Blast Induced Crack Propagation and Damage Accumulation in Rock Mass Containing Initial Damage

Blast induced rock mass damage and crack propagation play important roles in structure safety and stability in mining, quarrying, and civil constructions. This paper focuses on the effect of small blasthole diameter blast on crack propagation and damage accumulation in water-bearing rock mass containing initial damage composed of inherent geological discontinuities and previous multiblast induced damage. To elucidate this effect, theoretical analysis of calculation method for several important blast influencing factors is firstly presented. Secondly, definition of a practical damage variable using ratio of longitudinal wave velocity in rock mass before blast occurrence to that after blast occurrence and derivation of a damage accumulation calculation equation accounting for initial damage and blasting effect are described. Lastly, a detailed description of the conducted in situ blast tests and plan layout of the sonic wave monitoring holes is reported. The results indicate that blast activates and then extends the initial cracks in rock mass, leading to accumulation of rock mass damage. The rock mass damage accumulation can be conveniently quantified using the proposed damage variable. When the damage variable reaches its threshold of 0.19, occurrence of damage in the surrounding rock mass is indicated. It is also found that the blast induced rock mass damage extent and the blast induced vibration velocities decrease nonlinearly with increasing the distance between blast source and monitoring position