This paper presents a case study of geotechnical characterization used for underground mine design at the Murray Mine. The characterization approach is described and the distributions of the data are related to reliability requirements for underground production design. The conclusions from the characterization approach and the resulting recommendations are compared to the mining experience.
This paper describes the geotechnical characterization and mine structural design used for underground mining at the Jerritt Canyon Joint Venture (JCJV) Murray Mine, located approximately 80 km north of Elko, Nevada, USA, in the Independence Mountains. The JCJV is an operating joint venture between Anglogold (Jerritt Canyon) Corporation and Meridian Gold.
Beginning in 1993, mining at Jerritt Canyon began a transition from open pits to underground mines. The first significant underground mining was conducted in the West Generator Mine, developed from the bottom of the West Generator Pit, in a small deposit whose extraction would require an uneconomic lay-back of the existing pit wall. Rock mass quality at West Generator allowed the use of a sublevel stoping mining method with cemented rockfill. Successful application of the sublevel stoping mining method at West Generator and its relatively favorable cost made it the primary candidate at the Murray Mine, which was located in the same rocks approximately 0.5 km away.
Rock mass characterization data was generated from core drilling of the Murray ore body over a several-year period preceding the initial underground mining at West Generator. The drilling data was augmented by rock mass conditions exposed by limited development mining at the Murray Mine and from exposed rock mass conditions in the West Generator underground mine. Evaluation of core data from 16 core holes and mapping of early exposures in development headings were undertaken to confirm the suitability of the proposed mining method. Three additional core holes were drilled in 1994, specifically for structural confirmation of the data from earlier drilling. The three new Murray data sources were consistent and suggested that substantially lower rock mass quality and higher variability existed in much of the Murray ore body. Reductions in stope spans and adjustments in extraction/backfilling sequences would be required for application of sublevel stoping. Alternatively, a drit?-and-fill mining method would provide a fallback strategy.
Design recommendations were based on rock mass quality data distributions and were made to insure high levels of reliability in the structural stability of production excavations. High levels of reliability were required because of the direct access mining method. Probability distributions of the corebased geotechnical parameters rock quality designation (RQD), rock mass rating (RMR), Q, and uniaxial compressive strength (UCS) were developed from the data and used with empirical design methods (Barton et al. 1974, Mathews et al. 1980, & Potvin 1988). The data distributions and direct comparison with conditions at the Cannon (Brechtel et al. 1987) and West Generator underground mines were used for mining method evaluation and estimates of ground support requirements.
