Design criteria for production blasting in an underground quarry: an example of application of the BDI theory

Although not common, there are cases of underground quarries where the exploitation affects materials like limestone, when both technical and economic feasibility are guaranteed; the object of the present study is part of this case. The quarry is in operation since 1927: at present, the exploitation is carried out completely underground by sub-level stoping. The exploitation Involves two levels, with 4 rooms at the upper level, and 9 rooms at the lower level. The need to comply with the required production respecting the desired grain size distribution implies the adoption of 650-800 kg of explosives per blast. The vibrations, as well as the related unwanted effects, are then considerable, regardless of the timing system. The vibrations must therefore be reduced, through the analysis of the blasting pattern, a careful design and a proper performance of the blast. For this purpose, the theory of Blast Damage Index (BDI) was used: through the development of correlations between charge per delay, propagation velocity of p waves in the rock mass and peak particle velocity, it was possible to determine which pair minimum distance/maximum charge per delay is allowed.