retention curve and volumetric changes in the wetting and drying cycle has not been studied so far, we are faced with different microstructures in nature that are exposed to chemical compounds. The purpose of this study is to investigate the effect of these two factors on the soil-water retention curve and volume changes.In the present study, in order to achieve the desired goals, it is necessary to use different methods and devices. First, to create flocculated and dispersed microstructures in the soil, according to the soil compression curve, we use static compaction and slurry consolidation devices. Then, to apply and measure suction and volume changes, the axis translation method in the automatic unsaturated oedometer and filter paper with caliper is used, to evaluate the whole range of suction and volume changes for the samples. Finally, in order to qualitatively study two-dimensional and three-dimensional soil microstructure, two methods of scanning electron microscopy and micro-CT scan have been used.The results of this study in the low suction range show that the sample with dispersed microstructure has more retention than the sample with flocculated microstructure. The reason for this phenomenon is the small diameter of the pores and their uniformity in the dispersed microstructure. This uniformity in pore diameter reduces hydraulic hysteresis in a sample with dispersed microstructure compared to a flocculated sample. The importance of hydraulic hysteresis is in simulating and studying the effect of climatic processes of evaporation, transpiration and precipitation on soil deposits. Volumetric changes in saturation are greater for dispersed microstructure than for flocculated microstructure due to easier movement of soil particles. However, in the unsaturated state, due to the larger diameter of the pores, volumetric changes are predominant for the flocculated microstructure. In the presence of salt, the dispersed microstructure due to the effects of the diffuse double layer theory, such as changes in the forces of repulsion and adsorption between soil particles, leads to flocculated microstructure, this phenomenon reduces the volume changes for the sample with dispersed microstructure in the presence of salt. It was also shown that different cations, depending on the hydration radius and capacity, have different effects on the soil-water retention curve. In general, in the high suction range, the cation with higher capacity or larger hydration radius has more retention, but in the low suction range, the high-capacity cation has less retention due to the larger effect on the diameter of soil pores
  9. Keywords:
  10. Soil-Water Retention Curve ; Microstructure ; Hysteresis ; Scanning Electron Microscopy (SEM) ; Automatic Unsaturated Oedometer ; Micro CT scan ; Filter paper

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