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“Mining coal is easy.
Uncovering Coal is the Biggest Job
Coal seams #3 and #4 are exposed in this spectacular natural outcropping of the coal-bearing rocks north of Hoseanna Creek, near Healy, Alaska. The two coal seams are typical of the ultra-low sulfur subbituminous coal found in the Nenana coal field and mined at Usibelli Coal Mine, Inc.
At Usibelli Coal Mine, up to 100 feet of unconsolidated sandstone or overburden must be moved to uncover the top seam (seam number 6) of coal. Another 120 feet of interburden must be moved to uncover the second seam (seam number 4) while roughly another 80 feet of interburden must be removed to uncover the third seam of coal (seam number 3) reaching nearly 400 feet at its deepest point. Overburden is removed by four different methods:
Dragline does the "dirt" work
The most efficient machines for moving large volumes of dirt are draglines. In 1977, UCM made a major investment by purchasing a 1300W Bucyrus-Erie Walking Dragline. The dragline arrived in component parts on 26 railcars and 40 trucks during December 1977. It took 11 months to assemble the 2,100-ton machine. Named "Ace-in-the-Hole" by Healy school children, the dragline is the largest land mobile machine in Alaska. The acquisition of "Ace-in-the-Hole" made it possible to double coal production and initiate UCM's South Korean export contract in 1985. With its 325-foot boom, the dragline has a reach of 270 feet. The bucket weighs 32 tons and will hold 33 cubic yards of material. In one 24-hour period, the dragline can move 24,000 yards of dirt, leaving a strip of uncovered coal 145-feet (or more) wide. Stability for this heavy machine is achieved by the large steel plate (the tub) which rests on the ground during drag-line operations. Wheels or tracks would be impractical due to its extreme weight. The dragline moves by "walking." Shoes on both sides of the machine lift the base partially off the ground and drag it backward. Each step takes about 40 seconds and moves the dragline a distance of approximately 7 feet. While it takes only one person to operate the controls, there are two operators on duty at all times. One operates the dragline from the cab while the other does routine maintenance. For safety reasons, the operators switch positions every hour. Since the dragline was commissioned in 1978, it has operated for more than 110,000 hours of digging. It moves approximately 5 to 6 million cubic yards of overburden every year.
The dragline has several buckets, which are used on a rotational basis. UCM utilizes several different configurations of bucket including 33-, 35-, and a high-production 42 cubic-yard bucket. Each bucket is used on the dragline for approximately 4 to 6 months. After that period, the bucket must be serviced repairing critical surfaces of the bucket vulnerable to the abrasive excavated materials. The bucket is repaired by welding wear plates on vulnerable surfaces that are worn as a result of the abrasive characteristics of the materials that are excavated.
Blasting loosens materials, casts overburden
Both overburden and coal can be very difficult to excavate. Therefore, blasting is necessary in order to loosen the overburden and coal before it is moved by the heavy "yellow-iron" equipment. A specially designed powder truck transports, mixes and loads explosives into each drill hole. A technique called "cast blasting" is used to help remove the overburden. During cast blasting, the drill holes and explosive charges are designed so that a portion of the overburden is cast laterally by the force of the explosion into the adjacent mined-out pit. This technique, when utilized, reduces up to 25% of the amount of overburden that the dragline must handle. Cast blasting is a significant cost savings when compared to utilizing trucks and shovels. There is less wear and tear on equipment, less consumption of diesel fuel, and fewer employees are required. Cast blasts are conducted approximately 3 or 4 times a year.
Dozers move overburden short distances
UCM uses dozers including a Komatsu 475A-5 dozer and several large Caterpillar dozers including the D11R-CD, Carry Dozer. They are some of the largest tracked dozers manufactured in the world. The dozers move forward and backwards pushing overburden off the surface of a mining bench into the pit. The dozers also prepare the surface area in and around the dragline.
Shovels and backhoes strip overburden and load coal
While the dragline does the majority of the excavation work at the mine, additional excavators are needed. Four track-mounted O&K excavators, two RH120C backhoes and two RH170 shovels, are used to load coal and to strip overburden in areas difficult for the dragline to maneuver. The backhoes have 16 cubic-yard buckets and are powered by twin 567 horsepower diesel engines. The first large hydraulic shovel, an O&K RH-170, was added to UCM's fleet in August 1997 and the second in 2011. The shovel has a 26 cubic-yard bucket and can load the 150-ton trucks in four passes.
Trucks and other equipment support operations
A fleet of ten trucks is used to haul coal and overburden (dirt, topsoil, gravel and rocks). During 1995, the Caterpillar 785 haul truck was added to the UCM fleet. The truck has a capacity of 150 tons, more than 50% greater than the previous 95-ton Dresser HaulPaks. Local modifications are made to the 150 ton truck after delivery from the manufacturer. UCM customizes the truck bed by adding an additional 16 inches in height and 24 inches in width of the bed. The additional height and width allows a larger volume of coal (which is lighter than overburden) to be hauled. UCM currently operates a fleet of eight Caterpillar 785 (150 ton); two Caterpillar 777 (100 ton) and one Dresser HaulPak (95ton) haul trucks.
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PO Box 1000
100 River Road
Healy, AK 99743
Tel: (907) 683-2226
Fax: (907) 683-2253
100 Cushman St., Suite. 210
Fairbanks, AK 99701-4674
Tel: (907) 452-2625
Fax: (907) 451-6543