Impacts of prefabricated fissure on micro-macro damage behavior of conglomerate under freeze-thaw cycles

Active faults are widely distributed in the Qinghai-Tibet Plateau. To investigate the damage behavior of fissured conglomerates under freeze-thaw actions, taking the conglomerate from the Lagangcun landslide as an example, rock samples with 2 cm and 4 cm fissures were prepared. Triaxial unloading tests were carried out on rock samples without fissures and with fissures after 0, 20, 40, and 60 freeze-thaw cycles. The microscopic pore structure, porosity, permeability, fractal dimension, and grayscale values of rock samples were obtained using computed tomography (CT) scanning, and the evolutionary characteristics of microscopic freeze-thaw damage and mechanical properties were analyzed. Based on values of CT images, a freeze-thaw damage variable was defined, and a constitutive model accounting for freeze-thaw damage was constructed. The results show that freeze-thaw action leads to a reduction in the peak stress of the conglomerate. Fissures weaken the strength of the conglomerate. In addition, under freeze-thaw action, the conglomerate transforms from a single tensile failure to a complex tensile-shear mixed failure. After freeze-thaw action, the number of pores reduces, and the number of throats increases. Furthermore, the pore connectivity increases, and the average grayscale value decreases with the increase of the freeze-thaw cycles. After 60 freeze-thaw cycles, the permeability of rock samples without fissures, with a 2 cm fissure, and with a 4 cm fissure increases by 1.87%, 55.56%, and 183.90%, while the grayscale values decrease by 43.93, 50.96, and 73.53, respectively. The proposed damage constitutive model can accurately characterize the complete evolution process of conglomerate from deformation to failure under the freeze-thaw cycles. The performance can provide a reference for exploring the freeze-thaw damage mechanism of fissured rock masses in the Qinghai-Tibet Plateau.