Experimental Study On The Unsaturated Engineering Behavior Of The Cohesive Soil Of Hefei Area

The cohesive soil near one railway was selected for studying the unsaturated engineering behavior of the foundation soil in Hefei area. For the studied cohesive soil, it analyzed the geological characteristics; the composition features, which contains the granulometric composition characteristics, mineral composition, chemical composition and the mutual coupling between each other, the influence of the grain size distribution and mineral composition of matric suction characteristic and so on. Different from the conventional methods, the characteristics of SWCC were studied with considering the non-stress, net vertical stress and net mean stress by using the unsaturated soil pressure plate apparatus, quadruple direct shear apparatus of unsaturated soil and the unsaturated triaxial apparatus. With these tests, the characteristics of SWCC under different stress state were analyzed. The fractal characteristic of SWCC was studied by combing with the pore volume fractal model of unsaturated soil and the Van Genuchten model. For all the above, the soil water characteristic of the foundation soil was studied comprehensively. Then, the compression deformation and creep deformation in one-dimension and three-dimension were analyzed by the compression tests and creep tests with using the unsaturated soil consolidation apparatus and triaxial apparatus. Last, the triaxial drainage shear tests were conducted with considering the matric suction and net confining pressure by the unsaturated triaxial apparatus for the foundation soil, and the unsaturated strength characteristics were analyzed. What’s more, the influence of dry density on the strength indexes were analyzed. For the above, the unsaturated engineering characteristics were comprehensively analyzed. Through the above research, it mainly obtained these conclusions:(l)The main cohesive mineral composition of soil is montmorillonite and illite; the chemical composition is mainly made up of SiO2, Al2O3, Fe2O3. The residual moisture content showed linear positive correlation with the content of clay, but silt. The residual matric suction showed significant non-linear negative correlation with the content of clay, but silt. At the same time, the residual moisture content and residual matric suction of the foundation soil presents a significant linear correlation with the content of montmorillonite, but presents no significant correlation with the content of illite.(2)Under the same stress level, the SWCC with considering the net mean stress can be clearly divided into three sections, respectively represents the boundary effect, transition and unsaturated residual three sections. But the SWCC with considering the net vertical stress only contains the transition and unsaturated residual period, and without the stress, the SWCC only had obvious transition section with a straight line, the level of boundary effect and unsaturated residual period are not obvious. Then, a generalized SWCC model was put forward with considering the net vertical stress, net mean stress and matric suction. One theoretical solution of the unsaturated soil fractal dimension was obtained by the parameters of Van Genuchten model based on the fractal model of unsaturated soil pore volume and Van Genuchten model of SWCC.(3) The concept of suction tangent compression coefficient was put forward and it had exponential attenuation with the increase of net vertical stress. The yield net mean stress of unsaturated soil increased with the matric suction, and the yield suction decreased linear with the increase of net mean stress, and the yielding line of LC and SI were obtained based on the Barcelona model.(4)Under the condition of different matric suction, one-dimensional creep deformation variation with time can be divided into two stages:①The first stage, the growth of the deformation with time showed a trend of rapid growth;②The second stage, the growth of the deformation with time gradual stability. For the unsaturated foundation soil, the one-dimensional creep characteristics showed power relationships with time and with the increase of water content, the spring back deformation and compression deformation could be both caused.(5)It proposed the ratio between matric suction and net confining pressure as a new kind of stress level, and it found that the relationship between three-dimensional creep strain and time presented good hyperbolic function, but linear relationship in the semi-log coordinate under different stress level. The curves of ε-lgt under different F could be roughly as two straight lines, which represented the two stages of accelerated creep and steady creep, and the time nodes of two stages presented linearly decreased with F. Then, one three-dimensional creep model with considering the F for the unsaturated soil was obtained with considering the power function relationship between the creep deformation and time, the logarithmic function relationship between creep deformation and matric suction. Through the test validation, the model had better effect in describing the relationship between creep strain and time under the condition of different matric suction.(6)Under different conditions of matric suction, the curves between deviatoric stress and deviatoric strain could be divided into two stages:①The stage of deviatoric stress increased with axial strain;②The stage of deviatoric stress keep essentially constant with the increase of axial strain. There had good linear relationships between qfandpfunder different matric suction. The suction stress and shear yield stress were analyzed under the different matric suction, and the relationships between shear strength index, such as the c,φ’,φb between the dry density were analyzed.