Numerical analysis of rockfall hazard in open pit coal mines
Rockfalls are a significant safety hazard in open pit mines and underground mine entries from open cut highwalls that need to be rigorously managed when designing portal entries for punch longwalls. The installation of restraining nets is a common practice to mitigate this hazard. The protective system however does not totally eliminate the rockfall hazard as blocks can still detach and fall in between the net and the highwall. In such cases it is of prime importance to predict the rock fall trajectories and velocities behind the protective net in order to properly map and assess the residual hazard. An integrated approach combining field testing and DEM is currently being developed for which site specific knowledge of the fundamental characteristics of rockfalls is necessary. This work presents numerical analyses from which estimations of rockfall motion, trajectories, arrest zones and potential impacting energy on the protection structure are worked out. The study entails the estimation of the size distribution of unstable block first and then the simulation of their trajectories. The former requires an accurate description of the rock mass structure. By combining digital 3D photogrammetry analyses with Discrete Fracture Network modelling it is possible to generate a polyhedral model of the rock mass structure. A modeller capable to automatically identify complex polyhedra (rock blocks) has been used to represent a rock mass with finite persistence discontinuities. Size distribution of unstable blocks and trajectories are assessed performing Monte Carlo analyses where unstable blocks are detected using the key-block method for each realisation. Results are compared with the actual history of rockfall events. This site-specific knowledge will later be used for residual hazard assessment (i.e. trajectories and final velocities behind protective nets).