Abstract: As a basic portion of frozen soils, ice has a significant impact on the accelerated creep stage of frozen soils. Temperature gives rise to hardening and weakening of frozen soil structure by affecting the freeze-thaw process of ice in frozen soils, as well as its viscoplasticity flow, thus becoming one of the key factors to determine the behavior of frozen soils. Meanwhile, the external stress also causes hardening and weakening, affecting the creep of frozen soils. Both hardening variable and damage variable are introduced here to consider hardening and weakening of the frozen soils resulted from variation of temperature and stress. Among them, the hardening factor H represents the magnitude of hardening effect in the process of creep, while damage factor D represents the reduction ratio of related parameters of frozen soils caused by the weakening effect, and then an improved Nishihara model is developed, the expression of improved Nishihara model under complicated stress state is derived as well. The comparisons between model predictions and available experimental results show that the improved model can not only describe the initial creep stage and the stable creep stage well, but also agrees well with the strain law growing exponentially with time in the accelerated creep stage, which demonstrates its accuracy and usefulness.

Key words: hardening, weakening, temperature, stress, frozen soils, creep constitutive model