Numerical and field analysis of wave propagation and cracks caused by presplit blasting in conglomerate rock mass at Gotvand Olya dam

Controlled blasting is commonly employed in civil and mining engineering developments. This reduces maintenance and supporting system costs and improves the bench appearance. Safe maintenance of the wall and avoidance of damage caused by blasting is therefore important in all subsequent underground and surface excavations. Different types of controlled blasting nowadays include trim blasting, line drilling, cushion blasting, pre-split blasting, fracture controlled method and linear shaped charge. Although these approaches incur additional operational costs, but considering benefits associated with the safety condition necessary for such operations, increased production and faster progress resulting from more stable walls, improved waste/ore ratio, controlled ore concentration and the required size reduction of the rock for haulage and loading, these additional costs are justifiable. Controlled blasting is also common in open-pit mining, quarry mining, trenching and shaft drilling. In case spacing and charge quantity are evaluated based on engineering design principals, a uniform fracture with narrow width would result. This would dampen the transfer of the explosion wave outside the explosion block, when production blast holes are fired. In pre-split blasting, which is a more common technique often used in such operations, blast holes with smaller diameters and lesser spacing than normal production sizes, are applied in the last drilling row. The method could easily be applied to all types of rocks. However, drilling patterns and the required explosive charges should be determined based on rock mass characteristics, such as stiffness, roughness, existence of discontinuities. Also, in pre-split blasting, contrary to other methods, the controlled blast holes are fired 50ms sooner than the main production blast holes. In case this delay exceeds 50ms for whatever reason, the fracture produced by the controlled blast holes will be filled with post-explosion fragments and their ability in cushioning the explosive transmitted wave is seriously hampered. Hence, in applications of this technique for hard rock mass, it is customary to leave a proportion of the blast holes without charge. The blast holes diameter in this technique varies from 51 to 102 mm, with the diameter of charge ranging from 17 to 32 mm, hence decoupling is less than one. The explosive connection is often carried out by detonating cord and if wire and electric detonators are used, they should be fast triggered type of milli-second delay or better. Superposition of compression waves due to adjacent blasting holes lead to tension stresses, perpendicular to the direction of blast-hole lines. This results in tensional fractures within the rock mass. In this study, using UDEC distinct element software, the mechanism of crack propagation and superposition of pre-split blasting waves in three holes are investigated, and the data are compared with field data obtained on the conglomerate rock mass at Gotvand Olya dam. Blast holes of 76mm in diameter, 3m height, 8 cm spacing were drilled at the rock mass and charging included 7 Emulite cartridges with cortex blasting. Ground vibrations of 175.24 mms-1 and 77.33 mms-1 were recorded by two VIBROLOC seismometers, placed at 8m and 13m away from the blast hole centre, respectively. The results suggest that numerical simulation could be employed with sufficient accuracy for predicting presplit blasting.