Underground gold mines in Nevada are exploiting increasingly deeper ore bodies comprised of weak, to very weak rock masses. The Rock Mass Rating (RMR) classification system is widely used at underground gold mines in Nevada and is applicable in fair to good quality rock masses, but is difficult to apply and loses reliability in very weak rock mass to soil-like material. Because very weak rock masses are transition materials that border engineering rock mass and soil classification systems, soil classification may sometimes be easier and more appropriate to provide insight in material behavior and properties. The Unified Soil Classification System (USCS) is the most likely choice for the classification of very weak rock mass to soil-like material because of its accepted use in tunnel engineering projects and its ability to predict soil-like material behavior underground. A correlation between the RMR and USCS systems was developed by comparing underground geotechnical RMR mapping to laboratory testing of bulk samples from the same locations, thereby assigning a numeric RMR value to the USCS classification that can be used in spreadsheet calculations and geostatistical analyses. More broadly, this RMR/USCS correlation helps define the transition between engineering soil and rock mass classification systems, and may provide insight for geotechnical design in very weak rock masses.


Underground gold mines in Nevada are exploiting increasingly deeper zones composed of weak to very weak rock masses. Production zones within ore bodies are typically composed of intensely fractured and highly altered rock with Rock Mass Ratings (RMR) [1, 2] less than 45 [3, 4]. Access drifts and infrastructure often intersect faults and altered material of varying thickness and geotechnical quality ranging from blocky competent rock to soil-like material.

The RMR system [1, 2] is a numeric geotechnical classification system commonly used at underground gold mines in Nevada. RMR data is typically collected from drill core, underground mapping and surface mapping. Spreadsheet calcuations and geostatistical analyses are often perfomed on RMR data to construct predictive geotechnical models for a variety of uses including: a) mining method selection, b) stope design, c) ground support design, d) primary development and infrastructure planning, e) estimate development rates and costs, f) anticipate poor ground conditions, and g) quick reference to mine-wide geotechnical conditions.

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