Quarry conundrum

The 3M operation eats boulders for lunch but struggles with some little things.

By: Steve Rauscher
    MONTGOMERY — Driving along the quiet back roads that plunge through the wooded northeast of the township, or gazing at the deep green mass of the Sourland Mountain, you could be forgiven for being unaware that not too far away lies a tremendous chasm that is getting bigger almost every day.
   At the bottom of a pit sits a vast rock garden, packed with a mass of boulders, each about as large as a house, which might, in their final form, end up on top of yours.
   This is the quarry, dug out over 40 years by employees of the Minnesota Mining and Manufacturing Co., where great, big rocks are pulverized into the tiny pebbles used on roofing shingles.
   "We’re actually very early in our life," plant manager Leigh Otterlei said as she surveyed the canyon below her. "We’ve probably got another 100 years of mining left."
   Which will make for one very large hole in the ground. As of last week, the quarry encompassed 40 acres and was about 110 feet deep. It isn’t likely to get much wider, as 3M has reached the limits of its footprint on its 1,500-acre property spanning the Montgomery-Hillsborough township line. It is likely to get much deeper, though. As quarries go, this one is a bit of a pygmy.
   "Even the one at Trap Rock (in Franklin Township) is bigger," Ms. Otterlei said.
   3M’s Belle Mead quarry is one of five around the country from which the company manufactures roofing materials. It is perfectly placed atop a deep layer of diabase bedrock, amidst an extensive transportation network that allows the operation to provide rock for the entire Northeast roofing industry.
   But the rural landscape against which the 3M site was once set has become a fast-growing suburb, which has led to some problems. The only way to mine solid rock is with explosives, and explosions are loud. Most of 3M’s 1,500-acre property is used only as a buffer zone between the quarry and its neighbors. In fact, the whole site recently was certified a protected wildlife habitat by the National Wildlife Council.
   The company blasts about once a week, Ms. Otterlei said. Workers drill a 50-foot shaft into the rock, near the edge of the quarry, which is shaped something like an amphitheater with three levels. When the dynamite detonates, it cleaves thousands of tons of rocks onto the level below it. An industrial-strength backhoe then picks up the rocks and deposits them in the back of a dump truck with 8-foot wheels.
   There are two components to the blast’s fallout, Ms. Otterlei said, that affect those nearby. The first is the ground vibration or "deep boom," which sends vibrations through the rock that can potentially cause structural damage to homes. She said the company stays well within the state limits for such vibrations. What neighbors hear, and sometimes feel, is the air blast, which carries the sound of the explosion and can cause picture frames and china cabinets to wobble.
   "It’s annoying," she said. "But it doesn’t cause any damage."
   The blasting often releases boulders far too large for the on-site rock crushers. The boulders are taken to the Trap Rock Industries quarry in Franklin, where they are ground up and sold back to 3M.
   Those boulders that aren’t destined for the bottom of the quarry are hauled half a mile to a complex of rock crushers, which looks, with its network of oddly shaped towers and conveyor belts stretching every which way, like a Rube Goldberg machine designed by Wile E. Coyote.
   The first crusher operates like a giant mortar and pestle, pounding the rocks until they are about a foot wide. Then they are fed into a cone-shaped crusher, where they are whipped against the sides of the cone until they are about pebble-sized and small enough to pass through a screen at the cone’s bottom. The pebbles are passed into a third crusher, which pulverizes them into the small bits you see on roofing shingles.
   During the processing operation, however, tiny particles called mineral fines break off the rock. Most are collected and deposited in massive piles on the site. The fines are kept wet to keep them from blowing away in the wind, but the company has its hands full keeping the fines on site when a hard rain washes them away.
   The company has been fined several times, as recently as last summer, for failing to halt the runoff into local waterways. When present in large concentrations, the fines turn creeks an opaque white.
   Township Health Officer Dave Henry said the fines present no real danger, but could halt plant growth on the stream bed if present for too long.
   "It’s really an aesthetic thing," Mr. Henry said.
   The township and 3M have been at work on a stormwater management plan that would greatly reduce such instances, which Mr. Henry said have become less frequent in the last few years. The plan centers on building nine large detention basins in Montgomery and Hillsborough that would hold runoff long enough for the fines to settle out before the water discharges into surrounding streams.
   "I’m really optimistic about it," Ms. Otterlei said. Township Engineer Don Johnson said he could not comment on the project before it came before the Planning Board, which is expected to happen next month.
   Ms. Otterlei would prefer that people concentrate on the positive. The company employs 92 people, many of whom live in the township. It contributes, by her estimate, $1 million to $2 million to the local economy, and sponsors many local events.
   "And we have 1,000 acres of taxpaying open space," she said.
   In a nutshell, she might say, 3M rocks.