True triaxial experiment and FDEM simulation on the controlling effect of coal-measure rock interfaces on hydraulic fracture propagation

• The propagation path of hydraulic fractures at coal-measure rock interfaces is jointly controlled by interface strength coefficient (η), interface inclination angle (θ), and vertical stress difference coefficient (k). • When fractures propagate from soft to hard rock, interface strength dominates; larger η promotes interface penetration, whereas from hard to soft rock, vertical stress difference controls the path, with larger k favoring crossing. • Interface inclination angle θ influences the vertical stress component; smaller θ facilitates penetration, while larger θ leads to fracture propagation along the interface. • Fracture network complexity increases with higher k and θ, but the complexity of fracture morphology shows a non-monotonic trend, initially decreasing then increasing with rising k and θ.