ABSTRACT:

As shallower deposits in many mining jurisdictions are depleted, it is becoming more common to mine in deeper and more challenging ground conditions. This paper provides a discussion of empirical support design methods using the following rock mass classifications: Barton's Q System; Bieniawski's RMR System; and Warren's Weak-RMR. The Weak-RMR classification system, and empirical support design procedures proposed by Warren et al., 2018 are explored in more detail with a series of numerical models to test design sensitivities. These numerical models have been set up to evaluate the response of the empirical design of Warren et al., 2018, for a W-RMR range of 10-35. Based on these initial numerical models, comparisons are made to illustrate the effects of changing the excavation profile (i.e. square, circle, and horseshoe) as well as including floor support (i.e. cables, and invert).

1. Introduction

Many Nevada gold deposits are hosted in intensely fractured, faulted, and altered rock types, which are typically classified as very poor, or lower, rock mass conditions. Due to these conditions, there was an increase in the number of mining related fatalities and injuries in Nevada from 1985 to 2000 (Brady et al., 2005). Very poor ground conditions are defined as Q < 1, or when RMR or GSI < 40. Ground support design within these conditions is challenging due to uncertainties in material properties and boundary conditions.

In response to increased frequency of ground falls, a number of research efforts have been pursued to improve understanding of support design and performance in these conditions. These studies have produced:

• Updated design span curves (Ouchi et al., 2009)

• The Weak-RMR system of classification (Warren et al., 2016)

• Support bond strength recommendations for poor ground conditions (Pakalnis, 2014)

• Shotcrete sequence recommendations (Raffaldi et al., 2018)

• Empirical support design guidelines (Warren et al., 2018)

• Empirical support design capacity charts (Warren et al., 2018)

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