The study of the constitutive model under stress loading for rock after undergoing freeze-thaw weathering cycles (FTWCs) treatment has important theoretical significance for the site operation and numerical calculation of rock mass engineering in cold regions. In this work, a series of FTWCs treatment tests of sandstone were carried out, and the evolution characteristics of the T₂ spectrum distribution curve for sandstone before and after FTWCs treatment were analyzed utilizing nuclear magnetic resonance (NMR) technology. The schematic diagram of freeze-thaw damage evolution for sandstone was drawn, and the damage evolution mechanism was analyzed. Evolution laws of the stress-strain curves, peak and residual strength, peak and residual strain, and Young's modulus of sandstone under different FTWCs and confining pressures were analyzed by conventional triaxial compression tests. A piecewise constitutive model under triaxial stress loading for sandstone after undergoing FTWCs treatment was established, and the model considered the significant influence of FTWCs on the nonlinear deformation in the compaction stage and residual strength. By introducing error analysis indexes, the proposed model and published models were compared with the experimental data, it was found that the proposed model's performance is better than the published models, which indicated that the proposed model has good performance and strong universality.

In the process of ascending repetitive extraction of multilayer coal seams, the evolution and expansion of excavation-triggered cracks dominate the structural instability of overlying rock layers and the deterioration of overall rock mass quality. In this study, a similar material model containing two coal seams (No. 2 upper coal seam and No. 4 underlying seam) was constructed. Through model excavation, the entire process of overburden movement was captured, and a box-counting program was developed for quantitative analysis to investigate the dynamic evolution law of overlying strata fractures under upward repeated mining disturbances. The results show that during the mining of the No. 4 coal seam, the fractal dimension of mining-induced fractures initially increases and then decreases, with the peak value occurring at an excavation distance of approximately 40 cm. For the No. 2 coal seam, the fractal dimension exhibits fluctuating evolution characteristics, with the global peak appearing at approximately 55 cm and a local peak at approximately 70 cm. The rate of change in fractal dimension during coal seam mining exhibits alternating fluctuations of increase and decrease. The initially mined No. 4 coal seam, subject to higher in situ stress, exhibits rapid fracture development with an earlier peak, while the No. 2 coal seam, influenced by pressure relief, presents progressive fracturing characteristics with a delayed peak. The fractal dimension can effectively characterize the evolution characteristics of the mining-induced fracture network in overlying strata and provide reference for strata control and disaster prevention.