Three-Dimensional Morphological Characteristics and Shear Behavior of Yellow Sandstone Structural Surfaces under High-Temperature Conditions

Analyzing the impact of high temperature on morphological characteristics and shear strength weakening of rock structural surfaces is crucial for evaluating the stability of rock engineering in high-temperature environments and after fires. Field sampling was conducted on the yellow sandstone structural surfaces at the tunnel construction site. To enable repeated testing, three-dimensional laser scanning and fine carving techniques were employed to create yellow sandstone structural surface samples with identical morphology. Direct shear tests, three-dimensional morphological scans, and Scanning Electron Microscope (SEM) tests were performed on these samples treated at five different temperatures (20°C, 200°C, 400°C, 600°C, and 800°C). The influence of high temperature on the morphological characteristics and shear properties of the sandstone structural surfaces was analyzed. The experimental results indicate that the smoothing effect and softening effect on the structural surfaces caused by high temperature are two important factors in the mechanism of shear strength weakening induced by high temperature. A shear strength criterion for rock structural surfaces at high temperatures was developed based on the three-dimensional morphological parameters suggested by Grasselli. Comparing the experimental shear data from this study with existing literature demonstrated that the proposed criterion for peak shear strength of structural surfaces after high-temperature predictions is quite accurate. This indicates that the new criterion effectively estimates the shear strength of rough yellow sandstone structural surfaces at different treatment temperatures and can predict the shear strength of rock structural surfaces in high-temperature environments and after fires.