Influence of defect shape on the creep behavior and damage evolution of coal rock using an improved model

Deep coal-rock masses in complex geological environments often contain defects that threaten the long-term stability of engineering structures. To investigate the influence of defect shape on the creep behavior and failure mechanism of coal-rock, an improved creep model was developed by coupling the Kelvin-Voigt contact model with the parallel bond model in PFC. Based on calibrated mesoscopic parameters, simulations were performed on intact specimens and those containing rectangular, trapezoidal, inverted U-shaped, square, and circular cavities under stepwise loading. Results show that cavity defects significantly weaken the creep strength, strain, and elastic modulus of coal-rock, with degradation depending on cavity geometry. Rectangular cavities cause the largest drop in failure stress, inverted U-shaped cavities reduce the elastic modulus most, and square cavities lead to the greatest strain decrease. Under equal cavity areas, wider cavities promote greater compressive deformation. High-stress zones form and propagate differently across shapes: circular cavities generate symmetric stress fields and exhibit better structural stability, while other shapes develop stress zones outside the cavity that connect laterally. Cracks initiate in these high-stress regions and expand along stress paths, showing that cavity geometry dominates stress distribution and failure mode-rectangular and inverted U-shaped cavities favor local shear failure, whereas circular cavities tend toward global shear failure.