| Based on the previous academic research of Luochuan loess profile, this paper is considered from macroscopic, mesoscopic and microscopic ways,and systematically studied grain-size composition, physical property index, type of microstructure, structural parameter, compressive (collapsible) property, structural strength and shear strength of loess profile in Luochuan by means of routine soil test, high pressure consolidation test, triaxial shear test, laser particle size analysis technology, electron microscopic scanning and CT scanning tests. First, we qualitatively analyze the relationships between loess physical properties and microstructure under different sedimentary environment and stress history, and the differences on loess profiles. Following, on the basis of structural parameter and structural strength quantitative study, this paper focuses on the correlation between loess structural parameter and compressive (collapsible) property, and between structural strength and shear strength. The last but not the least, applying the model of soil microstructure and the principle of general effective stress, we reveal the mechanical mechanism of loess structural compressive failure and shear failure, which provides a method for profound study on the calculation of loess structural compression deformation, chosen of shear strength parameter and analysis of stability. So there is important academic significance and application value on solving loess project problem, and much contribution to the research and application of soil structure theory. The main contributions are as follows:(1) We take tests about grain-size composition, pore structural characteristics, microstructure and major physical property index. Result shows that the grain size of each layer is mainly coarse silt (content more than 50%), scattering in a fairly equal manner and showing no evident of grain size distribution. However, the differences of loess microstructure on the profile contribute to the sedimentary environment and stress history.The structure of Q4, Q3 neo-loess presents the bracketed-macropore weak-cemented structure and half-cemented structure characteristics, which reflects the result of interaction of dry and cold climate and short sedimentary history with low stress.The structure of Q2 old loess presents the inlaid-micropore weak-cemented structure and half-cemented structure characteristics, which reflects the material effect by long sedimentary history with high stress, while the tiny effect by dry and cold climate.The structure of Q2 paleosol presents the flocculated-cemented structure and clotted cemented structure characteristics, which reflects the result of interaction of wet and hot climate and long sedimentary history with high stress.There is a perfect correspondence between loess physical property index and microstructure along with the change on the profile, illustrating that loess'structure controls its engineering properties.(2) From high pressure consolidation test, we quantitatively analyze the relationship between the parameter of strain comprehensive structure potential (strain-CSP) and content of water for each loess layer. It finds that the deeper the loess profiles, the weaker the water sensitivity of Luochuan loess. This paper introduces the definitions and calculation methods of structural stability coefficient, structural variability coefficient and stress comprehensive structure potential (stress-CSP) based on stress, and redefines the concept of over-consolidation ratio. We quantitatively evaluate the structural stability and consolidated state of Luochuan loess profile using these new concepts and methods.The results indicate that structural stability of Q4, Q3 new-loess is poor and at under-consolidated state. Correspondingly, structural stability of Q2 old loess is relatively good and at normal consolidated or slightly over-consolidated state. the research results above comply with loess microstructural properties and engineering practice.(3) With the concept of strain-CSP, we deduce the equations between structural parameter and collapsible coefficient, between water sensitivity and compression coefficient, and quantitatively analyze the variation rules of between structural parameter and water sensitivity parameter on Luochuan loess profile.According to the interaction theory of loess three-phase material, we put forward loess microstructure concept model, and establish general effective stress model of unsaturated soil. Under these models, we explain the mechanical mechanism of loess under-consolidated property and structural compression failure, and conclude the calculation method of structural loess consolidation settlement.(4) We deduce the calculation formula of structural strength and the relation between structural strength and shear strength by the means of the theory of shear strength. Through the triaxial shear test of original loess and remolded loess, we quantitatively analyze the variation rules of loess structural strength and shear strength when profile changes, and the relationships with the content of water.Following, we fit the relationship between the shear strength parameters (i.e. cohesion, internal friction angle) of original loess and remolded loess and the content of water, and discuss the approximate method to estimate original loess strength parameters by remolded loess shear strength parameters, which provide new way to solve the problems that there is great data dispersion and strength parameters variability in original loess triaxial shear test. Meanwhile, we analyze the mechanical mechanism of loess structural compressive failure, according to the p-q stress paths of triaxial shear test.(5) With the help of loess CT-triaxial shear test, we analyze the CT values, damage variable and the whole damage evolution of loess mesostructure during the initial process, consolidation and shear process, and eventually establish evolution equations of loess structural damage variable with scale effect on the basis of principle of damage mechanics. Also, we make a quantitative analysis on the mechanism of loess mesostructural damage and harden yield during the triaxial shear process, based on the CT images processing.
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