Experimental Study On Nonlinear Stress Relaxation Characteristics Of Expansive Soils

Expansive soils is a special kind of disaster soil,it is widely distributed in our country.With the rapid development of China's economic construction,more and more infrastructures will be inevitably built on expansion soil area.However,due to the research on rheological properties such as creep and stress relaxation as well as long-term strength of expansive soils is relatively lagged behind,which hindered the development of engineering technology for the treatment of expansive soil disasters.Therefore,it is of great significance to study the stress relaxation characteristics of expansive soils for ensuring the safety and long-term stability of engineering construction in expansive soil area.The research aimed at Nanning expansive soils,and relying on the government's projects funding,the nonlinear stress relaxation characteristics of expansive soils have been studied in this thesis.All research works are supported by the following government's fund projects.These projects are included the National Natural Science Fund Project("Experimental study on nonlinear rheological characteristics of expansive soils",No.50978097),and the science&technology plan project which is supported by Hunan Science and Technology Department("Experimental study on constitutive model of nonlinear rheological properties of expansive soils",No.2010SK3047)etc.By reading a lot of research literature home and abroad,the relevant experimental research and theoretical analysis are finished.The main research works carried out are included as follows.(1)The main experimental research works? Basic physics and mechanical properties tests on expansive soilsAccording to the requirements of the test procedures,the basic physics and mechanical properties tests such as liquid limit tests,and plastic limit tests,and compaction tests,as well as swelling tests and so on,are carried out on expansive soils.The corresponding physical and mechanical properties indices of this soil are tested.? Triaxial compression tests on expansive soilsThe triaxial compression tests on expansive soils are carried out under different initial conditions,by using the unconsolidated-undrained module(UU)of unsaturated triaxial test system.? Triaxial stress relaxation tests on expansive soilsUnder different initial conditions,a series of triaxial stress relaxation tests on Nanning expansive soils are carried out by using triaxial rheological testing machine(TSS10)which is produced by Changchun Mechanical Science Research Institute Co Ltd.(2)The main theoretical research works? Based on the theory of soil rheology,the stress relaxation laws of expansive soils under different initial conditions are explored,by analyzing the results of triaxial stress relaxation tests.? According to the analysis of the tested results,the characteristic indexes of stress relaxation are determined.Through the analysis,the influence laws of various factors on the stress relaxation characteristics of expansive soils are explored,and the stress relaxation characteristics and its evolution laws of this kind of soils are discussed.? Based on fractional calculus theory and soil rheology theory,a fractional order software element is introduced,and the nonlinear stress relaxation model of expansive soil is modified,by analyzing the results of stress relaxation tests.On the other hand,by considering its viscosity degradation,the nonlinear stress relaxation analysis theory of expansive soil is established.Furthermore,the parameters of this model have been determined,and the experimental verification and comparative analysis are carried out.Through the above researches,the following research progresses and innovation conclusions are obtained.(1)Under different initial conditions,the stress relaxation laws of expansive soils are similar,and the stress relaxation curves have obvious nonlinearity.The stress relaxation procedures can be divided into three stage,these are the fast relaxation and the deceleration relaxation as well as the relaxation stability.For expansive soils,the stress relaxation is an incomplete attenuation relaxation.(2)Under different initial conditions,the stress-strain isochronal curves of expansive soils are shown as two different deformation stages.That is to say,when the strain level is low,the stress-strain relation is shown as a linear viscoelastic deformation stage.And with the increasing of strain level,the stress-strain relation is shown as a nonlinear viscoelastic deformation stage.The higher the strain level is,the more obvious the nonlinear characteristic is.(3)The higher the water content and the strain level are,the higher the degree of stress relaxation of expansive soil is.And the lower the confining pressure and the degree of compaction are,the higher the degree of stress relaxation of expansive soil is.(4)The ideal viscous element in the traditional model is replaced by fractional order software element,and the damage deterioration of the viscous coefficient is considered,so that the nonlinear stress relaxation characteristics of expansive soils can be more accurately reflected.(5)In the course of stress relaxation,the modulus of expansive soils is no longer a constant,but a variable related to the initial conditions.The essence is the accumulation of damage due to micro cracks occurring in the soil under the action of long-term strain.This rightly reflects the inherent mechanism of stress relaxation under the action of constant strain,i.e.,the modulus decreased with time increasing.As a result,the stresses decreased with the time increasing.(6)By improving the traditional integer order model,a fractional order St.Venant stress relaxation model is obtained.The theory of stress relaxation analysis for expansive soil is established,and the formulas for calculating relaxation modulus and relaxation stress of expansive soils are put forward.Furthermore,the parameters of this relaxation model are determined,and the physical meanings of these parameters are defined.Through comparing the calculated results obtained by this model with those of other models,the reliability and advantages of the proposed theory for the analysing stress relaxation of expansive soils are verified.In conclusion,the experimental and theoretical studies on nonlinear stress relaxation of Nanning expansive soils are studied in this thesis.The laws of nonlinear stress relaxation for Nanning expansive soils are explored,and the internal mechanism in the process of stress relaxation of this soil is revealed.Morever,an analytical model and the relevant computational theory are established respectively,which can better reflect the nonlinear stress relaxation characteristics of expansive soils.The research results have laid a theoretical foundation for the analysis of nonlinear stress relaxation of expansive soils,and it have an important guiding significance to the disaster prevention and the disaster control of expansive soil engineering.