Material-dependent interlocking mechanisms and AE precursor evolution of locked-patches instability in filled double-flawed sandstone

Filled flawed rock masses are common in rock engineering; however, the material-dependent interlocking mechanisms and AE precursor evolution of locked-patch instability remain insufficiently understood. This study investigates the mechanical behavior, deformation evolution, and precursor responses of filled flawed sandstone containing double parallel flaws (DPFS) using an AE–DIC monitoring system under uniaxial compression. Three representative filling materials—cement-based grout (HGC), epoxy resin (EP), and room-temperature vulcanizing rubber (RTV)—were examined. Laboratory experiments were integrated with grain-based DEM simulations and a multi-locked-patch fracture-threshold framework to clarify strengthening mechanisms and instability processes. Results show that filling significantly enhances load-bearing capacity, with compressive strength ranking as HGC > EP > RTV > unfilled specimens. Under the critical rock bridge configuration of β = 60°, the compressive-strength enhancements of HGC-, EP-, and RTV-filled specimens relative to the corresponding unfilled specimens were 40.6–78.2%, 36.0–48.9%, and 10.9–20.9%, respectively. The rock-bridge region acts as the dominant locked-patch controlling instability, while different filling materials regulate its failure through distinct mechanisms: HGC improves structural continuity and strength, EP enhances interfacial bonding and shear transfer, and RTV provides deformation buffering. Although EP has greater theoretical potential, HGC achieves superior reinforcement through more stable interlocking under high stress. AE responses exhibited pronounced material dependence. The intervals from the autocorrelation leap point to failure accounted for 19–25%, 14–19%, and 26–31% of the total loading time for specimens filled with HGC, EP, and RTV, respectively, while the corresponding dominant-frequency ratios λ f ranged from 0.806 to 1.016, 0.522–0.878, and 1.178–1.399. These characteristics corresponded to progressive-extension, delayed-burst, and persistent-fluctuation precursor modes, respectively. A multi-parameter precursor identification framework is proposed based on AE autocorrelation and the Ib-value. Instability onset is marked by cooperative anomalies in AE counts, energy, amplitude, RA/AF, and Ib-value, rather than a single-parameter mutation. These findings provide new insights into the mechanical regulation and precursor identification of filled flawed rock and offer a basis for early warning of instability in rock engineering.