Abstract: The interaction of pre-existing fissures and engineering cracks caused by excavation-induced disturbance, unloading and hydraulic fracturing is very common in rock engineering. Establishing a propagation criterion suitable for engineering cracks encountering pre-existing fissures is the key to explaining the propagation mechanism of engineering cracks and investigating the weak-surface shear-slip of rock mass. The study used two concrete filling materials with different strengths to simulate the pre-existing fissures of strong and weak cementation. The mode I static fracture toughness of the filled cemented rock was determined by the semi-circular bend method under the quasi-static tensile stress. By analyzing the propagation trajectory of engineering cracks, the relationship between the critical crack initiation angle and the stress approach angle was obtained; the interaction propagation mode and initiation criteria of engineering cracks interacting with preexisting fractures were further discussed. The results show that the interaction between engineering cracks and pre-existing fissures under tensile stress is jointly affected by the stress approach angle, the crack initiation approach distance, and the cementation strength of the filling. The static fracture toughness of the filled cemented sandstone first increases and then decreases with the increase of the stress approach angle. When the crack initiation distance is large, the fracture toughness changes little with the stress approach angle; while when the crack initiation distance is short, the fracture toughness first increases and then decreases with the increase of the stress approach angle. The stress approach angle affects the fracture toughness of the cemented rock filling, but the magnitude of the influence is not as great as those of the crack initiation approach distance and the cement strength of the fillings, and there is a limited influence distance. Even if the rock material is under tensile stress, there is also shear localization at the front of the crack. Therefore, whether or not an engineered crack crosses a pre-existing fissure depends on the shear strength of the cracks and the friction properties of the pre-existing fissure under the combined effect of the stress approach angle, the crack initiation approach distance, and the cementation strength of the filling materials.

Key words: semi-circular bend, rock fracturing, natural fracture, filling cement, fracture toughness