Effect of Shear Stresses on Pillar Stability: A Back Analysis of the Troy Mine Experience to Predict Pillar Performance at Montanore Mine

This paper describes the results of a back analysis of pillar failures at Troy Mine, Montana, and the use of this experience to make forward predictions on pillar stability in the nearby Montanore deposit which lies in a similar geomechanical setting. At Troy Mine, a progression of pillar failures in areas within the Middle Quartzite of the Revett formation led to the observed surface subsidence. The Troy Mine experience was used to understand the level of stresses and failure mechanism leading to the collapse of some pillars in the North Orebody to estimate pillar strength in quartzite beds within Troy’s mountainous terrain. The model elucidated that the dipping orebody geometry in relation to topography led to shear stresses in pillars at Troy Mine. Shear stresses resulted in significant loss of confinement in pillar cores (many theoretically in tension), even at width-to-height ratios that would be deemed stable under zero shear stress (flat seam under flat topography). A calibrated model was achieved, which allowed us to evaluate the impact that different pillar geometric characteristics (such as width, length, height, and shape) have on pillar performance under shear conditions for different depths and extraction ratios. Design charts were then generated to provide guidance on pillar geometry based on expected demand. Mine-wide models were developed to predict the level of vertical stress and horizontal shear stress for pillars in the different ore-bearing beds at Montanore. A sensitivity study was performed for various conditions, including extraction ratio, spatial location under the mountainous terrain, and local orebody geometry with the aim of performing a mine-wide evaluation of the factor of safety against shear. The results of the analyses performed in the present work show that the use of design methods that do not take the effect of shear stresses into account may result in under-designed pillars, while a false impression of rock mass strength could be derived from back analysis.