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

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.

Conveyors running along each side of a 50-foot-diameter pillar deliver material to the 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.