Abstract: To reveal the mechanical properties of grouted sandstone under incremental loading and to evaluate the reinforcement performance of sulfate aluminate cement (SAC), ultrafine cement (UFC), and epoxy resin (EPR), uniaxial cyclic loading–unloading tests were conducted. In combination with acoustic emission (AE), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) techniques, the mechanical response and pore structure characteristics of the grouted sandstone were quantitatively analyzed, and rock fracture precursors were identified. The results showed that: (1) Grouting reinforcement significantly improved the mechanical properties of fractured (F) sandstone. Compared with F specimens, the peak strength increased by 48.9%, 65.0%, and 72.8% for SAC, UFC, and EPR specimens, respectively; the secant modulus increased by 1.52, 1.84, and 1.84 GPa; and the peak strain increased by 0.10%, 0.13%, and 0.20%. EPR exhibited superior bonding and anti-decomposition properties, making it the optimal grouting material. (2) The Gaussian mixture model–support vector machine (GMM–SVM) classification method was suitable for incremental loading conditions. The proportions of shear cracks in F, SAC, UFC, and EPR specimens were 68.7%, 65.7%, 58.0%, and 70.1%, respectively, consistent with the shear-dominated macroscopic failure modes. (3) SAC and UFC materials increased pore complexity, whereas EPR grouting reduced it and effectively arrested crack propagation. (4) Under incremental loading, the window coefficient of variation outperformed the fusion index in capturing fracture precursor characteristics, demonstrating potential as a key early-warning indicator. The findings offer valuable guidance for the selection of grouting materials and the monitoring and early- warning strategies in complex stress zones of underground rock mass engineering.

Key words: incremental load, grouting materials, failure characteristics, mechanical properties, fracture precursor