Abstract

Many Nevada gold deposits are found in highly fractured, faulted, and argillised host rock with Rock Mass Rating (RMR76) mostly ranging from 10 to 50 classified as weak to very weak rock under Bieniawski’s (1976) rock mass classification system. Numerical modeling techniques provide a potentially useful means for evaluation of models for a number of different geological conditions, stress conditions, and support scenarios. The study focuses on application of numerical models for varying geological conditions and support scenarios for underground gold mines in Nevada. Time dependent numerical models are developed in 3DEC and calibrated to the displacement data procured by Multi-Point Borehole Extensometer (MPBX) arrays. Joints are embedded in the Numerical models by creation of a discrete fracture network. Parametric analysis of modeling parameters is proposed to determine confidence in these parameters at various underground locations. In the numerical models, the roof is supported by rock bolts to replicate the insitu squeezing behavior of the rock mass under stress. [2].

1. INTRODUCTION

Commonly, gold deposits in Nevada are found in highly fractured and faulted ground. Ground containing a large number of faults and fractures tends to be more susceptible to squeezing and moving. As ground squeezes and moves, safety begins to become a concern. In order to monitor this squeezing and movement of ground, Multi Point Borehole Extensometer (MPBX) arrays are commonly placed in areas of particular concern.

Multi Point Borehole Extensometers are composed of six anchor points and an integrated electronic readout head. The relative displacement between these anchor points is measured and recorded to determine displacements within the drift. These displacements can then be used to model typical behavior of the drift at the MPBX location.

The Discrete Fracture Network (DFN) approach is commonly used for joint generation in numerical models to simulate the behavior of fractured rock in an underground mine. The Discrete Fracture Network approach is well established and used primarily in petroleum engineering for permeability and fluid flow analyses. Methods by Snow (1965, 1969) determined the orientation and apertures of fractures in the field. Snow’s theory assumed fractures to be infinite, which would result in higher permeability values than the actual permeability of the fractured rock mass [1, 2]. Parsons (1966) used a regular fracture-matrix model with any number of fracture sets for the heterogeneous fracture system [3].

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