Study on Mechanical Properties and Crack Propagation Mechanisms of Saturated Red-Bed Sandstone Under Cyclic Loading–Unloading Conditions
摘要
The water–rock interaction is a significant factor inducing deformation and failure in tunnel surrounding rock, especially when the rock reaches a saturated state. Its mechanical properties and failure mechanisms require further in-depth study. Based on this, red-bed sandstone from the water conveyance tunnel in the Dali Section II of the Central Yunnan Water Diversion Project in Yunnan Province was selected. Seepage tests, cyclic loading–unloading experiments, and CT scanning for three-dimensional reconstruction were conducted on the red-bed sandstone. The experimental results indicate that: (1) With increasing confining pressure, the maximum water absorption and T₂ spectrum area of the red-bed sandstone show a negative correlation with confining pressure. As the bedding dip angle increases, the peak strength of the specimen first decreases and then increases, with the minimum peak strength observed at a bedding dip angle of 45°. (2) During crack propagation in red-bed sandstone, both volumetric strain and crack volumetric strain exhibit an initial increase followed by a decrease. The point at which volumetric strain and crack volumetric strain transition from increasing to decreasing corresponds to the initiation of unstable cracks in the specimen. During the plastic stage, the crack propagation rate of the specimen ranges from 2.373 × 10–6 s−1–1.747 × 10–5 s−1. (3) Three-dimensional reconstruction of failed specimens shows that, under constant moisture content and bedding dip angle, the maximum crack porosity decreases from 9.36% to 4.97% with increasing confining pressure. When the bedding dip angle ranges from 0° to 45°, the crack porosity of red-bed sandstone decreases with increasing dip angle; when the bedding dip angle ranges from 45° to 90°, the crack porosity increases with increasing dip angle. Relevant mathematical expressions are established. (4) Analysis of energy dissipation during specimen failure, based on the energy consumption ratio formula, establishes the energy consumption ratio rate distribution diagram for saturated red-bed sandstone. When the bedding dip angle ranges from 0° to 75°, the energy consumption rate ratio ranges from 4.87 × 10–4–4.58 × 10–3; when the bedding dip angle ranges from 75° to 90°, the energy consumption rate ratio ranges from 4.58 × 10–3–6.62 × 10–3, with the energy consumption rate ratio increasing as the dip angle increases.