Mining Project: Durable Roof Support for Underground Metal Mines

Research Summary

The NIOSH Mining Program and its past affiliation within the U.S. Bureau of Mines has established a long history of pioneering research in ground support. The literature gives many examples of this work that contributed to the knowledge of rock mass rating, rock mass strength, and geologic structure and its effect on the stability of underground openings. As a result of this research, design charts were developed that indicate the range of stable mine opening spans in various ground conditions. Additional research was conducted on the use of shotcrete in weak rock mass mines, and portable shotcrete test machines were developed to measure shotcrete properties (flexural, adhesion, and early strength) directly at the mine site. These tests were used to optimize the fiber content and to compare the performance of wire mesh composites to fiber-reinforced shotcrete. This research led to a better understanding of the role of shotcrete in multiple-component ground support systems being used in the mines, thereby increasing the stability of the ground and reducing the accident risk to workers.

This project sought to build on that past research to significantly reduce ground fall accidents in underground metal mines in the western U.S. The targeted ground fall accidents were directly related to falls of ground that could be measured using the MSHA database of Non-Fatal Days Lost (NFDL) and No Days Lost (NDL). Weak blocky ground, faults, ground support corrosion, and highly stressed ground were among the factors contributing to difficult ground control conditions considered in this study. These concerns were addressed by taking a holistic look at ground support through the mining cycle at mines experiencing these hazards. The problem was how to improve ground support for these mine hazards through a systematic design approach that reduced accident risk to miners.

The research focused on the more difficult ground conditions in which conventional ground support design required novel interventions. A major design concern was the loading and strengths associated with the overlying ground support system and its ability to withstand high-energy loads. By monitoring the load and displacement of the rock mass, more effective ground support systems could be selected and installed to create safer working conditions for miners. Additionally, the project utilized instrumentation in both laboratory tests and field evaluations that was capable of reading levels and/or rates of change of ground control system movement and degradation over time. These features were meant to be incorporated into mine monitoring and used to warn miners of rock mass instabilities.

The major tasks of this research and their associated outputs were as follows:

(1) Design and integrate novel ground control systems using results obtained from laboratory and field testing of composite support solutions.

Raffaldi MJ, Warren SN, Martin LA, Stepan MA, Pakalnis R, Sandbak LA. Reinforced Shotcrete Performance: Quantifying the Influence of Ground Support Installation Sequence. Paper presented at the 52nd US Rock Mechanics Symposium, Seattle, 2018.

(2) Develop advanced monitoring, sensing, and data transmission systems for improved ground control.

Benton DJ, Warren SN, Sunderman CB, Richardson JR. A Novel Application of Photogrammetry to Ground Convergence Monitoring in Underground Excavations. Proceedings of the Society of Photo-Optical Instrumentation Engineers (SPIE), Novel Optical Systems Design and Optimization XXI, 17 September 2018. 

(3) Evaluate support installation procedures for opportunities to modernize installation equipment and implement alternative methods to prevent support degradation.

Benton DJ, Warren SN, Sunderman CB, Richardson JR. A Novel Application of Photogrammetry to Ground Convergence Monitoring in Underground Excavations. Proceedings of the Society of Photo-Optical Instrumentation Engineers (SPIE), Novel Optical Systems Design and Optimization XXI, 17 September 2018. 

Clark CC, Benton DJ, Seymour JB, Martin LA. Jackleg Drill Injuries. Mining Engineering, Vol. 68, No. 8, pp. 57-62. August 2016.

(4) Assess and disseminate optimal ground control strategies identified within the rest of the research project through cooperation with collaborating mines.

Warren SN, Pakalnis R, Raffaldi MJ, Benton DJ Sandbak, Barnard CK. Ground Support Design for Weak Rock Mass: Quantifying Time-dependent Closure in Squeezing Ground. Proceedings of the Ninth International Symposium on Ground Support in Mining and Underground Construction, Australian Centre for Geomechanics, Perth, pp. 169-184. 2019.

The major outputs from this research were quantitative performance measures of shotcrete-based support systems, ground control design recommendations for very low Rock Mass Rating (RMR) rock masses, and adaptation of corrosion assessment and control methods to underground metal mining conditions. These findings will help lead to improved ground support in difficult mining conditions. Translation of this research to practice has the potential to reduce ground falls and accidents in western U.S. underground metal mines. This research may also have potential applications in other mining sectors and related industries.