When performing blasting operations in open pit mines, large amounts of energy is released and transmitted through the rock. The energy released can have significant impacts on mining operations and can adversely affect the mine production. This technical paper presents recommendations for blast design to prevent damage from blasting on underground concrete structures. Emphasis is placed in vibrations generated by blasting and its effects are studied referring to an existing hard rock mine expansion and asks whether production blasting would damage adjacent underground concrete structures due to blast stress waves. A set of criteria was developed to implement when blasting near-by underground structures. Knowledge of particle velocity and wave propagation theory for site-specific conditions to determine a safe level of vibration was recommended. The results suggest that an increase in structural capacity (dynamic capacity) is expected when structures are subjected to loads at very high strain rates, such as those of blasts.
When expanding the limits of a surface operation, generally there are no geometric constraints if the slope design, operability, and ore to waste ratios allow for a safe and economically feasible operation. However, geometric constraints can lead to issues when blasting rock in proximity to near-by structures as blast waves produce stresses on the structures that structures may or may not be designed to sustain. The enhanced dynamic capacity of the structure when subjected to high strain rate loading allows an elevated ground vibration (PPV) or particle velocity as it is investigated in this paper through a case study.
This case study involves a hard rock open pit mine located in western USA. At this site, the northwest area of the pit was undergoing drilling explorations when high-grade deposit was identified. The mineral evaluation indicated that the upper portion was mostly low grade and waste and the profitable portion was at the bottom of the pit, at approximately 300 meters deep (4200 level). Initial concerns with the proposed expansion were funded on the fact that the final crest would be set at approximately 50 meters from the ventilation exhaust system at the surface. The cross section of the vent shaft consisted of a 5 m (15 ft) by 5 m (15 ft), 0.91 m (3 ft) thick with #6 reinforcing bars 6-in o.c. of reinforced concrete wall that extends underground 340 m (1245 ft). The vent shaft fan operates in normal conditions at 18,000 rpm and moves an air flow Q of 70,800 m3/sec (2.5 million ft3/min).
