The new braces, called Atlas Cribs, are comprised of a mix of hardwoods and include a main lateral element made from a board with shorter boards nailed on both sides at both ends. They are much lighter than the traditional 6-inch square blocks of wood miners use now, making it easier for the miners to handle and stack them.
The new design also makes it easier to circulate air around the cribs, which could cut down on one of a mine’s biggest operating costs – circulating fresh air.
“Air has to circulate in a mine to dilute the methane gases and provide fresh air (to breathe),” explained Yoginder Paul Chugh, SIUC professor of mining and mineral resources engineering, who helped design the brace materials. “Moving air through a mine is the second largest energy consumer for a mine, next only to the transportation of coal out of the mine. That’s a pretty substantial amount of energy you’re spending on moving air.”
The new cribbing system takes up 41 percent less area than existing ones and may be up to 50 percent more efficient in terms of airflow, Chugh said.
Designed for Strength
The system’s strength comes from several design factors. First, the contact areas – the area where the braces touch each other and the load is actually borne – are equivalent to the traditional systems. This area is about 6 inches square.
Additionally, the shorter boards that are nailed to both sides of both ends of the lateral board are cut and positioned so that their grain runs vertically between the roof above and the floor of the mine. This axial grain orientation is much stronger than a parallel one.
“When you compress wood in a parallel direction it is very, very soft,” Chugh said. “So our central element has parallel grains, but the grain on the end pieces are axial to the load, and the strength of wood in this direction is about four times larger.”
For example, a traditional crib about 6 feet high will compress 12 to 14 inches under 70 tons of load. The same size crib made of Atlas Cribs will compress just 6 inches under more than twice the load – 150 tons.
The lighter weight also means lighter work for the miners charged with building the cribs. Several former miners worked with Chugh on the design with that perspective in mind. The new braces weigh 18 pounds, about half what a traditional cribbing timber weighs, so miners also can work faster.
“Sometimes miners have to carry the timbers a couple of hundred feet before stacking them,” Chugh said. “So this will be easier on their backs.”
The braces are made from readily available hardwoods including oak, sycamore, poplar and hickory. They come in several sizes, with various numbers of contact areas that allow for different cribbing configurations depending on the situation. A six-point configuration, for example, will carry 195 tons while a nine-point one would take about 300 tons of load.
Several mines are either testing or plan to test the method this summer, Chugh said, and are using it to brace mine seals, which are critical stability points. At the same time, several local sawmills also are putting in bids to manufacture the braces, which they could then sell under license to mines throughout the Midwest.
“One local mining company uses 400,000 of these each year,” Chugh said. “The total demand within Illinois, Indiana and western Kentucky is about 1.5 million a year.”
The initial price the university will provide to the mines is about 3 cents cheaper per brace than the current method.