Question: Estimate the hourly production and unit cost of rock excavation by drilling and blasting. The rock is a limestone having a seismic velocity of 6000 ft/s (1830 mls). Trial blasting indicates that 3~-in. (8.9-cm) holes drilled in an 8-ft (2.44-m) rectangular pattern will provide the desired fracturing. A hole depth of 20 ft (6.1 m) yields an effective depth of 18 ft (5.5 m). The average drilling rate is estimated to be 70 ftfh (21.4 m/h). A powder factor of 1 Ib/BCY (0.59 kg/BCM) of ANFO will be used.
