Hard rock miners commonly bore raises between levels underground, and now Raisebor, a division of Cowen & Co., has successfully used the technique to develop a shaft at a deep coal-mining operation in Alabama. Above: the Raisebor team in the foreground, the raise boring drill in the background.

Raisebor engineers the largest diameter raise in the Alabama coalfields

On December 7, 2014, Raisebor, a division of Cowin & Co., completed a 26-ft-diameter raise bore shaft to a depth of 1,440 ft, the largest diameter raise bore ever in North America, and appears to be the largest in the world. Raisebor performed the record-setting feat at the Jim Walter Resources No. 7 mine near Brookwood, Alabama, using an Atlas Copco Robbins 123R C raise boring machine and multimodular wing system (MMWS) reamer. The reamer was designed for this job in a collaborative effort with Atlas Copco design engineers.

The project started in 2009 when Rick Sidwell, general manager of Raisebor, started thinking about a larger raise bore. He knew the drill for this job needed to be special. “I see the industry going this way. It’s a safety factor that just makes sense,” Sidwell said. Years of planning and engineering followed.

The mine was sinking this shaft to offer miner and utility access closer to the working face. It was taking workers roughly an hour to reach the working face. Nearly 25% of each 8-hour shift was lost to commuting. The shaft was needed to make the mine more efficient.

The reamer was assembled on the surface custom to Raisebor’s specification, disassembled, taken underground and reassembled. Inset: Atlas Copco Secoroc cutters for the raise bore reamer.The reamer was assembled on the surface custom to Raisebor’s specification, disassembled, taken underground and reassembled. Inset: Atlas Copco Secoroc cutters for the raise bore reamer.

The 1,440-ft-long, 26-ft-diameter raise was actually the third project for the company’s Atlas Copco Robbins 123RH C raise boring machine. Two earlier, 20-ft raises were just enough for the crew to stretch its legs and get ready for the big pull.

The project was made more complicated than a typical raise by its size. The rebar-reinforced foundation for the drill was a massive 32-ft-deep concrete box built by Cowin & Co. at the surface that required the removal of 2,500 cubic yards of material prior to building the platform. The drill straddled an open box’s 1-ft-thick concrete liner, which was drilled out when the raise bore’s head reached the surface.

Drill operator James Bass said there is more power behind the 123RH C compared to other units he has used in the past. “I can feel more power and better control. And when I make a correction, there’s almost an anticipation of my actions. I can feel the changes in the formation as it happens and change the drilling parameters smoothly and as necessary.”

The formation’s high angle fractures with varying changes throughout the formation made it difficult to ream. “Because the head is so wide, I could be pulling though hard and soft formations at the same time,” Bass said. “Pulling slowly through it, though, I can feel the torque change. The rpm and force show constantly on the control panel, and I hear and feel what needs to be adjusted more than I see it.”

Rick Sidwell, general manager of Raisebor, a division of Cowin & Co. Inc.
Rick Sidwell, general manager of Raisebor, a division of Cowin & Co. Inc.

Average drilling parameters put 2.5 rpm of head rotation at the pipe. The average thrust was from 350,000 to 700,000 lb, with torque at or below 350 klbf. The system operates off two hydraulic systems, with RCS-based monitoring, for high machine availability.

An external loop cooling system maintains optimum drive and thrust system temperature. The drill runs on a 480-volt drive pack and a 700-hp hydraulic motor turning the drill str