To investigate the radial fracture and damage under confining pressure and repeated blasting, the high-speed (HS) camera and strain gauges were used to test the dynamic response, meanwhile a damage model is established to analyze the damage evolution. The study reveals that the compressive stress is formed by initial confining pressures, and it effectively prevents radial cracks growing, reduces the damage accumulation speed and increases the ability of rock against blast loading. Under the repeated blasting, the elastic deformation appear and the initial microcracks do not have enough energy to develop into fractures after the first loading, the rock deformation linear increase with loading times and failure suddenly by the last loading. The blast induced damage was successfully assessed by strain variables in the damage model. The damage (D) is less than 0.4 and a few micro cracks are generated by elastic plasticity deformation after first loading, slowly increase to 0.6 with the number of repeated blasting. At last, D exceed 0.8 and cause the rock broken completely under the last loading. The study results clearly indicate that the repeated blasting and initial in situ stress should be taken into consideration for proper assessment in underground excavations.
As the deep tunneling and underground space creation is rapidly growing, blasting as an inexpensive method is widely used in underground engineering. Unlike the surface rock, underground rocks naturally stressed by gravitational and tectonic stress, moreover, the stress redistribute is formed along blast created excavation boundaries, and the failure mechanism is become complicated due to the combined effects of stress redistribution and repeated blasting (Liu et al. 2011; Ramulu et al. 2009; Zhang & Zhao 2014). In general, blast-induced rock damage results from stress waves and subsequent explosion gas expansion, the explosion gas plays a more important role on broken zone around borehole. The lower stress waves pass through the medium with the spreading distance increase, and the further rock mass is not completely broken under single blasting. After repeated blasting, the irreversible damage accumulate and induce the cracks development, and leading to safety and stability problems in underground engineering at last (Ricketts 1988; Verma et al. 2018; Zhang et al. 2017).