A tractor-mounted ripper will be used for excavating a limestone having a seismic velocity of 6000 ft/s (1830 m/s). Field tests indicate that the ripper can obtain satisfactory rock fracturing to a depth of 2 ft (0.61 m) with one pass of a single ripper shank at 3-ft (0.91-m) intervals.

Average ripping speed for each 400-ft (122-m) pass is 1.5 mph (2.4 km/h). Maneuver and turn time at the end of each pass averages 0.3 min. Job efficiency is estimated at 0.70. Machine cost including the operator is $130/h. Estimate the hourly production and unit cost of excavation.