Shear failure behavior and mesoscopic mechanism of en-echelon fractures in marble

En-echelon fractures commonly exist as intermittent joint sets in rock masses, and their shear failure significantly controls the stability of rock engineering. In this study, numerical models of marble with en-echelon fractures of different inclination angles were constructed using the discrete element code PFC2D. A series of direct shear tests under varying normal stresses was conducted to explore shear fracture evolution and reveal the mesomechanisms. Tensile wing cracks dominated crack initiation, accompanied by shear and mixed tensile-shear cracks. Three coalescence patterns were identified including tensile, shear and mixed tensile-shear coalescence, which were highly influenced by fracture inclination angle and normal stress. Mesoscopic analysis showed that tensile crack initiation was controlled by concentrated tensile force chains, while shear cracks were induced by compressive force chains. Two rock bridge failure modes were observed after coalescence, namely rolling-crushing and sliding-rolling. The study improves the understanding of shear failure mechanism in en-echelon fractured rock masses and provides a theoretical basis for engineering stability assessment.