Mesostructure and strength characteristics of granite under freeze-thaw cycles based on CT scanning

Objective With the rapid increase in construction projects in the western regions in recent years, the impact of seasonal freeze-thaw cycles in the high-altitude areas of western China has become more pronounced. Conducting research on the microscopic characteristics and strength degradation properties of rocks under freeze-thaw cycles is crucial for guiding engineering construction in these cold, high-altitude regions. Methods To study the influence of freeze-thaw cycles on rock structure and mechanical properties, we collected diorite samples from a tunnel in the Kangding area and examined the effects of freeze-thaw cycles on their microstructure and mechanical characteristics. Firstly, thin rock sections were observed under a polarizing microscope to obtain mineral compositions and microstructures. Then, CT scanning technology was used to scan the granite samples after freeze-thaw cycles, and the scanned layers were binarized using threshold segmentation. The scanning images of different layers were binarized using threshold segmentation, and high-resolution 3D data and images of the internal and external structures of the samples were obtained by stacking the binary image layers. Fractal theory was applied to calculate the box-counting dimension of the images and quantitatively assess their complexity. This analysis allowed us to examine the evolution and distribution characteristics of the internal structure of granite under freeze-thaw cycles. Results Under a polarizing microscope, the rock exhibits a block-like structure with a patchy, coarse-grained, and unequal-grained granite texture, with locally visible metasomatic worm structures. The main phenocryst minerals are alkaline feldspar. Other minerals range in size from 0.25 to 4.0 mm and primarily include quartz, plagioclase, and alkaline feldspar. Secondary minerals include biotite and epidote, while accessory minerals comprise apatite, zircon, and pyrite. Microscopically, the rock is identified as porphyritic, coarse-grained, and unequal-grained biotite diorite granite. Freeze-thaw cycles were applied to the granite samples in the laboratory to study the strength evolution and explore the relationship between structural evolution and strength. The results indicate that the freeze-thaw cycle effect leads to an overall increase in the internal porosity of the granite's microstructure, though the rock's permeability changes minimally, with an increase of only 0.003×10−3 μm2. The internal pore development is uneven, primarily due to the emergence of new micropores, causing changes in the overall structure of the sample. After freeze-thaw cycles, the complexity of the internal structure of the rock decreases, but the overall integrity remains good, with the fractal dimension staying at a high level. Fractal analysis shows that 20 freeze-thaw cycles do not cause significant changes in the structural complexity of granite. However, the overall mechanical properties of the sample decline, viscosity increases, and long-term strength shows significant attenuation, raising the strain threshold for entering the creep test stage. Conclusion When evaluating the safety of rocks with dense primary structures, considering only their structure may lead to deviations from the actual situation. It is essential to combine necessary strength indicators for a comprehensive evaluation. After undergoing freeze-thaw cycles, rocks tend to exhibit more significant deformation while maintaining lower strength. Therefore, appropriate treatments are required for construction in high-altitude areas. [ Significance ] This study provides a reference for applying fractal theory to the evolution of rock microstructure and the relationship between rock microstructure and strength evolution. It also offers valuable guidance for engineering construction in high-altitude and cold regions.