Study On Direct Shear Creep Characteristics And The Long-term Strength Of Expansive Soil

With the development of economy and social progress, more and moreinfrastructural facilities needed to build on expansive soil. However,expansive soil is a kind of special soil, have a lot of multi-cracks, swelling,dehydration shrinkage and rheological properties. For the reason of inlacking of knowledge on rheological characteristics of expansive soil, sothe strength and deformation of the foundation in long-term loaded can notbe predicted, thus often result in engineering accident. Therefore,researching on the rheology properties of expansive soil became animportant and hot problem.On the basis of the research results of soil rheology at home andabroad, supported by the project of National Natural Science Founding,through experimental study and theoretical analysis as well as numericalcalculation, a series of research on some rheological properties such ascreep model and the long-term strength of Nanning expansive soil havebeen made. The major progresses have been made in this thesis are as thefollowing.1) By the indoor direct shear creep test, the study on shear creepcharacteristics of expansive soil under different moisture content anddifferent vertical pressure is made. The research results show that thevertical stress and moisture content as well as the level of shear stress makea very obvious influence to the direct shear creep characteristics of this soil.Under a certain stress level, with the moisture content is increase, thenonlinear creep characteristics of expansive soil is more obvious, and thetotal deformation of shear creep is increase. Under a certain moisturecontent, with the vertical stress is increase, the nonlinear creepcharacteristics of this soil is weaken, and the total deformation of shearcreep is decrease. Under a certain vertical stress, with the shear stress isincrease, the nonlinear creep characteristics of this soil is more obvious,and the total deformation of shear creep is increase. Under a certainmoisture content and a certain vertical stress, with the shear stress isincrease, the shear creep of this soil will be presented as different formrespectively, such as attenuation creep and constant speed creep as well asacceleration creep and so on. 2) According to the theory of experience rheology, a nonlinear directshear creep model of expansive soil is established. This model can bedescribed as a form of logarithmic or power function. Compare to thetested results and calculated results, it is shows that the nonlinear creepproperty of this soil is reasonably simulated by power function model.Further more, the parameters of this model are very simple, and it isconvenient to use. This will provide a reliable theoretical basis for thedeformation calculation of expansive soil foundation.3) By the analysis of tested results, the characteristics of shear stress-strain isochronous curve of expansive soil and its variation are researched.The method that to define the rheology parameters such as shear modulusand viscid coefficient of this model is presented, and the influencing factorson these parameters are discussed. The research shows, that theisochronous curve will be near to the strain axis as the increases of timeand shear stress, and nonlinear rheology characteristics are shows clearly.Under the same moisture content, the shear modulus of this soil will bedecreases as the shear stress is increases and time goes on. Under the sameshear stress, the viscid coefficient of this soil will be increases as thevertical stress is increases. Under the same vertical stress, the viscidcoefficient will be decreases as the shear stress is increases.4) Base on the failure criteria of long-term loaded soil and itslong-term strength theory, and combined with the testing results, thelong-term strength formula is established. This formula can be described asthe form of power function, it can be used to simulate the rheologyproperties and long-term strength behavior of expansive soil accurately. Bythis theory, the calculated results is close to the same with tested results,thus proves that theory method is reliable. The formula have simpleparameters and easy to use. It provided a reliable theoretical basis for theanalysis of strength and stabilization of expansive soil foundation.