The Deserado mine in North West Colorado has been using an innovative Fletcher drilling feedback system to help improve roof bolting. Jeff Dubbert, director of engineering at the mine, described the system at a technical briefing session at MINExpo in Las Vegas in October.
The 2 million ton per annum Deserado mine supplies steaming coal to the Bonanza power plant. Mining is by continuous miner and one longwall at a seam height of 2.13-3.35m. Mining in the B-seam has proven to be relatively easy, even though the shallow cover, averaging 107m, results in the longwall shields being in constant yield. Controlling the roof during CM development has been a tremendous challenge. It is intensive, expensive and a limiting factor in development rates.
Roof material is extremely laminated and the first 30-60cm falls immediately upon mining. Immediate roof consists of layers of shales, siltstones and mudstones with periodic lenses of cideritic siltstone, and occasional extremely hard bands of siderite.
The mine operates two Fletcher dual boom CHDDR roof-bolting machines. Drilling is done dry, utilising a vacuum dust collection system. Bit consumption can vary from 1-2 metre holes per bit to three bits per hole depending on the presence and thickness of the cideritic limestone. The average is about one bit per hole.
In 1999, the mine agreed to work with JH Fletcher to help further develop their micro processor based feed back drilling system. Together with West Virginia University, Fletcher has been developing the system since 1988 but the system had not been tested in a production environment.
After a dual boom roof bolter was rebuilt with the feedback drilling system installed the machine began operation on March 6, 2000 on a two shift per day basis. Today it operates in 38-45cm wide entries, 3-3.65m high, installing 60cm mechanical bolts, 2m resin bolts, roof trusses, and 4m cable bolts.
Dubbert said the mine's initial goals were to reduce the frequency of plugging of the vacuum system; reduce the time required for a new operator to become proficient; maintain consistency in roof bolt installation; decrease operator fatigue by use of the hands off drilling ability; and, identify and map roof lithology.
While the system has enabled operators to become proficient much faster, an unexpected spinoff was the response of operators who had spent years learning to drill safely and efficiently. Said Dubbert: "One of our more experienced operators, Jerry Eliason, commented: 'You are not making the beginner as good as me, you are making me as bad as the beginner'.
"Fortunately, Jerry has become the most ardent supporter of the system and gone far beyond the call of duty to learn the intimate details of the system. Jerry has taken it as a personal challenge to fully understand all of the intricacies of the system and to help to establish its full potential. He has been a driving force in most of the hardware and software modifications that have been made," Dubbert said.
While the system allows the operator to drill a hole in manual, the electrical joystick only requires 3.5 pounds of force, compared with 15 pounds for the manual unit.
"When you consider that advancing the section 60m requires 1600 joystick operations while in the manual control, a tremendous reduction in physical effort can be obtained by operating in the feedback operation mode," Dubbert said.
The system's pre-cleaners collect the coarse cuttings and discard them onto the ground, while the feedback system's microprocessor automatically controls dumping of the pre-cleaners. The result has been that operators can bolt about 60m before emptying the dust boxes. This is not only a significant savings in time but also significantly reduces the operators’ exposure to dust.
The drill control unit, which comprises the microprocessor, display screen, toggle switches, and push buttons, the vacuum feedback sensor and the speed sensor for monitoring drill rotational speed have performed flawlessly, according to Dubbert.
A major benefit has been the reduction of plugging the vacuum hose even in wet drilling conditions. Dubbert said on a test run the same operator plugged 20 times running on manual mode and the next day, on the feedback system there was no plugging.
Dubbert said most of the mine's goals with the system had been met. Once the roof mapping software has been enabled, it is hoped the system will provide invaluable information to mine management on roof lithology, he said.
Apart from operating at Deserado, the feedback system is also in operation at Monterey mine in Illinois, and on the Quad Ranger four head roof bolter at BHP San Juan Mine in New Mexico. Monterey has ordered two new machines with feedback and one retrofit kit for an existing machine, which will fit their entire bolter fleet with feedback systems.
