The wealth of Wydoak.
Wyoming is the leading coal producing state in the U.S. and its 1993 output of some 182 Mt would have placed it in the top ten of all world producing countries. It boasts three coalfields, the Powder River Basin, Hanna and Bighorn, of which the former is the largest, extending from Douglas in the south to 30 km north of Gillette.
Worked by more than ten companies in 17 active operations, the Powder River Basin follows the course of the Wydoak formation, one of the most important coal-bearing seams in the U.S. The 112 km long Wydoak seam is estimated to contain sub-bituminous coal reserves of 64,000 Mt, of which 21,000 Mt can be recovered by existing surface mining techniques.
The top four U.S. surface coal mines are all Wyoming operations; Black Thunder, Jacobs Ranch, Rochelle and Caballo. Cyprus Amax Coal's top producers are both mining the Wydoak and rank 10th and 8th respectively among U.S. coal open pits; Belle Ayr and Eagle Butte. Belle Ayr opened in 1972 and has one working seam some 22 m thick of Wydoak coal; it produced 11.8 Mt in 1992. Belle Ayr and the nearby Eagle Butte (12.4 Mt in 1992) have together produced more than 400 Mt in the first ten years of production.
Belle Ayr's 8,450 Btu coal is dug out by two Bucyrus-Erie 295 30 |m.sup.3~ face shovels loading a fleet of nine Dresser Haulpak 120 t trucks. The average stripping ratio is 1.8:1.
Overburden is removed with Marion 301 and B-E 295 face shovels equipped with 42 and 20 |m.sup.3~ buckets, respectively. The five-truck overburden fleet is made up of 217 t Unit Rigs which haul the waste some 1,100 m to the dump.
Arriving at a rate of around 35 truck loads per hour, equivalent to 2 Mt of overburden per month, the material dumped is levelled by a 90 t Tiger 690B wheel dozer. The Tiger is equipped with a 16.5 |m.sup.3~ capacity straight blade and spreads each 217 t load in just three passes, pushing every load of material over a push-distance of around 70 m. Belle Ayr's production manager Steven Rennell reports that the 690B was chosen for its ability to match the 7,620 t/hr dump input levels. "A wheel dozer is much faster over the ground than a tracked machine of equivalent size," says Rennell. "We chose the Tiger 690B because it has the speed and power to handle the loads on the dump."