When I arrived about 10 in the morning on Friday, October 19, the muddy gravel parking lot was full of cars and a line of waiting visitors stretch nearly a block out of the hospitality tent. I learned later they started arriving at about 3 a.m., even though the announcement had said the event would run from 5 a.m. to 3 p.m. All of us had been invited to attend another of Martin Marietta Aggregates' Underground Tours of their Ames, Iowa, mine.
Fortunately, I got to go right into the tent, where all the coffee and donuts awaited the visitors. My companion, Midwest Contractor Sales Rep Mandy Preston, placed the bag full of canned goods she had brought in a donation bin — a donation to the homeless and hungry requested in the invitation.
According to the literature, "The original quarry started on this site in the 1960s, operated by Ray Cook Construction. The initial underground opening, or portal, was started in 1978 by E. I. Sargent Companies, which was later acquired by Martin Marietta Aggregates in 1982. The current mine covers approximately 620 acres, and extends 1.4 miles from north to south and 1.5 miles wide from east to west."
The mine was dark except for Martin Marietta Manager Mark Worrall's pickup lights and the occasional self-powered lighting tower. We stayed inside the truck, but a slightly damp chill crept in. It was quiet except for the rumble of the engine and Mark's explanations. Most of the work was suspended for the day, though we did see a few machines doing some minor chores. We drove off the route the rented school buses were taking the other visitors, driving down broad tunnels with surprisingly high ceilings: I had expected the low roof of a coal mine. The "roads" have street signs and even a stoplight or two.
"The Ames Mine has a number of avenues or streets on each level, which divide that part of the mine into modules," the handout went on. "A mining module or Mod is about 640 feet by 640 feet or 9.4 acres. The avenues make up the main haul roads in the mine. Each avenue is 40 feet wide. Each Mod is made up of rooms (areas that are mined out) and pillars (support structures). The pillars are 40 feet by 40 feet and the rooms are 40 feet wide. This room and pillar mining pattern allows for removal of about 75 percent of the stone.
"The Ames Mine has three different mining horizons. The upper mining horizon is the Gilmore City Limestone formation, and varies in thickness from 22 feet to 37 feet. The mine ceiling at this level is 220 feet to 240 feet below the ground surface. The Gilmore City formation is named after the outcrop area near Gilmore City, Iowa, where Martin Marietta operates an open pit quarry. The majority of the stone previously sold from the Ames Mine came from the Gilmore City Formation. The middle mining horizon is the Eagle City and Maynes Creek members of the Maynes Creek Formation and consist of dolomite which contains numerous chert nodules (fine crystalline silica, i.e. flint). The lowest mining level is the Lime Creek Formation (currently 380 feet to 420 feet below the surface), which is now the active source of limestone for all materials currently supplied from the Ames Mine. The Lime Creek is a Devonian limestone approximately 370 million years old. The stone from the Lime Creek is used to produce concrete stone, asphalt stone, base material, and ag-lime."
The air was fairly fresh, not stale or dusty. Mark told us that for the most part, the natural, but engineered, venting keeps the air moving. The mine chambers are designed to be closed off once work is done in that area. That keeps a clear channel for air to circulate throughout the mine and from there to the surface. "The mine has two 12-foot-diameter airshafts or fan shafts. The two fan shafts allow 400,000 cubic feet per minute of airflow, to provide fresh air into the mine."
We talked a little about the blast process. I was surprised that blasting takes place nearly every day and that work in other parts of the mine are not affected nor shut down. "The limestone headings are drilled in a specific pattern with a 2-inch bit, loaded with explosives and detonated. A scaler, which has a long hydraulic boom with a pick on the end, is used to remove any loose stone from the mine roof and walls. A large pit loader is used to load the blasted material into a 60-ton haul truck, which carries the material through the series of haul roads to a primary crusher. The limestone is crushed down to a 6-inch to 9-inch size, and is conveyed from the mine to a surge pile on the surface. The stone is then conveyed to a secondary crusher for final crushing, and is then conveyed to a series of screens for final sizing. Haul trucks and conveyors then move the finished materials to a stockpile, where they are eventually loaded into customer trucks, and hauled across the scale systems."
As I love statistics, aggregates by the numbers got my attention. "Materials produced at the Ames Mine are primarily used in construction for road base, concrete and asphalt. Another very common use of limestone is to neutralize soil acidity, which, when finely ground, is commonly referred to as aglime. In fact, crushed stone is one of the most commonly used minerals on earth. Nearly 80 percent of all concrete and asphalt roadways are made up of aggregates. About 10 tons of aggregates per person are used annually in America. Every mile of interstate contains 38,000 tons of aggregates; about 400 tons of aggregates are used in construction of the average home. Limestone is also used in the production of glass, paper, steel, PVC plastics, shingles, and other common household products, for air and water purification, and erosion control along dam, lakes and waterways."
Finally, the "Ames Mine is the largest single source of aggregate supply in Iowa, and provides one of the highest quality sources of limestone available. The market area is generally within approximately 60-mile radius, including the Ames and Des Moines areas, although, due to the high quality, materials are shipped considerably further for specific uses. The Ames Mine is part of the Midwest Division of the nation's second-largest producer of construction aggregates. The company has more than 300 quarries and distribution facilities in 28 states, the Bahamas and Nova Scotia."