Numerical modelling of the effects of air decking/decoupling in production and controlled blasting

A series of numerical simulations were performed to investigate the physical processes and effects in rock blasting with air decked/decoupled explosive charges. A newly developed continuum damage model for blasting analysis was successfully used to model the movement of detonation products in the borehole chamber with an air space. The model evaluates rock damage according to constitutive relationships and the concepts in statistical fracture mechanics, and predicts fragment size distribution considering the equilibrium between kinetic energy and fracture surface energy. It is shown that a secondary loading wave is generated due to the repeated movement of the detonation products within the borehole chamber when blasting with an air deck. As a result, the potential energy retained in the explosion gas is released and a significant part of it is transmitted into the rock burden. It is found that there exists a minimum beneficial air deck length which is determined by the balance between the energy loss from primary loading to stemming and the energy gain from secondary loading. Blasting results are modified with an air deck longer than the minimum beneficial length. The best blasting results are obtained with the longest air deck provided that the shortened stemming can confine the explosion gas in the borehole chamber. It is concluded that with the use of air decking, the volume of broken rock can be increased or the explosive usage can be reduced. The simulation results have been verified by experiments in model scale. It is also revealed that some variations to top air decking such as bottom and mid-column air decks do not benefit production blasting in the axisymmetric case of the simulation conditions. Three charging strategies, i.e., stemming decking, decoupling and air decking which are used in controlled blasting have been modelled. It is shown that air decking has the maximum damage potential to rock mass and it is recommended for both production and controlled blasting. Simulation results also suggest that the use of air decking for non-ideal explosives like ANFO is much more important than it is for some more ideal explosives like emulsion in maximizing energy utilization.