Freezing-dynamic combined damage characteristics of carbonaceous phyllite in cold region

The surrounding rock of tunnel in cold region would produce serious damage and performance degradation under the action of freeze-thaw and dynamic loads. Therefore, it is of great significance to study the deterioration characteristics of rock under the coupling effect of freeze-thaw and dynamic loads. In this paper, the MTS815 loading system was used to carry out the triaxial multi-stage cyclic loading and unloading tests on the carbonaceous phyllite samples under 0, 30, 60 different freeze-thaw cycles. The effects of freeze-thaw and cyclic loading on the rock mechanical parameters and deformation were studied. Based on energy calculation, the energy evolution law of rocks with different freeze-thaw cycles under dynamic load was analyzed. Finally, the failure modes of rocks under static load and freeze-dynamic coupling were compared and analyzed from macro and micro perspectives, and the corresponding failure modes were revealed. The results are as follows: ① Compared with the compressive strength of static load, the strength of rock under the combined action of freeze-thaw and cyclic load is significantly reduced, and it shows obvious plasticity. Meanwhile the more freeze-thaw cycles, the more significant the phenomenon; ② With the increase of freeze-thaw cycles, the elastic modulus will gradually decrease and the Poisson's ratio will gradually increase; ③ During the cyclic process, the axial and hoop irreversible strains increase gradually, and the strain increments of both are the largest in the initial cycle of each level of stress and then decrease gradually, while the irreversible volume strain increases first and then decreases, that is, the volume is mainly compacted first and then gradually transformed into expansion; ④ The rock is mainly composed of elastic energy, and the proportion of dissipated energy decreases rapidly in the first cycle and increases stepwise in the subsequent cycle and then tends to be stable; ⑤ From the perspective of macro and micro analysis, the failure mechanism of monotonic loading and cyclic loading is different, but both are the shear failure of inclined section, but the degree of cyclic loading and unloading is greater.