Effects of Blast Induced Wave Propagation on the Degradation of Jointed Rock Masses

In open pit mines, repetitive blast-induced ground vibration can increase the risk of pit wall instability due to strain accumulation along discontinuities. Presence of discrete fractures or fracture networks in a rock mass can influence the propagation of blast-induced shock waves in the rock and consequently the degradation of shear strength of the jointed rock mass. Quantification of blast-induced rock mass degradation is essential for prediction of potential risk of pit slope failure. This paper presents the results of a series of numerical experiments that examine the effects of ground vibration from a single row blast on a jointed rock mass, simulated using the Particle Flow Code (PFC2D). Discrete Fracture Network (DFN) was used to generate two joint sets in a rock block of 4 m x 8 m. Two different scenarios were considered: a) two orthogonal joint sets (one horizontal and one vertical set), b) two inclined joint sets. For each joint orientation scenario, five different fracture intensities (P₂₁) were generated, varying between 0.4 and 5 m^-1. Recorded blast vibration history from a quarry was applied to the 2D jointed rock mass samples and wave propagation was monitored along the rock blocks. Results show that the first fractures along the wave propagation path have experienced more degradation (damage) in the form of micro-crack generation along the fractures. Damage was only developed along the horizontal joint set for the rock mass model with orthogonal sets, whereas in the rock mass with two inclined joint sets, damage was accumulated on both joint sets. Rock mass degradation increases as the fracture intensity increase.