Study On The Mechanical And Rheolgical Properties Of Yan'an Q₂ Loess

The loess is a special kind of regionally distributed soil whose physical mechanics nature varies significantly in different regions. Taking the Q₂ loess of Yan'an as the researching material while focusing on the structural and the rheological properties of the loess, this paper has carried on a series of research tests and experiments, namely, the conventional physical feature test of the Yan'an Q₂ loess with different water content, the conventional unaxial compression test, the conventional triaxial compression test, the unaxial creep experiment, the observation through scanning electronic microscope as well as the triaxial creep experiment. A systematic research has also been conducted on Q₂ loess, through mechanics analysis and numerical analysis, to discover the mechanics parameters and the law of the characteristic change of the structural damage due to the change in the water content, and the destruction characteristics of detrusion and total creep, the constitutional model, and so on, which has an important theoretical and practical significance on the designing of ground projects of all kinds on the Loess Highlands area.The main achievements of this paper include:(1) Researching into the basic physical target characteristics of the granular compositions and the mineral components by way of the conventional physical feature test, researching into the law of change of mechanics parameters (mainly include structure strength of the loess, the cohesive force, the friction coefficient) owing to the change in water content.(2) It is indicated from the CU shear test that stress-strain relations of the Q₂ loess assume the strong conditioning to the stronger conditioning. This construction relation can be described by the strong conditioning model proposed by Professor Liu Zudian (1996). Discussions have also been conducted about the law of the change of those model parameters with the change in the water content and confining pressures, and about the influences of the strain speed on the stress-strain curve based on the results of the test. Summaries have been reached on the whole distortion way due to the loess shearing destruction, the type of destruction and the mechanics pattern of the distortion destruction.(3) The unaxial compression test indicated that stress damage threshold value of Yan'an Q₂ loess decreases exponentially with the increase in water content, while, the damage strain threshold value of the loess has little relation with the water content, and the initial elasticity module decreases by power function with the increase in the water content. By means of the assumption of the rigidity damage and the intensity damage and statistical method of measuring the damages, this research paper seeks for the expressions of the macroscopic parameters and the microscopic structure change for the loess damage, and establishes models of rigidity damage, intensity damage and statistical damage of the loess, and compares the features between these models.(4) It is indicated from the result of a massive amount of creep tests that the unaxial failure stress of the Yan'an Q₂ loess decreases exponentially with the increase in the water content, the ratio of the creep failure stress to structural intensity under the same water content is approximately 0.8; while in triaxial creep test, the water content and confining pressure have tremendous influence on the creep failure stress of the loess. Based on the testing result, it is also discussed the law of the change in the creep failure stress of the loess with the changes in water content and confining pressure. The whole distortion characteristics and shape of the triaxial creep leoss have been summarized as well.(5) It is indicated through the observational analysis by scanning electronic microscope into the Yan'an original Q₂ loess and the unaxial creep damaged loess that the structure of the original loess is featured primarily, before the creeping, by supportive megapore cemented structure and mosaic micropore cemented structure, while after the creeping it transfers to the micropore cemented structure and congeal cemented structure primarily. The essence of the loess creep process is that the soil granular structure connection is changing, with the increasing in the outside load, from structure hardening to structure conditioning.(6) Based on the result of the creep test, the soft rheological constitutional model is established exploratorily through the Riemann-Liouville score step calculus operator theory, and the changing law of the model parameters has been explored as well. The visco-elasticity-plasticity rheological model of the loess is established by the modeling theory; with this basis , two kinds of rheological constitutional models are established, one considers loess statistical damage caused by instant damages by introducing statistical damage theory, and the other considers creep damages while the loess accelerating creep process can be well reflected.(7) This paper carries out the preprocess using ANSYS, gets related information needed in computing by the APDL language, and compiles the plane strain finite element computational program with MATLAB6.5 language. By using the established finite element program, finite element analysis is carried out on the creep test of the Yan'an Q₂ loess whose result shows that this model can well reflect the rheological characteristics.