Mark Board (Hecla Mining Comopany), Bob Golden (Hecla Mining Company)
This paper provides a description of and preliminary results from an intensive rock mechanics research program currently underway at Hecla Ltd’s Lucky Friday mine in Mullan, Idaho, US. The Lucky Friday mine, about 70 years old, is one of the deepest (approx. 2400m) mines in the western hemisphere, extracting narrow, vertical silver, lead and zinc veins using underhand cut and fill mining. The program described here is aimed at understanding seismic source mechanisms, and developing proactive measures for reducing their occurrence and the potential damage that can result from interaction of the generated seismic waves and the mining excavations.
The basis for Hecla’s rockburst mitigation strategy is twofold: a) use of “destressing” strategies for advancing mining fronts and pillars based on creation of “stress shadowing” excavations on parallel vein structures to divert stresses from the primary 30 Vein stopes or use of intensive blasting of slots in advance of mining using large-diameter holes, and, b) installation of dynamic ground support methods in critical areas to retain potential seismically-induced damage to excavation surfaces. Extensive numerical modeling was employed in the design of these proactive measures which are now in progress.
A cooperative project was developed in 2012 with the mining researchers at the NIOSH Spokane Research Center (SRC) for installation and assessment of extensive rock mass instrumentation as well as large-scale quasi-static laboratory testing of ground support. Field instrumentation, including biaxial stressmeters, stope closure instruments, paste fill pressure gauges and fault displacement gauges have been installed to monitor the impact of stress shadowing of stopes mining downward from the 5500 level. Real-time seismic monitoring using a mine-wide waveform-digitizing seismic system, a surface-based network of seismometers, as well as detailed lithology and structural mapping, are used to understand seismic source mechanisms. Finally, periodic three-dimensional photogrammetry at strategic fault-excavation intersections surrounding the stoping area is used to provide deformations to understand the large-scale kinematics of fault and rock mass movements. All of this data is used for verification and updating of numerical stability predictions.
To understand the large-deformation response of ground control systems employed at the mine, NIOSH is performing full-scale loading tests on mesh and fiber-reinforced shotcrete panels supplemented by a new generation of dynamic, high strain capacity grouted rockbolts (the D-Bolt). This testing provides data on the energy dissipation capacity of the support and guides the support design in critical areas underground.
