September 23, 2015
Venture into an underground aggregate mine and you might think you’ve entered Moria, the underground dwarf kingdom in J.R.R. Tolkein’s The Lord of the Rings.
by Kerry Clines, Senior Editor
Editor’s Note: This article originally appeared in the April 2010 issue of Aggregates Manager.
An underground aggregate mine is something to behold. It’s not for the claustrophobic, that’s for sure, but it is said to be where aggregate mining is headed in the future. With permitting becoming more difficult and the ever-present NIMBY proponents, it seems to be a logical response to harvesting more aggregate without requiring more surface space.
Underground mining presents a unique set of challenges, and no one knows that better than Sam Van, plant superintendent at Sterling Materials’ underground mine located near the small town of Verona, in northern Kentucky. Van comes from a long line of miners — his father worked at Pitcher Mine, a zinc mine, and his grandfather and great-grandfather were miners — so you might say mining is in his blood.
Digging out the mine
The idea for the Sterling Materials underground quarry started in 1989, but the location was up in the air. For the next eight years, the company went through multiple zoning cases on a couple of different properties, neither of which was successful. The current location was the third attempt and was chosen for its good road access and nearness to the market. “It was about nine years in the making before we put a spade in the ground in 1998, and the rest is history,” says Alex Boone, president of Sterling Materials.
The land was originally a farm. Van spent a month designing the mine, which he insists is still a work in progress. “When you’re underground, you’re always developing,” Boone says. “The mine plan is that you always move in a circle. You keep going around the outside making it bigger. The drill moves either clockwise or counter-clockwise around the mine. Behind the drill comes the powder crew. Behind the powder crew comes the blast. Behind the blast comes the mucking out, then the trimming and bolting, and then back to drilling. It keeps running in that cycle.”
“We follow the contour of the deposits,” Van says. “The drills are constantly drilling in different areas for blasts, and we shoot every day at about 5 o’clock.”
During the drilling and blasting process, 50-foot-diameter pillars are left in place to support the ceiling. If you could look down through the ground at the mine, it would look like a giant three-dimensional checkerboard, with the pillars as white spaces and the open areas as black spaces. A scaler comes along behind the mucking crew to knock loose any jagged rock on the pillars and walls to leave a smoother surface.
The ceiling comes out amazingly straight after a blast because it usually follows the seam of a deposit. “The roof stays basically flat, but some places take a little bit of work,” Van says. That’s where the roof bolter comes into play. He chips away the uneven rock, drills holes into the ceiling, inserts 6-foot-long roof bolts coated with resin into the holes, and holds them in place for 40 seconds, essentially gluing the roof together.
The mine has three levels of operation. The first level is about 400 feet below ground level and produces aggregate for construction purposes. Level two produces a mix of aggregate and chemical lime. The third and lowest level of the mine is about 900 feet below ground level, which is 140 feet below sea level, and produces chemical lime only.
“The first two levels are fairly flat,” Van says, “but on the third level, we have grades of 25 to 30 percent. We’re mining on top of an old eroded surface 450 million years old. We have hills and valleys just like up on the surface. We follow the chemistry stone, so we go up, down, wherever it goes.”
The mined aggregate and chemical lime is hauled directly to underground crushers where the material is crushed to the desired size. From the crusher, it may go through a secondary crusher before being conveyed directly to the surface. The mine can split the material and send it to the lime plant located on the property or to the aggregate processing plant above ground.
“This mine was designed to do 3 ½ million tons plus per year,” Boone says. “We could do that without making any significant changes, but the market hasn’t required us to do that yet.”
It takes a special person to work underground, especially in wintertime when the days are short. Employees working the regular 10-hour shift go down into the mine before the sun comes up and return to the surface after it has set. “They never see the light of day in the winter,” Boone says. “You can’t take the first man or woman off the street and turn them into a miner. They have to want to do that.”
When aggregate demand is low in the winter, the quarry runs mostly the chemical lime level and employs about 39 people; about 25 of those employees work underground. In the summertime, when demand for aggregate increases, the quarry employs about 50 people and runs both the chemical and aggregate levels of the mine at the same time.
Probably one of the most unusual things about Sterling Materials’ employees is that all the underground truck drivers and most of the underground crusher operators are women. “They keep their equipment clean and in good working order,” Boone says. “They seem to have more pride in what they’re doing than the typical male we’ve had previously.”
“Ten years ago, you’d have never thought it would happen,” Van says. “Back then, if a woman went underground, that mine was jinxed. Women have come a long way.”
Van says that the miners are like family. “They know each other well,” he says. “When the weather’s bad, one person will go around and pick up some of the employees that live further out and bring them in.”
The processing plant
The only part of the operation above ground is the aggregate processing plant. This looks like any other processing plant, but the screens are housed inside buildings to protect them from the elements. After the product is processed through the screens, conveyors run the finished, sorted material out to the various stockpiles to await loadout.
“When we built this, we came out to the hilltop, cut off the top, and set up this processing plant,” Van says. “We’ve got 8s, 57s, DGA, crushed stone base, and 2s in different stockpiles around the rim of the hill.” Tunnels were built underneath the stockpiles with conveyors running product out to the “race track” that was built around the hill. The race track and loadout bins allow trucks to come in, pull right up, and load themselves.
“I’ve got these loadouts set for 10 tons a minute, so if the truck driver wants 25 tons, he runs 2 ½ minutes,” Van says. “The trucks just pull up under whichever type rock they want and load up.”
Underground mining is highly regulated, like any quarry. “We get four mandatory MSHA (Mine Safety and Health Administration) visits per year,” Boone says, “but we usually get even more [visits] because we’re in proximity to the Lexington office.”
Air quality is the major issue. “Air regulations are getting more stringent every year, but we meet whatever the next requirements are,” Boone says. “We burn biofuel as much as possible — up to 90 percent — which helps, and we have very good ventilation throughout the mine, especially in work areas.”
Large air tubes run throughout the mine for ventilation and lead to air shafts that go up to the surface. “We pull the good air in and pull the bad air back out through the tubes,” Van says. In an emergency, employees are instructed to follow the tubes to the air shaft and wait for rescue.
“If we hit water down there, there’s usually a lot of it,” Van says. “We have to grout it out to push it out away from us. If we hit water on the third level, it’s high pressure, about 250 pounds per square inch.”
In the winter, the mine is dry and dusty, but during the summer rainy season, water seeps into the mine turning the dirt floors into mud.
Underground mining does have its bright spots, however. One advantage to being underground is that you can mine without changing the surface of the land. The mine is underneath the processing plant and all that shows are several small air shaft openings, which are mostly hidden by the trees and other growth. Wildlife flourishes in the area.
Another advantage to working underground is that the temperature in the mine remains at about 68 degrees all year round. The heat of summer and the cold of winter don’t have any effect on the workers in the mine or the mining equipment.
The bottom line
Despite the differences between surface and underground mines, a goal-oriented focus remains constant.
“Never take your eyes off the ball, it’s a moving target,” Boone says. “You’re always looking for roof issues, whether or not you’re having pressure and what direction the pressure’s going, whether or not you have water issues. It’s like any other business — you pay attention to the details and things take care of themselves, but you never take your eye off the details.” AM
Underground Safety Training
The unique conditions found in underground mining require unique safety training. Mine Safety and Health Administration (MSHA) requirements demand that everyone who goes underground, even visitors, receive this training. “We provide this training to each person who goes underground at our mine, and it’s good for one year,” says Gail Goodnough, safety director for Sterling Materials’ underground mine.
“We use the MSA W-65 Self-Rescuer as a part of our personal protective equipment in case of an emergency,” Goodnough adds. The self-rescuer is a breathing apparatus that is rather heavy and awkward to tote around, but everyone is required to carry it on their person or have it within 25 feet of them at all times.
A brass tag system is used to keep track of who is in the mine. Each person going underground is required to sign in and pick up a brass tag before entering the mine and then sign out and return the tag as they leave. This allows the company to know, at all times, who is in the mine. If the mine has to be cleared for any reason, and someone is still tagged in, employees know to look for them.
Equipment Line Up
Mack water trucks (3)
Ford F-150 pickup
Ford F-150 parts truck
Ford F-250 pickups (2)
Ford F-250 superduty truck
Ford F-250 4×4 powder truck
Ford F-250 4×4 man transport
Ford F-350 pickups (2)
Ford F-350 superduty boom truck
Ford F-350 service trucks (2)
Ford F-350 utility truck
Ford F-450 utility elec.
Ford F-450 superduty truck
Ford ¾-ton truck
Ford F-800 welding truck
Ford F Series fuel truck
Chevy K15 suburban
Peterbilt (ANFO) 6wd
Chevy Silverado 2500 4wd (2)
Caterpillar 74D articulated haul trucks (4)
Midwest Machine Jackleg
Cannon Drill DP 12-HD
Cannon Drills DPI-HD (2)
Ingersoll Rand ECM590 rock drill
Ingersoll Rand 1-inch impact wrench
International powder monkey
Process Machinery mine conveyors
Process Machinery plant conveyors
High calcium crushing plant
Grove RT58D crane
Street Master sweeper
Case GXR 686 forklift
Kubota L4300DT 4wd tractor
Symon cone crusher/Metso screen
Portable screen #2
Nordberg 4-¼ crushers (2)
Hazemag impact crusher
Cedarapids roll crusher
Nordberg screen wash plant – 8’s
Telsmith RB jaw crusher
Telsmith vibrating grizzly feeder
Allied 2215 Power Boom pedal hammer
Seco screens (5)
Nordberg wash plant – 57’s
Metso screen #1-FS 303
Komatsu WA500 loader
Komatsu WA500-3LK loader
Caterpillar D8K dozer
Volvo L220E loader
Volvo L110E loader
Caterpillar 980H loader (3)
John Deere 770A grader
Snorkel TB60 manlift
Bobcat S150 skid steers (2)
Bobcat S250 skid steer
Caterpillar 330LC track hoe
Caterpillar 330 excavator
Caterpillar 330L excavator
JCB JS330 excavator
Caterpillar 320 CL excavator
Weatherhead hose machine
Leica TCR307 transit
Hotsy 790SS pressure washer
Hotsy 795 pressure washer
Hotsy 1280SSG pressure washer
Hotsy HSS-503089E pressure washer
AQ-3 rock grinder
AQ-4 rock grinder
Miller welder/Dimension NT 450
Diesel air compressor