Experimental Study On Nonlinear Creep Characteristics Of Expansive Soil Based On Fractional Calculus

Expansive soil is a special kind of disastrous soft soil, and its rheological characteristics are very obvious. Therefore, study on nonlinear rheological properties of this kind of soils, investigating its constitutive relation in which the time factor is considered, and exploring its laws of strength and deformation have important theoretical significance and engineering application value. The research aimed at Nanning Guangxi expansive soil, and relying on the government’s projects funding, the nonlinear compression creep behavior of expansive soils have been studied in this thesis. All the research works are supported by the following government’s fund projects, these are include the National Natural Science Fund Project, " Experimental study on nonlinear rheological characteristics of expansive soil " (No:50978097), and the plan project of science and technology supported by science and technology department of Hunan Province, " Experimental study on constitutive model of nonlinear rheological properties of expansive soils " (No:2010SK3047) etc. Through a series of indoor triaxial compression creep tests, the creep deformation mechanism of expansive soil is studied under the conditions of different confining pressures. Based on the existing theories of soil rheology, using fractional order calculus method, a nonlinear creep model which can reflect the characteristics of expansive soil is constructed, and a corresponding nonlinear creep equation is proposed. The research results provide a theoretical basis for calculating the deformation of expansive soil foundation under long term load, and it also have reference significance to the further exploration of the rheological properties of expansive soil, such as stress relaxation and long-term strength. Through the research, the main research achievements and the related innovative conclusions are have been obtained as follows.(1) Experimental study on triaxial creep properties① Under different initial conditions, the creep laws of expansive soils are similar. Namely, in the moment of the deviatoric stress loaded, instantaneous deformation is produced in expansive soil, and with the increase of time, the creep phenomenon of soil sample is appeared. When the stress level is lower, the instantaneous deformation is relatively small, the main performance is the attenuation creep. Under the action of higher stress level, it is very fast to reach the stage of rapid flow, shear failure is occursed in soil samples.② Under the condition of consolidated undrained, creep deformation of expansive soil and pore water pressure have the same variety laws with time.③ Based on the fractional calculus theory, a software element is used to replace the ideal viscous element in the traditional component model, which can more accurately reflect the nonlinear rheological properties of this expansive soil. Through the analysis of the experimental data, a nonlinear creep model consisting of four elements is constructed, which is suitable for Nanning expansive soil analysing. Meanwhile, the corresponding creep constitutive equation is proposed.④ In the creep model, the viscosity coefficient of constant parameter is modified to be a variable coefficient related to the stress level, so that The nonlinear creep properties of expansive soils can be simulated more accurately.⑤ Utilizing the proposed model to fit the experimental data of Nanning expansive soil, the results show that the improved creep model can effectively simulate the whole process of the nonlinear creep of expansive soil, by adjusting the fractional order β1 and β2. Moreover, This model has simple structure and few parameters to be determined, and so it is convenient for practical application.(2) Experimental study on triaxial creep properties under the condition of graded loading and unloading① Through the triaxial creep tests under the condition of long term graded loading and unloading, the research results show that the attenuation creep characteristics is emerged obviously in Nanning expansive soil. After loading-unloading, viscoelastic rebound deformation will be produced in soil. With the increasing of the load, the rebound deformation increased. And with the increasing of confining pressure it decreased.② By comparing and analyzing the creep curves under the same deviatoric stress leavel before and after unloading, it is found that the axial strain produced by re-loaded deviatoric stress is greater than before unloading. Therefore, it is known that the creep deformation of expansive soil has a cumulative effect, under the repeated loading.③ Through analysing the experimental results, it is found that the deformation of expansive soil in the creep process is mainly composed of four parts, that are the instantaneous elastic deformation, the instantaneous plastic deformation, the viscoelastic deformation and the visco-plastic deformation. After completely unloading, the instantaneous elastic deformation is equivalent to the instantaneous rebound deformation, and the viscoelastic deformation is equivalent to the delayed rebound deformation.④ Based on the analysis of experimental data, a nonlinear creep model consisted of six elements is established, which can reflect the creep characteristics of expansive soil. Moreover, the corresponding creep equations are put forward, which provides a theoretical basis for the calculation of creep deformation of expansive soils.