Damage evolution characteristics of freeze–thaw rock combined with CT image and deep learning technology

The surrounding rock of tunnel engineering in an alpine mountainous environment is prone to frequent freeze–thaw action due to fissure water and temperature differential, which leads to crack propagation and even failure in rock. Freezing sandstone CT damage-free scanning studies were conducted. Based on deep learning theory, the U-Net network technique is utilized to naturally merge high-resolution properties of frozen rock CT images in the shrinking path with low-resolution characteristics in the expansion path. Intelligent detection of freezing rock fissures and geometric information parameters at the pixel level has been accomplished. The primary fracture structure and its parameters of the sandstone with natural damage during the freeze–thaw process are obtained, and the pixel-level intelligent identification of the meso-structure and geometric information parameters of the freeze–thaw rock fracture is realized. This justifies the classification of naturally cracked rock under load and freeze–thaw as a discrete time-dimensional evolution system. The dynamic process and mechanical characteristics of meso-damage propagation of naturally fractured rock under freeze–thaw and compression load are investigated using Casrock numerical computation software, which is based on the cellular automata theory. The results reveal that when the number of freeze–thaw cycles rises, the random rate of fracture network structure distribution increases, the uniformity of fracture distribution increases, and the dominating direction decreases. The sandstone's secondary fractures progressively increase as the fracture dominant angle rises, and the rock sample's failure mode eventually shifts from tensile failure to compression-shear mixed failure. When the comprehensive dominant angle of fracture is 60°, the fracture of freeze–thaw rock is more prone to expansion and its mechanical strength deteriorates more. The fractured rock creates narrow strip directional damage along the end of the original fracture when subjected to compressive load, exhibiting typical localization features. The main crack and the secondary crack dominate the crack progression. The number of secondary fractures inside sandstone steadily grows as the fracture's comprehensive dominant angle increases. The direction of the crack penetration development is determined by the comprehensive dominating angle of the fracture.