Monolithic machines and men in hard hats are creating a subterranean world just north of the East-West Tollway in North Aurora.
The men, 18 in all, do two shifts a day at the Conco-Western Stone Co. limestone mine, 250 feet below the surface. Material from the 92-acre mine, used to make cement, asphalt, road base, backfill and landscaping stone, among other products, is gathered by drilling, blasting and crushing in the cool, dry caverns laced with electrical conduits and the occasional portable toilet.
The North Aurora mine has been operating obscurely for six years, except for the occasional blast that rattles nearby residents. About 4,500 tons of limestone is removed per day from the mine, which could be in operation for 30 more years.
In Aurora, the city next Tuesday is expected to approve a contract with Conco-Western that will allow it to bore three tunnels under the East-West Tollway to reach another part of the same limestone deposit on the other side. At least 50, and as much as 71 acres, of underground area will be mined in Aurora.
The city, which owns that land, would receive about $2.9 million in royalties and sales taxes during the expected 22 years of operation on its side, said attorney John Duggan, who negotiated the lease for the city. In Aurora, the land lies below a tapped-out surface limestone mine, as does the one in North Aurora.
“These days, it’s very, very difficult to get a permit for surface work,” said Mike Dunn, Conco-Western’s general superintendent. “So I think in the future more and more people will be going underground.”
It took crews three months in late 1992 to bore through 156 feet of shale to reach the limestone, part of a massive dolomite deposit called the Galena-Platteville Formation mined in several Illinois locations, including the tapped-out Elmhurst-Chicago Stone Quarry in Elmhurst.
To get to the Aurora side, it will take about a year and a half, Dunn said.
Once the crews reached the limestone through a 12-foot-wide, 18-foot-tall adit–an exploratory person-size tunnel–they blasted out four main roads, mining the limestone as they went. Each road is 40 feet wide and 23 feet high. An 8,000-square-foot maintenance area was then created near the adit.
From those roads, tunnels are carved out, with limestone pillars at least 40 feet by 40 feet left in place for support, Dunn said.
Once each tunnel is mined out, workers blast another 27 feet below the 23-foot-high tunnels, creating a mined area 50 feet high. There are 3 to 3 1/2 levels to be mined, and Conco-Western has another year and a half before it finishes the first level, Dunn said. In the end, about 70 percent of the limestone can be removed, he added.
To loosen the rock, a 13 1/2-foot hole is drilled. It is stuffed with ammonium nitrate, a small stick of dynamite and a blasting cap and is detonated from another part of the mine. Then the stone is removed and crushed at another site in the mine. From there, it’s lifted out through conveyers and deposited in heaps above ground.
Safety is paramount. About 230,000 cubic feet of air is pumped in to the mine each minute. An escape tunnel has been constructed.
Each worker carries an emergency breathing apparatus that can convert carbon monoxide to less-harmful carbon dioxide for one hour should a fire break out. That’s never happened, Dunn said.
Once the mines are empty, they can be adapted for several uses, including storm-water storage, garbage disposal, document storage and, in some cases, manufacturing operations for which the cool, dry conditions are ideal. In Kansas City, Mo., the Internal Revenue Service stores documents in an old mine, Dunn said.
In North Aurora, the village will be given the mine at the end of the operation. In Aurora, the city may get access to portions of the mine before it is tapped out, under the lease agreement.
