Abstract: On the basis of a series of indoor shear stress relaxation tests for unsaturated remolded expansive soils by using a quadruple load equal strain-controlled direct shear apparatus，the shear stress relaxation characteristics of this kind of soils were researched. Many useful conclusions are obtained. The results show that shear stress relaxation curves under different initial conditions are all incomplete attenuation type; but the shear stress decreased gradually and eventually tending to stable values. The relaxation curves can be clearly divided into three stages, i.e. the instantaneous relaxation stage, attenuation relaxation stage and stable relaxation stage. Shear stress-strain curve at different moments is nonlinear under different initial conditions, which indicates that expansive soil in Nanning is nonlinear, indeed bomb, sticky, as well as plastic. The conclusions show that the soil is the higher the nonlinear degree of the rheological properties of expansive soil with higher the moisture content, or the smaller the vertical load, or the greater initial strain, or the longer the time. Based on the analysis of tested results, an empirical shear stress relaxation equation of Nanning expansive soils under the condition of direct shearing has been established by improving the linear relationship between logarithm of shear stress and time. The regression curves of empirical relaxation equation are in good agreement with those of the observed data. The strain level of stress relaxation was taken into nonlinear empirical models. It intuitively reflect the nonlinear stress relaxation properties of Nanning expansive soils. The model is more corresponded with the actual engineering, so as to provide a theory for analytical calculation of nonlinear stress relaxation problems of expansive soils.

Key words: expansive soil, stress relaxation, nonlinear rheology, relaxation equation