Numerical Investigation of the Strength and Failure Mechanism of Hard Rock Pillars Under Compressive and Shear Loading Conditions

Abstract

Pillars are important support elements in underground mines. A reliable design of hard rock pillars requires a thorough understanding of their strength and failure mechanisms. While several investigations have been conducted to predict the rock mass strength near excavation boundaries (low confinement), only a few studies have attempted to estimate the rock mass strength at high confinement, which is relevant for the design of the core of wide pillars. Recognizing this research gap and the limitations of conventional design approaches, the aim of this study is to enhance the current understanding of the strength and behavior of hard rock pillars using numerical modeling. For this purpose, this study employs different numerical methods to simulate the failure of hard rock pillars, focusing on a well-known case of pillar failures at the Quirke Mine, Ontario, Canada. The Quirke Mine is a typical example of a mine that experienced a chain reaction of pillar failure due to shear loading. The rib pillars at this mine were initially laid out parallel to the dip direction of an inclined orebody. In the central part of this mine, the pillars were re-oriented at 45° to the orebody dip direction. These pillars failed due to unfavorable shear loading, causing the failure of those in compression. Numerical simulations were conducted using: (1) 2D continuum-based heterogeneous models; (2) plane strain discontinuum models; and (3) 3D continuum and discontinuum models. The plane strain continuum and discontinuum heterogeneous models were used to estimate the strength of Quirke Mine pillars. The calibration of the rock mass models was based on an empirical (tri-linear) brittle failure criterion originally developed for low confined problems. Next, the impact of shear loading on pillar stability was examined using 3D continuum and discontinuum models. The results of numerical simulations highlight the importance of the rock mass strength envelope in determining the strength of hard rock pillars. The findings indicate that an appropriate rock mass model may need to consider a strength envelope that matches the in situ damage initiation stress level at low confinement but is significantly lower than that of the tri-linear envelope at high confinement.