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- Type of Document: Ph.D. Dissertation
- Language: Farsi
- Document No: 49369 (09)
- University: Sharif University of Technology, Hong Kong University of Science and Technology
- Department: Civil Engineering
- Advisor(s): Jafarzadeh, Fardin; W. W. NG, Charles
- Abstract:
- Loess soil layers in semi-arid to arid areas have been deposited through the accumulation of aeolian sediments, resulted in a special metastable and open structure being usually unsaturated in nature. Since naturally occurred loess is widespread, it has been recently considered as a promising resource for constructing earth-fill structures. However, due to different microstructures of intact and recompacted loess, different responses to hydraulic and mechanical loading would be expected. Up to now, the hydro-mechanical behavior of loess from different perspectives has not been linked to its microstructural characteristics. The current research, therefore, aims to provide insight into the behavior of unsaturated loess at relatively dry states, the dominant in-situ conditions which have been received less attention from both scientific and practical points of view. More importantly, the significance of microstructure on water retention curve (WRC), volumetric behavior, dilatancy, shear strength, and dynamic properties of loess is revealed and highlighted. To achieve these goals, a comprehensive series of laboratory element tests as well as microstructural investigations were conducted. A new shear box chamber was specifically developed for high suction range based on the principles of vapor equilibrium technique. In addition, a modified pressure plate, an environmental chamber, a resonant column device and an oedometer cell were utilized. The complementary microstructural studies included scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests. Results of WRC tests showed that hysteresis was significant in the suction range of zero to 140 MPa. However, it was more pronounced for intact loess at suction around the air-entry value (AEV) as a consequence of constricted large pores associated with the ink-bottle effects, being supported by microstructural observations. A conceptual model was proposed to link a WRC to the corresponding pore size density (PSD) function and hence to capture the key features of the WRC. Compared with recompacted loess, intact loess shrunk less (by 50%) and yielded at a higher suction. Differences in terms of water retention and volumetric characteristics between intact and recompacted loess were attributed to different stress and suction histories, which in turn resulted in two distinct microstructures. Recompression index, κ, was reduced by one order of magnitude as suction increased from zero to 230 MPa, implying the enhancement of material rigidity due to aggregate stiffening. The peak shear strength continuously increased with suction even at high range (8 to 230 MPa) but at a reduced rate, which cannot be predicted by existing single effective stress concept or two independent stress state variables approach. This is due to the observed linear increase in dilatancy with suction, which is likely caused by the progressive aggregation of silt-clay particles as suction increased. Moreover, recompacted loess showed higher dilatancy than that of intact loess under identical conditions. This is because of more non-uniform distribution of voids in the latter than in the former. For the first time, the macrovoid ratio was introduced as a new state parameter instead of void ratio for describing shear behavior of unsaturted fine-grained soils like loess in a more rational way. Differences in the elastic shear strain threshold are negligible for specimens with different microstructures at the initial state (close to saturation). However, the threshold strain decreased up to one order of magnitude and both damping ratio and shear modulus degradation attained higher rates with respect to shear strain as suction increased to 40 MPa. This is due to suction-induced silt-clay aggregation as demonstrated by SEM photos. The influence of high suction on shear modulus degradation and damping of unsaturated loess was revealed to be as important as the effects of plasticity index (PI) on dynamic properties of saturated soils. In other words, desiccated fine-grained soils like loess at high suctions behave similar to non-plastic coarse-grained saturated soils. This observation, in fact, makes the response of a relatively dry geo-structure to dynamic loading very different from the response of the same soil under saturated condition. This crucial finding has not been considered in geotechnical earthquake engineering design so far
- Keywords:
- Shear Strength ; Dynamic Properties ; Dilatancy ; Hydromechanical Behavior ; Microstructure ; Unsaturated Soil ; Loess Soil ; Soil-Water Retention Curve
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