Experimental Study On Nonlinear Shear Stress Relaxation Characteristics Of Expansive Soil

Shear stress relaxation of expansive soil under long-term loading is one of the main components of study on its rheological characteristics. And the rheological characteristics of expansive soil are quite obvious with high clay content and plasticity index. Besides, when absorbing water it will expand in volume and decrease in strength while dehydration it will shrink and recover some strength. These natural characteristics make its rheological characteristics more complicated than general soft soil. Therefore it is of great theoretical value to, explore the shear stress relaxation model and relaxation laws of expansive soil under long-term loading, and develop regulation of long-term shear strength under the condition of stress relaxation. Through relevant research, it provides reliable the theoretical basis for stress, strength, deformation and stability analysis of expansive soil engineering, builds the theoretical foundation for the treatment of landslide, debris flows and other geological hazards of expansive soil. In this way, based on the existing achievements domestic and overseas, in this paper, the research is carried out on the stress relaxation properties of expansive soil nonlinear shear, which is funded by the National Natural Science Foundation-funded project "Researched on Nonlinear Rheological Characteristics of Expansive Soil"(No:50978097) and Science and Technology Department-funded project in Hunan province "Researches on Nonlinear Rheological Characteristics of Constitutive Model of Expansive Soil"(No:2010SK3047). Through a series of researches on tests of indoor and shear relaxation as well as theoretical analysis and numerical analysis, investigations are conducted on shear stress relaxation properties of expansive soil under different initial conditions. The importantly conclusions and innovations are made in the following:(1) Researches on the shear stress relaxation tests.①The expansive soils represent the similar stress relaxation law under different initial conditions. The shear stress decreases quickly the moment shear strain applies as time increasing. The stress relaxation rate is gradually reduced until to a certain value. It belongs to "Not Completely Loose" ②Relaxation curves of expansive soils can be divided into three stages: instantaneous relaxation stage, attenuation relaxation stage, stable relaxation stage.③Shear stress-strain curves at different moments are non-linear under different initial conditions, which indicates that expansive soil in Nanning is nonlinear, including bomb, sticky, as well as plastic.④Stress relaxation properties of expansive soil relates to initial shear strain, moisture content and vertical load. The expansion of the rheological properties of soil and the degree of nonlinearity are more obviously under the condition of the higher the moisture content, the smaller the vertical load, the greater initial strain, or the longer the time.(2) Researches on shear stress relaxation theory model:Through the analysis of the shear stress relaxation properties of expansive soils, the selection of H-(H|N)-(N|V) of the five-element model and its relaxation equation are used to describe the expansion of the shear stress relaxation properties of the soil. The theoretical values are proved to be in good agreement with the experimental values. What’s more, the model can reflect the law of shear stress decaying with time in stress relaxation process of Nanning expansive soil. It is some significance in engineering applications.(3) Researches on empirical model of stress relaxation properties of the nonlinear shear.Empirical model of nonlinear stress relaxation has been built by improving the linear relationship between logarithm of shear stress and time.The comparisons have also been made with the theoretical model of the five-element. Nonlinear empirical models are better in the phase of the attenuation relaxation. Nonlinear empirical models takes the strain level of stress relaxation into consideration, intuitively reflecting the nonlinear stress relaxation properties of Nanning expansive soils. The models are more corresponded with the actual engineering. It provides a theory for the analysis of calculation of nonlinear stress relaxation problems of expansive soils.(4) Researches on long-term shear strength equation of expansive soils.Based on expansive soil shear stress relaxation tests, equation of long-term expansive soil shear strength and empirical equation of strength indicator (cohesion and angle of internal friction) have been referred. Through numerical analysis, the relevant parameters are determined. These parameters are proved to be the goodness of fit of experimental values. It provides a theoretical basis and reference to stability calculation analysis of various geotechnical engineering in areas of expansive soil, which is of certain theoretical value and engineering significance.