Abstract

Assumed stress states can have significant implications to the feasibility of mining projects, as stope design, strategic sequencing and permanent infrastructure siting are major controls on development costs and production rates, and henceforth life of mine economics. The stress state used for numerical modelling purposes is typically a global assumption for the entire mining area of interest, when in fact the actual magnitude and orientation of the stress tensor may vary by localized geological controls. This paper provides a discussion on how to minimize uncertainty imn pre-mining stress state assumptions by utilizing analytical and numerical methods to back-calculate far field stresses from observations of borehole breakout. A case history is presented examining breakout observed in acoustic televiewer data in a deep shaft pilot hole at KGHM International’s Victoria Project in Sudbury, Ontario, Canada. In addition to the use of conventional analytical methods, the available data are analyzed based on a series of two-dimensional numerical models. Ultimately, a stress model is developed to demonstrate the proposed analysis methodology. The proposed model shows the local variability in stress magnitudes across different lithologies while generally agreeing with a regional stress model from the literature

1. INTRODUCTION

When designing excavations at depth, the stress model used in empirical, analytical, and numerical tools plays a significant role in the outcomes of analyses. In the field of mining geomechanics, the stress state can have significant implications for the feasibility of projects, as major controls on life of mine economics such as stope design, excavation sequencing, and permanent infrastructure siting are all dependent on predicted stress re-distribution patterns.

The most recent ISRM Suggested Method for rock stress estimation provides a strategy for stress model development which integrates geology and tectonic setting, stress indicators, stress measurements, and numerical modelling into a final model [1]. One of the preliminary stress estimation techniques recommended by Stephansson & Zang [1] is the analysis of borehole breakout data. Borehole breakout data provides a convenient way to estimate the in-situ stress tensor, as all the necessary information can be obtained simply by logging existing boreholes using a device such as an Acoustic Televiewer (ATV).

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