Abstract: The rock bridge in rock mass controls the stability of slope. The failure of rock bridge generates loading rate effect due to the influence of excavation speed. It is of great significance to explore the deformation and failure characteristics and crack propagation mechanism of rock bridge in slope under the influence of loading rate. By prefabricating cracks at the end of rock-like material, we prepared test specimens with middle rock bridge and examined the failure characteristics, crack initiation, propagation and penetration laws of specimens with different rock bridge lengths under varying loading rates with the aid of digital imaging. The crack propagation mechanism was also revealed in line with the theory of fracture mechanics. Results manifested that: (1) The stress-strain curve displayed post-peak fluctuation and stress slump characteristics. With the rising of loading rate, peak strength swelled whereas peak displacement shrinks. (2) Five patterns of crack propagation were identified: the upper crack ran through the lower end face; the rock bridge penetrated through; the lower crack ran through the left end face; the lower crack ran through the lower end face; and the lower crack ran through the upper end face. (3) During the failure of specimen, the number of cracks declined with the rising of loading rate. Cracks started from the tip of the lower crack. (4) The expression of stress intensity factor at crack tip considering closure effect under uniaxial compression was derived, and the theoretical calculation and experimental results of crack initiation angle were both within the allowable range of error. (5) The change of surface strain field of specimen intensified with the rising of loading rate, and the failure of sample was caused by the accumulation of damage in the early stage.

Key words: rock bridge, crack propagation, crack initiation angle, loading rate, digital image