OCR

Selection of Equipment The decision of whether to use shovels or draglines to strip the overburden was carefully weighed. Four important factors enter into the picture in the selection of this equipment: production capacity required, range in overburden digging depths needed, flexibility to handle multiple seam mining, problems and comparative fixed and operating costs. Studies indicated a 40 cu. yd. dragline with a 250 ft. boom was best suited for the initial stripping tool. This boom, if it stood upright, would match the height of the smoke Stack at the Four Corners Power plant. Production from this dragline is more than 16,000 cu. yds. of waste each working shift. Every six months this 40 cu. yd. Marion dragline will move as much material as there is in Hoover Dam--or enough to bury a football field under two-thirds of a mile of rock. The "box cut" now being dug is 100 feet wide; the coal at this point is from 26 to 32 feet below the surface. The Marion 7900 walking dragline with present boom and bucket can handle overburden up to 80 feet in normal straightaway stripping. On an outside curve and spoiling up to the top of the coal, it can go to 100 feet. A 50 ft. extension section using a smaller bucket is available for even thicker overburden. Electrical equipment is 7,200 volts. Dragline's First Job The first job of the dragline was to make a 100-foot wide box cut in the Dodge pit. A. Marion M del 5323 shovel carrying a 22 cu. yd. dipper began the Dodge Pit box cut in the shallow overburden area while the 7900 was still being erected. This shovel was moved from the company's worked-out Ozark Philpott operation in Arkansas and will be used at Navajo whenever it is necessary to assist the 7900 in this multi-pit strip operation. Overburden drilling is being carried on with a Bucyrus-Erie 6OR, machine making 12-1/2 In. diameter holes. A. Joy Champion 58BH drill serves as a spare. Holes are drilled on 33 ft. centers in the 100 ft. wide box cut, and are loaded with from . 2 lb. / cy to .5 lb. /cy of "Geneva prills," supplied by U.S. Steel. Charges are detonated with one 2x8 stick of 60 per cent dynamite with electric cap. Two men handle all the details of charging and firing holes, while two men drill the holes and a D8 bulldozer with operator prepares the drill bench and assists in preparing the dragline grade. Coal Quality Stabilized By using two loading shovels and by working in two or three pits, coal quality is stabilized. Two Marion 151-M shovels with 11 cu. yd. buckets load the coal into 10 converted DW 20 Caterpillar bottom-dump tractor-trailer units, expanded to 50 tons through the addition of a 3-foot side board and a 3-foot mid-section, for haulage about four miles to the crusher. The units average 20 mph and deliver up to 1,000 tons per shift.