Investigating the Energy Balance of Two Mining Methods Set in a Deep Underground Metal Mine in the U.S.

Hecla Mining Company (Hecla) operates the Lucky Friday Mine since 1958, which is an underground silver, lead, and zinc mine near Mullan, Idaho, USA. In the mid-1980s until 2020, the company relied on the Lucky Friday Underhand Longwall (LFUL) method for seismic control to mine (first) the Lucky Friday and (then) the Gold Hunter ore bodies. However, engineering challenges related to high in-situ rock stresses, squeezing ground conditions and seismic occurrences continued to increase as the production levels deepened to depths greater than 2,000 m below ground surface. In 2020, Hecla developed a new mining method called UCB (Underhand Closed Bench) which proactively induces fault slip in proximity of the mining front, improving safety and productivity at the mine. UCB was fully implemented at the Gold Hunter in 2021. Although preventing faults to slip at their critical load was identified as the main seismic control measure, the contribution of fragmented ore presents another component capable of reducing the seismic energy release measured at the mine. As part of the seismic hazard research initiative, the National Institute for Occupational Safety and Health (NIOSH) collaborated with Hecla to develop a series of numerical models using a 3-dimensional distinct element method (3DEC) to quantitatively investigate and compare the differences in the energy balance characteristics between the LFUL and the UCB methods. As a result, the numerical models were sensitive enough to capture the differences between the mining methods and were in good agreement with the field observations, recognizing the influence of blasted ore in seismic control. 1. INTRODUCTION As part of the project, managing ground support for long-term stability, the National Institute for Occupational Safety and Health (NIOSH), is researching more effective seismic energy analysis methods and improved forecasting of induced seismicity around mining areas. The Lucky Friday Mine (Hecla) has recently implemented a new mining method, Underhand Closed Bench (UCB), that has yielded favorable results regarding seismic energy release and control when compared to the Lucky Friday Underhand Longwall method (Board et al., 2022; Armatys, 2023). This collaborative opportunity was realized and leveraged NIOSH's modeling expertise and Hecla's proprietary mining method, which is beneficial for both parties.