Abstract: The freeze-thaw cycle represents one of the most common physical weathering processes in cold regions, which can significantly affect the deformation characteristics of soils. An improved constitutive model with double yield surfaces, which can take the freeze-thaw effect into consideration, was developed based on the traditional elliptic-parabolic yield surfaces model in this study. First, a set of freeze-thaw cycling tests and consolidated-drained(CD) triaxial tests were conducted on silty clay from Qinghai-Tibet Plateau. The test results showed that the stress-strain behaviors exhibited strain hardening and shear-shrinkage characteristics. The volumetric strain increased while the failure strength decreased during the shearing process. Then, the elliptic equation and parabolic equation were adopted to fit the volumetric yield surface and the shear one in p-q plane, respectively. The incremental deviatoric stress-axial strain relation and deviatoric stress-volumetric strain relation were subsequently established by following the associated flow rule. The parameters of above relations were valued according to the tested deviatoric stress-axial strain-volumetric strain relationship, and were all found varying by the number of freeze-thaw cycles with certain regularity. The c、φ、h、K、M1、M2 and a all kept decreasing with increasing the number of freeze-thaw cycles, while the t and n kept increasing, and they can all be fitted by the Logistic function. At last, the expressions of above parameters with number of freeze-thaw cycle as variable were included into the incremental constitutive equations, thus an improved model considering freeze-thaw effect was established. The deviatoric stress-axial strain curves and volumetric strain-axial strain curves predicted by the proposed model agreed basically with the experimental results.

Key words: silty clay, freeze-thaw cycle, consolidated-drained triaxial test, stress-strain relation, constitutive model with double yield surfaces, parameters of constitutive model, Logistic function