Strength Deterioration Model of Soft Rock Considering Mesoscopic Bonding–Expansion Coupling Mechanism under Freeze–Thaw Cycles

The mechanical deterioration of soft rocks under freeze–thaw cycles is caused by the accumulation of mesoscopic damage. However, the current freeze–thaw deterioration model for soft rocks does not adequately consider the multiscale correlations, which makes the strength calculation results differ greatly from the test results and cannot fully reveal the damage mechanism of soft rocks under freeze–thaw cycling conditions. In this paper, the bond damage and pore ice expansion laws are considered from the soft-rock mesoscopic bond unit and a multiscale strength deterioration model is proposed. The freeze–thaw deterioration model is extended to intact and cracked soft rocks by the Discrete Element Method (DEM). The results are validated by laboratory tests. The peak strengths of intact soft rocks are calculated within 10% error for different numbers of freeze–thaw cycles, and the macroscopic crack development simulation results are consistent with the laboratory tests. The joints have a significant effect on the damage evolution: the freeze–thaw-induced mesoscopic damage in cracked rocks accumulates at a uniform rate, while the damage in intact soft rocks grows exponentially; the freeze–thaw cracks in cracked soft rocks are distributed between 60 and 90°, with a tensile–shear damage ratio of 1:2; the freeze–thaw cracks in intact soft rocks are distributed around 90°, with a tensile–shear damage ratio of 1:3. The deterioration model proposed in this paper can fully consider the multiscale damage correlations, which renders it easy to promote the application in the freeze–thaw hazard problem of soft rock engineering.