Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R01-OH-008709, 2008 Dec; :1-114
Scaling is the activity of removing loose and unstable rock from underground mine openings, and is a fundamental activity necessary to promote employee safety and minimize the inherent risk associated with ground falls in underground metal/nonmetal mining operations. Unfortunately, a significant number of accidents occur while scaling. While mechanized scalers are common in most large operations, manual scaling is still the standard throughout the U.S. underground hardrock industry. A job safety analysis (JSA) study performed as part of this research identified three major areas of risk associated with hand scaling: 1) The immediate proximity of the miners to the working face and unstable/unsupported ground, 2) Working on uneven and irregular floors/surfaces, and 3) Physical fatigue caused by the strenuous nature of manual scaling. The most logical conclusion to mitigate and/ or eliminate these potential hazards involve automating the process and removing the miner from the operating environment through the use of an alternative technology or technique. This report summarizes research performed at the Colorado School of Mines, Edgar Experimental Mine with the primary goal of evaluating the effectiveness of using waterjet scaling as part of a mechanized scaling system. It is believed that proper implementation of waterjet scaling could significantly reduce the number of accidents and injuries that result as a consequence of scaling. A prototype waterjet scaling system was built and used for the tests conducted in this project. A pump configured to operate at 24 MPa (3500 psi) and a flow rate of 0.11 m3/min (30 gpm) was used to provide high pressure water for scaling. A donated shotcrete truck served as the carrier vehicle. The hydraulically actuated boom of the vehicle was used to sweep the waterjet nozzle across the area to be scaled. An electronic hydraulic valve bank was installed on the rig that allowed for remote tethered operation, thus allowing miners to operate the rig at a safe distance away from the area being scaled. As part of this research, five different nozzle designs were evaluated: a single orifice continuous jet, a dual orifice self-rotating jet, an acoustically pulsed, single orifice jet, and two types of mechanically oscillated single orifice jets. All of the nozzles tested performed well. However, based upon empiric results and observations made during testing, a preferred nozzle design was selected. The researchers chose the hydraulically powered, mechanically oscillated, single orifice jet for a variety of reasons. The design of the oscillating unit is a device similar to those commonly used to provide nozzle rotation in conventional shotcrete applications. Using the rotational unit with the single orifice continuous jet meant the power and performance of the single orifice jet could be maintained, while maximizing surface area coverage. The simplicity, low cost, and robust design of this unit proved to be a definite advantage in the harsh working environments often encountered in underground mining. An unplanned additional benefit of this research was to evaluate the effects of smoothwall blasting on the amount of material scaled. With smoothwall blasting, scaling times were shorter, and the total amount of material scaled was on average about one-third the volume of when perimeter holes were fully charged with ANFO. This research demonstrated that the safety advantages of waterjet scaling were pronounced and it represents a viable alternative to conventional scaling techniques. Furthermore, there appears to be tremendous potential benefits that could dramatically improve miner safety as a consequence of integrating waterjet scaling with other advanced underground excavation practices, including precision drilling, engineered round design, smoothwall blasting, and the utilization of new techniques and products related to shotcrete and rock support systems.