THE CREEP CHARACTERISTICS AND DAMAGE MODEL OF SANDSTONE UNDER FREEZE-THAW CYCLES

The long-term stability of rock in the alpine area of Qinghai-Tibet Plateau is significantly affected by freeze-thaw action. Accordingly it is necessary to study the time-dependent mechanical characteristics of the rock under freeze-thaw cycles. In this study,we conducted the uniaxial creep experiments on the red sandstone samples subjected to different freeze-thaw cycles. We further analysed the previous studies about quartz sandstone and investigated the effect of freeze-thaw cycles on each creep stage of sandstone. The results show the follows. The creep time of the samples during the deceleration creep decreases with an increasing number of freeze-thaw cycles under the condition of non-yield stress,corresponding to the increase of creep deformation and rate. The threshold stress for the rock samples entering accelerated creep decreases gradually with increasing freeze-thaw cycles under the condition of yield stress,the corresponding time from stable creep to acceleration creep gets more shorter,and the accumulated strain becomes more greater when rock failure. The macro-failure mode of the samples gradually evolves from a single oblique shear failure mode to a conjugate section tension-shear composite failure mode with increasing freeze-thaw cycles. The damage evolution equation about unsteady viscoelastic coefficient with the characteristics of freeze-thaw damage and time-dependent deterioration was proposed based on the experimental results. We introduced the Nishihara model to establish the creep constitutive model of sandstone considering freeze-thaw damage. The parameters of the model were identified by using the creep experimental data of red sandstone. The comparisons verify the correctness and applicability of the model. By fitting and analysing the identified parameters of the model,the law of viscoelastic coefficients varying with freeze-thaw cycles was revealed. The damage parameters under long-term load which plays an important role in controlling the amplitude of accelerated creep was shown. The research results have certain significance for long-term stability evaluation of rock mass in alpine area.