Mechanical response and damage constitutive model of layered-grouting-reinforced fractured coal mass

• This study pioneers the investigation into the mechanical influence of inclined cemented interfaces formed by layered grouting, identifying 20° as the critical inclination where the failure mode transitions from tensile to shear dominance. • For the first time, a macro-meso damage-coupled constitutive model was established for grouted coal with inclined interfaces, effectively characterizing its complete stress-strain response. • A novel specimen preparation method was developed to fabricate standard cylinders containing multi-angle, persistent inclined interfaces, enabling controlled laboratory testing. To investigate the mechanical behavior of a fragmented coal mass reinforced by layered grouting, an acoustic emission (AE) testing system and a non-contact full field strain measurement system (VIC) were employed to record AE signals corresponding to the instantaneous release of elastic waves and full-field strain data on the specimen surface during axial compression. The results indicate that the uniaxial compressive strength (UCS) of the grouted coal first decreases with an increase in the dip angle of the inclined plane, reaches a minimum at 20°, and then gradually recovers as the dip angle further increases. This variation exhibits a non‑monotonic trend, yet the UCS remains consistently lower than that of the specimen with a 0° dip angle. When the inclination is ≥ 20°, increasing the inclination enhanced the void filling effectiveness of the grout, thereby increasing the shear capacity and improving the compressive strength. Moreover, tensile failure dominated for inclination angles < 20°, whereas shear failure along the inclined plane prevailed for inclination angles ≥ 20°. Finally, based on Lemaitre's strain-equivalence principle and the concept of macro-meso coupling, a macro-meso damage variable D was defined for the coal mass. An axial-compression constitutive model grounded in meso-damage was established for grouted coal containing an inclined interface, and the procedures for determining its parameters were specified. Validation by comparing theoretical and experimental curves confirmed that the model predictions—including those for the peak strength and other key mechanical stages—exhibit good agreement with the experimental results. The findings provide significant engineering implications for evaluating the stability of coal masses reinforced by layered grouting in the presence of an inclined cemented interface.