In Carlin Style gold deposits challenging ground conditions can be correlated to the alteration type and intensity as well as ground disturbance (i.e. faulting or collapse breccias). In these conditions, there are significant limitations in the application of conventional rock mass rating systems (i.e. RMR or Q) because these systems emphasize fracture frequency and fracture condition rather than the condition of the matrix material. As matrix strength is reduced, the condition of fractures and the fracture frequency become less relevant as indicators of geotechnical ground behaviour. As part of this work, a classification scheme was developed to incorporate the intensity of rock alteration as well as the degree of ground disturbance. This scheme provides a hazard classification system for rocks that fall below the practically useful range of conventional systems.
Further to the classification system, this study uses geological data cross-referenced to geotechnical data to verify both data sets, and to increase geotechnical knowledge about a rock mass. This is achieved by the MDEng Weights of Evidence Method which uses multiple parameters from the geology log to assign indicator scores for geotechnical condition to each logged interval of a drill hole database. Indicator scores are processed by a mathematical algorithm in order to assign hazard rankings for each interval of geologically logged core. This analytical approach produces a non-subjective, repeatable and semi-automated way to evaluate expected geotechnical conditions from existing geological logs which, typically, would otherwise be overlooked by geotechnical practitioners.
During all phases of mine design (from exploration to operations), geological logging data is generally significantly more abundant and often more detailed than geotechnical records. When geotechnical parameters are collected as part of an exploration campaign, they are often limited to RQD and occasionally a strength and/or weathering estimate. As mine design advances, geotechnical data collection generally expands to cover the necessary parameters for joint characterization and rock mass classification. Detailed geotechnical data is generally collected on far fewer drill holes than geological data, resulting in broader inference of ground conditions.
Newmont's Leeville/Turf operation, located along the Carlin trend in North East Nevada, has such a database with extensive geology logs and (comparatively) limited geotechnical data. In addition, the classification systems that were used in the geotechnical logs, the RMR (Bieniawski, 1976) and MRMR (Laubscher and Jakubec, 2001) systems were not practically useful for all of the ground conditions present on site.