WANG Miao-miao, ZHU Lin-xuan, ZHOU Zhi-jun, XU Jiang-bo, REN Yu-bo, HAN Jing, FENG Hong-ming
Engineering rock masses in cold regions are affected by external loads and chemical solutions during freeze-thaw action, which is directly reflected in changes in the pore structure that determine the damage mechanism. To study the evolution characteristics of sandstone pore structures under the combined action of stress, chemical solution, and freeze-thaw cycles, sandstone freeze-thaw cycle tests were conducted at different stress levels and solutions. The T2 spectra and microscopic images of a sandstone under different conditions were obtained using nuclear magnetic resonance and scanning electron microscopy techniques, respectively. The pore size distribution, porosity, pore size uniformity coefficient, permeability, tortuosity, and microstructural changes in the sandstone were analyzed. The influence and control mechanism of stress levels and chemical solutions on the pore structure evolution process of the sandstone during freeze-thaw cycles were further explored. The results show that as the number of freeze-thaw cycles increases, the number of pores of different sizes increases, and the expansion rate of mesopores accelerates, making the expansion into macropores with larger sizes easier. The porosity, uniformity coefficient, and permeability of sandstone increases, whereas tortuosity decreases with freeze-thaw cycling. The acidic solution promotes the development of micropores into mesopores and macropores. When the sandstone is at low stress, such as at 0.3σf, the pores perpendicular to the stress direction close during the initial stage of the freeze-thaw cycles, suppressing the exertion of chemical corrosion and frost heave force. This leads to a decrease in the number of pores of different sizes compared with a sandstone with no stress, resulting in a decrease in porosity, uniformity coefficient, and permeability, and an increase in tortuosity. Subsequently, with the alternating action of chemical corrosion and frost heave forces coupled with the additional effect of axial stress, the number of pores of different sizes increases. When the sandstone is at high stress, such as at 0.7σf, and mainly controlled by stress, the mesopores and macropores increase significantly, and intergranular and transgranular cracks initiate and propagate rapidly. This results in a rapid increase in porosity, uniformity coefficient, and permeability, and a decrease in tortuosity, and finally resulting in rock failure.
