Increasing mining productivity is a goal to be achieved in many different ways, employing talents, techniques, and standards from many different areas, both divisional and geographical. The high quality backfill at Barrick’s Turquoise Ridge Mine (TRJV) has presented the opportunity to test the potential to optimize the dimensions of drifts mining below backfill. Other mines in Nevada have successfully mined entire levels using an optimized mining sequence that employs the use of 30’ wide undercuts beneath a cemented rock fill sill. The average Rock Mass Rating of the rock mass at TRJV is about 40% less than that of other mines in the areas implementing the same undercut dimensions. Given this situation, the question of increasing the undercut dimensions has been raised and backed with enthusiasm. The problems that may be faced are both operational and geotechnical, and have presented the opportunity to cooperate between departments around the mine site. These undercuts are planned to be mined in March of 2015. This paper will cover the analyses undertaken to determine the viability of mining these undercuts.
The Turquoise Ridge Joint Venture (TRJV) underground mine is located about 50 km (30 mi) northeast from Winnemucca, NV, USA, at the foot of the Osgood Mountains. The ore body at TRJV is highly disseminated over a large area and gold is extracted using underhand cut and fill (UHCF) mining in order to gain the highest possible ore recovery. The rock in these ore bodies is of exceptionally poor quality (RMR 20-35) and requires the implementation of a highly competent backfill. TRJV has been involved in several studies involving backfill in the past decade, namely a study in 2007 involving NIOSH (National Institute of Occupational Safely) as the primary researchers. This study involved successfully instrumenting and mining a 13.7 m (45’) wide undercut below a 4.6 m (15’) thick layer of backfill. This particular study will differ from the previous in that the wide spans will be left open for an extended period of time. This drift will eventually be implemented as a tire shop. There will be two 30’ wide undercuts that will be separated by a 20’ wide backfill pillar. This pillar will be monitored for stability as it is mined and for long term stability after this area is abandoned. The previous 45’ wide span was opened up in an area with relatively high RMR values under a singular 15’ tall backfill sill above. The location of the 30’ wide cuts at the center of this study are located in the High Grade Bullion (HGB) zone of the mine, located approximately 2200’ from ground surface. Mining these 30’ wide cuts will mark the first time in the history of TRJV that a 30’ wide cut will be implemented in to the normal mining cycle in a highly active mining zone.