Set in Concrete
Right now, the haul distance from the pit to the gyratory crusher is more than a mile because of the depth of the pit and the number of benches, but work is being done on some of the benches to shorten the haul route, which will reduce fuel usage and improve cycle times.
A 42-inch-wide belt with a 400-horsepower motor carries material from the gyratory to the surge pile. Two 900-foot-long, clean-stone blending tunnels have 36-inch-wide conveyors that run through them. One conveyor feeds the wash screen on top of the bin tower. A quaternary crusher recrushes the material into half-inch and larger, all the way up to 3s, which are ballast stones.
At the south end of the plant, a 500-foot tunnel joins the two blending tunnels underground. “Back in the day, they did some blending between the two, but the tunnel was difficult to clean, so we pulled the conveyor out,” Podrazik says. “It’s an access way and escape tunnel now.”
Only two wheel loaders are used in the plant. One operates on the yard and the other is used for stockpiling.
The plant is not totally automated as many are today. One person in the loadout control tower monitors cameras in the tunnels and brings product up from the stockpiles. A conveyor on top of the bins runs on a rail-type system that allows it to reverse and move from one bin to another. Only the pressure circuit with the bins is automated.
“That’s one of the things we’d like to do,” Podrazik says, “get this plant more automated. But automating a plant from the ’60s isn’t the easiest thing in the world.”
The primary plant can handle about 1,400 tons per hour; the secondary handles about 750 to 800 tons per hour. The primary plant has one conveyor. The secondary plant has 45 conveyors, 10 screens, and 45 feeders — 35 of which are vibrating or belt-type screens and 10 of which are gate feeders. The last major upgrade done to the secondary plant was the addition of a recrushing circuit and screen that were added in 1988.
Between the primary and secondary plants, there are more than 16 motors of 250 horsepower or greater. Nine pumps in the wash circuit — pit dewatering and slurry circuit — pull out all the wash water and recirculate it through seven settling ponds on the property.
The plant has cable-suspended stackers. They have to be inspected regularly, but they allow more room for other equipment. “You don’t have to worry about loaders banging into towers,” Podrazik says. “They give you plenty of room to stockpile in just one bin.”
Contract blasting is provided by Austin Powder Co. “They do a good job for us,” Podrazik says. “We use a down-the-hole hammer drill to drill a 6-inch hole in a 13 by 17 pattern. Average shot size is between 30,000 and 40,000 tons. The drill is used by two or three different locations, so when it gets through drilling here, it moves to another plant. We shoot electronics now, but historically, the blasts have been non-electric.”
Smith says the quarry does have rail service, but that it hasn’t been used much lately. “We don’t load a lot out by rail currently,” he says, “but we have that potential.”
Stress on safety
“Martin Marietta prides itself on safety,” Podrazik says. “In our industry, everyone is improving on it every day. At this location, the last lost-time incident we had was 600,000 man-hours ago in 2001. The last reportable we had at this location was in 2006.”
Arrowood Quarry has an internal safety center that is used by everyone at the plant. The center features an overhead projector in each room, allowing easy access to safety information for all its employees.